This article aims at exploiting GPGPU (GP^{2}U)
computation capabilities to improve physical and scientific simulations. In
order for the reader to understand all the passages, we will gradually proceed
in the explanation of a simple physical simulation based on the well-known Wave
equation. A classic CPU implementation will be developed and eventually another
version using GPU workloads is going to be presented.

PDE
stands for “Partial differential equation” and indicates an equation which has
one or more partial derivatives as independent variables in its terms. The
order of the PDE is usually defined as the highest partial derivative in it.
The following is a second-order PDE:

Usually a PDE of order *n* having *m* variables *x*_{i} for *i=1,2…m*
is expressed as

A compact form to express is *u*_{x} and the same applies to (*u*_{xx} )
and
(*u*_{xy} ).

The wave equation is a second order partial
differential equation used to describe water waves, light waves, sound waves,
etc. It is a fundamental equation in fields such as electro-magnetics, fluid
dynamics and acoustics. Its applications involve modeling the vibration of a
string or the air flow dynamics as a result from an aircraft movement.

We can derive the one-dimensional wave equation (i.e.
the vibration of a string) by considering a flexible elastic string that is
tightly bound between two fixed end-points lying of the *x* axis

A
guitar string has a restoring force that is proportional to how much it’s
stretched. Suppose that, neglecting gravity, we apply a *y-direction* displacement at the string (i.e. we’re slightly
pulling it). Let’s consider only a short segment of it between *x* and *x+*
*: *

Let’s write down for the small red segment in the diagram
above. We assume that the string has a linear density (linear density is a measure of mass per unit
of length and is often used with one-dimensional objects). Recalling that if a
one-dimensional object is made of a homogeneous substance of length *L* and total mass *m* the linear density is

we have *m= * .

The forces (as already stated we neglect gravity as
well as air resistance and other tiny forces) are the tensions *T* at the ends. Since the string is
actually waving, we can’t assume that the two *T* vectors cancel themselves: this is the force we are looking for. Now let’s make some
assumptions to simplify things: first we consider the net force
we are searching for as vertical (actually
it’s not exactly vertical but very close)

Furthermore
we consider the wave amplitude small. That is, the vertical component of the
tension
at the *x+ *
* *end of the*
*small segment of string is

The slope is
if we
consider *dx* and *dy* as the
horizontal and vertical components of the above image. Since we have considered
the wave amplitude to be very tiny, we can therefore assume
. This greatly helps us:
and the
total vertical force from the tensions at the two ends becomes

The equality becomes exact in the limit
.

We see that *y*
is a function of *x*, but it is however
a function of *t* as well: *y=y(x,t)*. The standard convention for
denoting differentiation with respect to one variable while the other is held
constant (we’re looking at the sum of forces at one instant of time) let us
write

The final part is to use Newton’s second law
and put everything together: the sum of all
forces, the mass (substituted with the linear density multiplied by the segment
length) and the acceleration (i.e. just
because we’re only moving in the vertical
direction, remember the small amplitude approximation).

And we’re finally there: the wave equation. Using the
spatial Laplacian operator, indicating the *y*
function (depending on *x *and* t*) as *u* and substituting
(a fixed constant) we have the common compact
form

The two-dimensional wave equation is obtained by
adding a second spatial term to the equation as follows

The constant *c*
has dimensions of distance per unit time and thus represents a velocity. We
won’t prove here that *c* is actually the velocity at which waves
propagate along a string or through a surface (although it’s surely worth
noting). This makes sense since the wave speed increases with tension
experienced by the medium and decreases with the density of the medium.

In a physical simulation we need to take into account
forces other than just the surface tension: the average amplitude of the waves
on the surface diminishes in real-world fluids. We may therefore add a viscous
damping force to the equation by introducing a force that acts in the opposite
direction of the velocity of a point on the surface:

where the nonnegative constant
represents the viscosity of the fluid (it
controls how long it takes for the wave on the surface to calm down, a small
allows waves to exist for a long time as with
water while a large
causes them to diminish rapidly as for thick
oil).

To actually implement a physical simulation
which uses the wave PDE we need to find a method to solve it. Let’s solve the
last equation we wrote with the damping force

here *t*
is the time, *x* and *y * are the spatial coordinates of the 2D wave, *c*^{2} is the wave speed and
the damping factor. *u=u(t,x,y)* is the function we need to calculate:
generally speaking it could represent various effects like a change of height of
a pool’s water, electric potential in an electromagnetic wave, etc..

A common method to solve this problem is to use the
finite difference method. The idea behind is basically to replace derivatives
with finite differences which can be easily calculated in a discrete algorithm.
If there is a function *f=f(x)* and we want to calculate its derivative
respect to the *x* variable we can write

if *h* is a
discrete small value (which is not zero), then we can approximate the
derivative with

the error of such an approach could be derived from
Taylor’s theorem but that isn’t the purpose of this paper.

A finite difference approach uses one of the following
three approximations to express the derivative

Let’s stick with the latter (i.e. central difference);
this kind of approach can be generalized, so if we want to calculate the
discrete version of *f''(x)* we could first write

and then calculate *f''(x)* as follows

The same idea is used to estimate the partial derivatives
below

Let’s get back to the wave equation. We had the
following

let’s apply the partial derivative formula just
obtained to it (remember that *u=u(t,x,y)*,
that is, *u* depends by *t,x* and* y*)

This is quite a
long expression. We just substituted the second derivatives and the first
derivatives with the formulas we got before.

If we now consider ,
we are basically forcing the intervals where the derivatives are calculated to
be the same for both the *x* and the *y* directions (and we can greatly
simplify the expression):

To improve the readability, let us substitute
some of the terms with letters

and we have

If you look at
the *t* variables we have something
like

This tells us that the new wave’s height (at *t+1*) will be the current wave height (*t*) plus the previous wave height (*t-1*) plus something in the present (at *t*) depending only by what are the values
around the wave point we are considering.

This can be visualized as a sequence of time frames
one by another where a point on the surface we are considering evolves

The object

has a central dot which represents a point (*t,x,y*) on the surface at time *t*. Since the term we previously called *something_at_t(x,y)* is actually

the value of the central point is influenced by five
terms and the latter is its same value (-4*u*_{t,x,y} )
multiplied by -4.

As
we stated before, the wave PDE can be effectively solved with finite difference
methods. However we still need to write some resolution code before a real
physical simulation can be set up. In the last paragraph we eventually ended up
obtaining

we
then simplified this expression with

This is indeed a recursive form which may be
modeled with the following pseudo-code

for each t in time

{

u_{t+1 }<- u_{t}+u_{t-1}+something_at_t(x,y)

// Do something with the new u_{t+1}, e.g. you can draw the wave here

u_{t-1}<- u_{t }

u_{t }_{<- }u_{t+1 }

_{} }

The above pseudo-code is supposed to run in
retained-mode so the application might be able to draw the wave in each point
of the surface we’re considering by simply calling *draw_wave(u*_{t+1}) .

Let assume that we’re modeling how a water wave
evolves and so let the *u *function
represent the wave height (0 – horizontal level): we can now write down the
beginning of our pseudo-code

// Surface data

height =
500;

width =
500;

// Wave parameters

c = 1; // Speed of wave

dx = 1; // Space step

dt = 0.05; // Time step

k=0.002 // Decay factor

kdt=k*dt; // Decayment factor per timestep, recall that q = 2 -
kdt, r = -1 +kdt

c1 = dt^2*c^2/dx^2;
// b factor

// Initial conditions

u_current_t
= zeros(height,width); // Create a height x width
zero matrix

u_previous_t
= u_current_t;

We basically defined a surface where the wave
evolution will be drawn (a 500x500 area) and initialized the wave parameters we
saw a few paragraphs ago (make sure to recall the *q*,*r* and* b* substitutions we did before). The
initial condition is a zero matrix (*u_current_t*)
so the entire surface is quiet and there’s no wave evolving.

Given that we are considering a matrix surface (every
point located at (*x;y*) coordinates is
described by a *u* value indicating the
height of the wave there) we need to write code to implement the

line in the for cycle. Actually the above
expression is a simplified form for

and we need to implement this last one. We may
write down something like the following

for(t=0; t<A_LOT_OF_TIME; t += dt)

{

u_next_t =
(2-kdt)*u_current_t+(kdt-1)*u_previous_t+c1*something_at_t(x,y)

u_previous_t
= u_current_t; // Current becomes old

u_current_t = u_next_t; // New becomes current

// Draw the wave

draw_wave(u_current_t)

}

that is, a for cycle with index variable *t* increasing by *dt* at every step. Everything should be familiar by now because the
(*2-kdt*),(*kdt-1*) and *c1* terms are
the usual *q*,*r* and* b* substitutions.
Last thing we need to implement is the something_at_t(x,y) term, as known as:

The wave PDE we started from was

and
the term we are interested now is this:

that, in our case, is

Since we have a matrix of points representing
our surface, we are totally in the discrete field, and since we need to perform
a second derivative on a matrix of discrete points our problem is the same as having
an image with pixel intensity values *I(x,y)* and need to calculate its Laplacian

This is a common image processing task and it’s
usually solved by applying a convolution filter to the image we are interested
in (in our case: the matrix representing the surface). A common small kernel
used to approximate the second derivatives in the definition of the Laplacian
is the following

So in order
to implement the

term, we need to
apply the *D* Laplacian kernel as a
filter to our
image (i.e. the *u_current_t*):

u_next_t=(2-kdt)*u_current_t+(kdt-1)*u_previous_t+c1* convolution(u_current_t,D);

In fact in the element we saw earlier

the red dot elements are weighted and calculated
with a 2D convolution of the *D* kernel.

An important element to keep in mind while
performing the *D* kernel convolution
with our *u_current_t* matrix is that
every value in outside halos (every value involved in the convolution but
outside the* u_current_t* matrix
boundaries) should be zero as in the image below

In the picture above the red border is the *u_current_t* matrix perimeter while the
blue 3x3 matrix is the *D* Laplacian
kernel, everything outside the red border is zero. This is important because we
want our surface to act like water contained in a recipient and the waves in
the water to “bounce back” if they hit the container’s border. By zeroing all
the values outside the surface matrix we don’t receive wave contributions from
the outside of our perimeter nor do we influence in any way what’s beyond it.
In addition the “energy” of the wave doesn’t spread out and is partially
“bounced back” by the equation.

Now the algorithm is almost complete: our PDE
assures us that every wave crest in our surface will be properly “transmitted”
as a real wave. The only problem is: starting with a zero-everywhere matrix and
letting it evolve would produce just nothing. Forever.

We will now add a small droplet to our surface
to perturb it and set the wheels in motion.

To simulate as realistic as possible the effect
of a droplet falling into our surface we will introduce a “packet” in our
surface matrix. That means we are going to add a matrix that represents a
discrete Gaussian function (similar to a Gaussian kernel) to our surface
matrix. Notice that we’re going to “add”, not to “convolve”.

Given the 2D Gaussian formula

we
have that *A* is the Gaussian amplitude
(so will be our droplet amplitude), *x*_{0} and *y*_{0} are the
center’s coordinates and the
and are the
spreads of the blob. Putting a negative amplitude like in the following image
we can simulate a droplet just fallen into our surface.

To generate
the droplet matrix we can use the following pseudo-code

//
Generate a droplet matrix

dsz = 3; // Droplet size

da=0.07; // Droplet amplitude

[X,Y] =
generate_XY_planes(dsz,da);

DropletMatrix = -da *
exp( -(X/dsz)^2 -(Y/dsz)^2);

*da* is the droplet amplitude (and it’s
negative as we just said), while *dsz*
is the droplet size, that is, the Gaussian “bell” radius. *X* and *Y* are two matrices
representing the X and Y plane discrete values

so
the X and Y matrices for the image above are

And the final DropletMatrix is similar to the following

where the central value is -0.0700. If you drew
the above matrix you would obtain a 3D Gaussian function which can now model our
droplet.

The final pseudo-code for our wave algorithm is
the following

//
Surface data

height = 500;

width = 500;

// Wave
parameters

c = 1; // Speed of wave

dx = 1; // Space step

dt = 0.05; // Time step

k=0.002 // Decay factor

kdt=k*dt; // Decayment factor per timestep, recall that q = 2 -
kdt, r = -1 +kdt

c1 = dt^2*c^2/dx^2; // b factor

//
Initial conditions

u_current_t=zeros(height,width);
// Create a height x width zero matrix

u_previous_t =
u_current_t;

//
Generate a droplet matrix

dsz = 3; // Droplet size

da=0.07; // Droplet amplitude

[X,Y] =
generate_XY_planes(dsz,da);

DropletMatrix = -da *
exp( -(X/dsz)^2 -(Y/dsz)^2);

// This
variable is used to add just one droplet

One_single_droplet_added
= 0;

for(t=0; t<A_LOT_OF_TIME; t = t + dt)

{

u_next_t = (2-kdt)*u_current_t+(kdt-1)*u_previous_t+c1*convolution(u_current_t,D);

u_previous_t = u_current_t; // Current becomes old

u_current_t
= u_next_t; // New becomes current

// Draw the
wave

draw_wave(u_current_t)

if(One_single_droplet_added
== 0)

{

One_single_droplet_added = 1; // no more droplets

addMatrix(u_current_t,
DropletMatrix, [+100;+100]);

}

}

The variable *One_single_droplet_added* is used to check if the droplet has
already been inserted (we want just one droplet). The *addMatrix* function adds the *DropletMatrix*
values to the *u_current_t* surface
matrix values centering the *DropletMatrix*
at the point (100;100), remember that the *DropletMatrix*
is smaller (or equal) to the surface matrix, so we just add the *DropletMatrix*’s values to the *u_current_t*’s values that fall within
the dimensions of the *DropletMatrix*.

Now the algorithm is complete, although it is
still a theoretical simulation. We will soon implement it with real code.

We will now discuss how the above algorithm has
been implemented in a real C++ project to create a fully functional openGL
physical simulation.

The sequence diagram below shows the skeleton
of the program which basically consists of three main parts: the main module
where the startup function resides as well as the kernel function which creates
the matrix image for the entire window, an openGL renderer wrapper to encapsulate
GLUT library functions and callback handlers and a matrix hand-written class to
simplify matrix data access and manipulation. Although a sequence diagram would
require following a standard software engineering methodology and its use is
strictly enforced by predetermined rules, nonetheless we will use it as an
abstraction to model the program’s control flow

The program starts at the main() and creates an
openGLrenderer object which will handle all the graphic communication with the
GLUT library and the callback events (like mouse movements, keyboard press
events, etc.). OpenGL doesn’t provide routines for interfacing with a windowing
system, so in this project we will rely on GLUT libraries which provide a
platform-independent interface to manage windows and input events. To create an
animation that runs as fast as possible we will set an idle callback with the* glutIdleFunc()* function. We will
explain more about this later.

Initially the algorithm sets its initialization
variables (time step, space step, droplet amplitude, Laplacian 2D kernel, etc..
practically everything we saw in the theory section) and every matrix
corresponding to the image to be rendered is zeroed out. The Gaussian matrix
corresponding to the droplets is also preprocessed. A structure defined in
openGLRenderer’s header file contains all the data which should be retained
across image renderings

typedef struct kernelData
{
float a1;
float a2;
float c1;
sMatrix* u;
sMatrix* u0;
sMatrix* D;
int Ddim;
int dsz;
sMatrix* Zd;
} kernelData;

The structure is updated each time a time step
is performed since it contains both current and previous matrices that describe
the waves evolution across time. Since this structure is both used by the
openGL renderer and the main module to initialize it, the variable is declared
as *external* and defined afterwards in
the openGLrenderer cpp file (so its scope goes beyond the single translation
unit). After everything has been set up the openGLRenderer class’
startRendering() method is called and the openGL main loop starts fetching
events. The core of the algorithm we saw is in the main module’s kernel()
function which is called every time an openGL idle event is dispatched (that
is, the screen is updated and the changes will be shown only when the idle callback
has completed, thus the amount of rendering work here performed should be
minimized to avoid performance loss).

The kernel’s function code is the following

void kernel(unsigned char *ptr, kernelData& data)
{
sMatrix un(DIM,DIM);
un = (*data.u)*data.a1 + (*data.u0)*data.a2 + convolutionCPU((*data.u),(*data.D))*data.c1;
(*data.u0) = (*data.u);
(*data.u) = un;
matrix2Bitmap( (*data.u), ptr );
if(first_droplet == 1)
{
first_droplet = 0;
int x0d= DIM / 2;
int y0d= DIM / 2;
for(int Zdi=0; Zdi < data.Ddim; Zdi++)
{
for(int Zdj=0; Zdj < data.Ddim; Zdj++)
{
(*data.u)(y0d-2*data.dsz+Zdi,x0d-2*data.dsz+Zdj) += (*data.Zd)(Zdi, Zdj);
}
}
}
m_renderer->renderWaitingDroplets();
}

The pattern we followed in the wave PDE
evolution can be easily recognized in the computational-intensive code line

un = (*data.u)*data.a1 +
(*data.u0)*data.a2 + convolutionCPU((*data.u),(*data.D))*data.c1;

which basically performs the well-known
iterative step

All constants are preprocessed to improve
performances.

It is to be noticed that the line adds up large
matrices which are referred in the code as *sMatrix*
objects. The *sMatrix* class is an
handwritten simple class that exposes simple operator overrides to ease working
with matrices. Except that one should bear in mind that large matrix operations
shall avoid passing arguments by value and that to create a new matrix and copy
it to the destination a copy constructor is required (to avoid obtaining a
shallow copy without the actual data), the code is pretty straight forwarding
so no more words will be spent on it

class sMatrix
{
public:
int rows, columns;
float *values;
sMatrix(int height, int width)
{
if (height == 0 || width == 0)
throw "Matrix constructor has 0 size";
rows = height;
columns = width;
values = new float[rows*columns];
}
sMatrix(const sMatrix& mt)
{
this->rows = mt.rows;
this->columns = mt.columns;
this->values = new float[rows*columns];
memcpy(this->values, mt.values, this->rows*this->columns*sizeof(float));
}
~sMatrix()
{
delete [] values;
}
sMatrix& operator= (sMatrix const& m)
{
if(m.rows != this->rows || m.columns != this->columns)
{
throw "Size mismatch";
}
memcpy(this->values,m.values,this->rows*this->columns*sizeof(float));
return *this;
}
float& operator() (const int row, const int column)
{
if (row < 0 || column < 0 || row > this->rows || column > this->columns)
throw "Size mismatch";
return values[row*columns+column];
}
sMatrix operator* (const float scalar)
{
sMatrix result(this->rows, this->columns);
for(int i=0; i<rows*columns; i++)
{
result.values[i] = this->values[i]*scalar;
}
return result;
}
sMatrix operator+ (const sMatrix& mt)
{
if (this->rows != mt.rows || this->columns != mt.columns)
throw "Size mismatch";
sMatrix result(this->rows, this->columns);
for(int i=0; i<rows; i++)
{
for(int j=0; j<columns; j++)
result.values[i*columns+j] = this->values[i*columns+j] + mt.values[i*columns+j];
}
return result;
}
};

The
convolution is performed with the following code (a classic approach):

sMatrix convolutionCPU(sMatrix& A, sMatrix& B)
{
sMatrix result(A.rows, A.columns);
int kernelHradius = (B.rows - 1) / 2;
int kernelWradius = (B.columns - 1) / 2;
float convSum, convProd, Avalue;
for(int i=0; i<A.rows; i++)
{
for(int j=0; j<A.columns; j++)
{
convSum = 0;
for(int bi=0; bi<B.rows; bi++)
{
for(int bj=0; bj<B.columns; bj++)
{
int relpointI = i-kernelHradius+bi;
int relpointJ = j-kernelWradius+bj;
if(relpointI < 0 || relpointJ < 0 || relpointI >= A.rows || relpointJ >= A.columns)
Avalue = 0;
else
Avalue = A(i-kernelHradius+bi,j-kernelWradius+bj);
convProd = Avalue*B(bi,bj);
convSum += convProd;
}
}
result(i,j) = convSum;
}
}
return result;
}

After calculating the system’s evolution, the
time elapsing is simulated by swapping the new matrix with the old one and
discarding the previous state as we described before.

Then a *matrix2Bitmap()*
call performs a matrix-to-bitmap conversion as its name suggests, more
precisely the entire simulation area is described by a large* sMatrix* object which contains,
obviously, float values. To actually render these values as pixel units we need
to convert each value to its corresponding RGBA value and pass it to the
openGLRenderer class (which in turn will pass the entire bitmap buffer to the
GLUT library). In brief: we need to perform a float-to-RGBcolor mapping.

Since in the physical simulation we assumed
that the resting water height is at 0 and every perturbation would heighten or
lower this value (in particular the droplet Gaussian matrix lowers it by a
maximum -0.07 factor), we are searching for a [-1;1] to color mapping. A HSV
color model would better simulate a gradual color transition as we actually
experience with our own eyes, but this would require converting it back to RGB
values to set up a bitmap map to pass back at the GLUT wrapper. For performance
reasons we chose to assign each value a color (colormap). A first solution
would have been implementing a full [-1;1] -> [0x0;0xFFFFFF] mapping in
order to cover all the possible colors in the RGB format

RGB getColorFromValue(float value)
{
RGB result;
if(value <= -1.0f)
{
result.r = 0x00;
result.g = 0x00;
result.b = 0x00;
}
else if(value >= 1.0f)
{
result.r = 0xFF;
result.g = 0xFF;
result.b = 0xFF;
}
else
{
float step = 2.0f/0xFFFFFF;
unsigned int cvalue = (unsigned int)((value + 1.0f)/step);
if(cvalue < 0)
cvalue = 0;
else if(cvalue > 0xFFFFFF)
cvalue = 0xFFFFFF;
result.r = cvalue & 0xFF;
result.g = (cvalue & 0xFF00) >> 8;
result.b = (cvalue & 0xFF0000) >> 16;
}
return result;
}

However the above method is either
performance-intensive and doesn’t render very good a smooth color transition:
let’s take a look at a droplet mapped like that

looks more like a fractal rather than a droplet,
so the above solution won’t work. A better way to improve performances (and the
look of the image) is to hard-code a colormap in an array and to use it when
needed:

float pp_step = 2.0f / (float)COLOR_NUM;
unsigned char m_colorMap[COLOR_NUM][3] =
{
{0,0,143},{0,0,159},{0,0,175},{0,0,191},{0,0,207},{0,0,223},{0,0,239},{0,0,255},
{0,16,255},{0,32,255},{0,48,255},{0,64,255},{0,80,255},{0,96,255},{0,111,255},{0,128,255},
{0,143,255},{0,159,255},{0,175,255},{0,191,255},{0,207,255},{0,223,255},{0,239,255},{0,255,255},
{16,255,239},{32,255,223},{48,255,207},{64,255,191},{80,255,175},{96,255,159},{111,255,143},{128,255,128},
{143,255,111},{159,255,96},{175,255,80},{191,255,64},{207,255,48},{223,255,32},{239,255,16},{255,255,0},
{255,239,0},{255,223,0},{255,207,0},{255,191,0},{255,175,0},{255,159,0},{255,143,0},{255,128,0},
{255,111,0},{255,96,0},{255,80,0},{255,64,0},{255,48,0},{255,32,0},{255,16,0},{255,0,0},
{239,0,0},{223,0,0},{207,0,0},{191,0,0},{175,0,0},{159,0,0},{143,0,0},{128,0,0},
{100,0,20},{80,0,40},{60,0,60},{40,0,80},{20,0,100},{0,0,120}
};
RGB getColorFromValue(float value)
{
RGB result;
unsigned int cvalue = (unsigned int)((value + 1.0f)/pp_step);
if(cvalue < 0)
cvalue = 0;
else if(cvalue >= COLOR_NUM)
cvalue = COLOR_NUM-1;
result.r = m_colorMap[cvalue][0];
result.g = m_colorMap[cvalue][1];
result.b = m_colorMap[cvalue][2];
return result;
}

Creating a colormap isn’t hard and different
colormaps are freely available on the web which produce different transition
effects. This time the result was much nicer (see the screenshot later on) and
the performances (although an every-value conversion is always an intensive
task) increased substantially.

The last part involved in the on-screen
rendering is adding a droplet wherever the user clicks on the window with the
cursor. One droplet is automatically added at the center of the surface (you
can find the code in the *kernel()*
function, it is controlled by the *first_droplet*
variable) but the user can click everywhere (almost everywhere) on the surface
to add another droplet in that spot. To achieve this a queue has been implemented
to contain at the most 60 droplet centers where the Gaussian matrix will be
placed (notice that the matrix will be added to the surface values that were
already present in the spots, not just replace them).

#define MAX_DROPLET 60
typedef struct Droplet
{
int mx;
int my;
} Droplet;
Droplet dropletQueue[MAX_DROPLET];
int dropletQueueCount = 0;

The
queue system has been implemented for a reason: unlike the algorithm in
pseudo-code we wrote before, rendering a scene with openGL requires the program
to control the objects to be displayed in immediate-mode: that means the
program needs to take care of what should be drawn before the actual rendering
is performed, it cannot simply put a droplet to be rendered inside the data to
be drawn because it could be in use (you can do this in retained-mode).
Besides, we don’t know when a droplet will be added because it’s totally
user-dependent. Because of that, every time the *kernel()* finishes, the droplet queue is emptied and the droplet
Gaussian matrices are added to the data to be rendered (the surface data). The
code which performs this is the following

void openGLRenderer::renderWaitingDroplets()
{
while(dropletQueueCount > 0)
{
dropletQueueCount--;
addDroplet(dropletQueue[dropletQueueCount].mx,dropletQueue[dropletQueueCount].my);
}
}
void addDroplet( int x0d, int y0d )
{
y0d = DIM - y0d;
for(int Zdi=0; Zdi< m_simulationData.Ddim; Zdi++)
{
for(int Zdj=0; Zdj< m_simulationData.Ddim; Zdj++)
{
(*m_simulationData.u)(y0d-2*m_simulationData.dsz+Zdi,x0d-2*m_simulationData.dsz+Zdj)
+= (*m_simulationData.Zd)(Zdi, Zdj);
}
}
}

The code
should be familiar by now: the *addDroplet*
function simply adds the *Zd* 2D
Gaussian matrix to the simulation data (the kernel data) at the “current” time,
a.k.a. the *u* matrix which represents
the surface.

The code loops until the keyboard callback handler
(defined by the openGLrenderer) detects the Esc keypress, after that the
application is issued a termination signal, the loops ends. The resources
allocated by the program are freed before the exit signal is issued, however
this might not be necessary since a program termination would let the OS free
any of its previously allocated resources.

With the droplet-adding feature and all the
handlers set the code is ready to run. This time the result is much nicer than
the previous, that’s because we used a smoother colormap (take a look at the
images below). Notice how the convolution term creates the “wave spreading” and
the “bouncing” effect when computing values against the padded zero data
outside the surface matrix (i.e. when the wave hits the window’s borders and is
reflected back). The first image is the simulation in its early stage, that is
when some droplets have just been added, the second image represents a later
stage when the surface is going to calm down (in our colormap blue values are
higher than red ones).

Since we introduced a damping factor (recall it
from the theory section), the waves are eventually going to cease and the
surface will sooner or later be quiet again. The entire simulation is (except
for the colors) quite realistic but quite slow too. That’s because the entire
surface matrix is being thoroughly updated and calculated by the system. The *kernel()* function runs continuously
updating the rendering buffer. For a 512x512 image the CPU has to process a
large quantity of data and it has also to perform 512x512 floating point
convolutions. Using a profiler (like VS’s integrated one) shows that the
program spends most of its time in the *kernel()*
call (as we expected) and that the convolution operation is the most
cpu-intensive.

It is also interesting to notice that the
simulation speed decreases substantially when adding lots of droplets.

In a real scientific simulation environment
gigantic quantities of data need to be processed in relatively small time
intervals. That’s where GPGPU computing comes into the scene. We will briefly
summarize what this acronym means and then we will present a GPU-accelerated
version of the wave PDE simulation.

GPGPU stands for General Purpose computation on Graphics Processing Units and indicates using graphic processors and devices to perform high parallelizable computations that would normally be handled by CPU devices. The idea of using graphic devices to help CPUnits with their workloads isn’t new, but until recent architectures and frameworks like CUDA (©NVIDIA vendor-specific) or openCL showed up, programmers had to rely on series of workarounds and tricks to work with inconvenient and unintuitive methods and data structures. The reason why a developer should think about porting his CPU-native code into a new GPU version resides in the architecture design differences between CPUs and GPUs. While CPUs evolved (multicore) to gain performance advantages with sequential executions (pipelines, caches, control flows, etc..), GPUs evolved in a many-core way: they tended to operate at higher data bandwidths and chose to heavily increase their execution threads number. In the last years GPGPU has been seen as a new opportunity to use graphical processing units as algebraic coprocessors capable of handling massive parallelization and precision floating point calculations. The idea behind GPGPU architectures is letting CPU handling sequential parts of programs and GPU getting over with parallelizable parts. In fact many scientific applications and systems found their performances increased by such an approach and GPGPU is now a fundamental technology in many fields like medical imaging, physics simulations, signal processing, cryptography, intrusion detection, environmental sciences, etc..

We chose to use the CUD-Architecture to
parallelize some parts of our code. Parallelizing with GPGPU techniques means
passing from a sequentially-designed code to a parallel-designed code, this
also often means having to rewrite large parts of your code. The most obvious
part of the entire simulation algorithm that could benefit of a parallel
approach is the surface matrix convolution with the 2D Laplacian kernel.

Notice: for brevity’s sake, we will assume that
the reader is already familiar with CUDA C/C++.

The CUDA SDK comes with a large variety of
examples regarding how to implement an efficient convolution between matrices
and how to apply a kernel as an image filter. However we decided to implement
our own version rather than rely on standard techniques. The reasons are many:

- we wanted to show how a massively parallel algorithm is designed and implemented
- since our kernel is very small (3x3 2D Laplacian kernel, basically 9 float values) using a FFT approach like the one described by Victor Podlozhnyuk would be inefficient
- the two-dimensional Gaussian kernel is the only radially symmetric function that is also separable, our kernel is not, so we cannot use separable convolution methods
- such a small kernel seems perfect to be “aggressively cached” in the convolution operation. We’ll expand on that as soon as we describe the CUDA kernel designed

The most obvious way to perform a convolution
(although extremely inefficient) consists in delegating each GPU thread multiple
convolutions for each element across the entire image.

Take a look at the following image, we will use
a 9x9 thread grid (just one block to simplify things) to perform the
convolution. The purple square is our 9x9 grid while the red grids correspond
to the 9x9 kernel. Each thread performs the convolution within its elements,
then the X are shifted and the entire block is “virtually” moved to the right.
When the X coordinate is complete (that is: the entire image horizontal area
has been covered), the Y coordinate is incremented and the process starts again
until completion. In the border area, every value outside the image will be set
to zero.

The code for this simple approach is the
following where A is the image matrix and B is the kernel.

__global__ void convolutionGPU(float *A, int A_cols, int A_rows, float *B, int B_Wradius, int B_Hradius,
int B_cols, int B_rows, float *result)
{
int threadXabs = blockIdx.x*blockDim.x + threadIdx.x;
int threadYabs = blockIdx.y*blockDim.y + threadIdx.y;
int threadXabsInitialPos = threadXabs;
float convSum;
while(threadYabs < A_rows)
{
while(threadXabs < A_cols)
{
convSum = 0.0f;
#pragma unroll
for(int xrel=-B_Wradius;xrel<(B_Wradius+1); xrel++)
{
#pragma unroll
for(int yrel=-B_Hradius;yrel<(B_Hradius+1); yrel++)
{
float Avalue;
if(threadXabs + xrel < 0 || threadYabs + yrel <0 || threadXabs + xrel >= A_cols || threadYabs + yrel >= A_rows)
Avalue = 0;
else
Avalue = A[ (threadYabs+yrel)*A_cols + (threadXabs + xrel) ];
float Bvalue = B[ (yrel+B_Hradius)*B_cols + (xrel+B_Wradius) ];
convSum += Avalue * Bvalue;
}
}
result[threadYabs*A_cols + threadXabs ] = convSum;
threadXabs += blockDim.x * gridDim.x;
}
threadXabs = threadXabsInitialPos;
threadYabs += blockDim.y * gridDim.y;
}
}

As already
stated, this simple approach has several disadvantages

- The kernel is very small, keeping it into global memory and accessing to it for every convolution performed is extremely inefficient
- Although the matrix readings are partially coalesced, thread divergence can be significant with threads active in the border area and threads that are inactive
- There’s no collaborative behavior among threads, although they basically use the same kernel and share a large part of the apron region

Hence a way better method to perform GPU
convolution has been designed keeping in mind the points above.

The idea is simple: letting each thread load
part of the apron and data regions in the shared memory thus maximizing
readings coalescence and reducing divergence.

The code that performs the convolution on the
GPU version of the simulation is the following

__global__ void convolutionGPU(float *A, float *result)
{
__shared__ float data[laplacianD*2][laplacianD*2];
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
const int x0 = threadIdx.x + IMUL(blockIdx.x,blockDim.x);
const int y0 = threadIdx.y + IMUL(blockIdx.y,blockDim.y);
int x,y;
x = x0 - kernelRadius;
y = y0 - kernelRadius;
if(x < 0 || y < 0)
data[threadIdx.x][threadIdx.y] = 0.0f;
else
data[threadIdx.x][threadIdx.y] = A[ gLoc - kernelRadius - IMUL(DIM,kernelRadius)];
x = x0 + kernelRadius + 1;
y = y0 - kernelRadius;
if(x >= DIM || y < 0)
data[threadIdx.x + blockDim.x][threadIdx.y] = 0.0f;
else
data[threadIdx.x + blockDim.x][threadIdx.y] = A[ gLoc + kernelRadius+1 - IMUL(DIM,kernelRadius)];
x = x0 - kernelRadius;
y = y0 + kernelRadius+1;
if(x < 0 || y >= DIM)
data[threadIdx.x][threadIdx.y + blockDim.y] = 0.0f;
else
data[threadIdx.x][threadIdx.y + blockDim.y] = A[ gLoc - kernelRadius + IMUL(DIM,(kernelRadius+1))];
x = x0 + kernelRadius+1;
y = y0 + kernelRadius+1;
if(x >= DIM || y >= DIM)
data[threadIdx.x + blockDim.x][threadIdx.y + blockDim.y] = 0.0f;
else
data[threadIdx.x + blockDim.x][threadIdx.y + blockDim.y] = A[ gLoc + kernelRadius+1 + IMUL(DIM,(kernelRadius+1))];
__syncthreads();
float sum = 0;
x = kernelRadius + threadIdx.x;
y = kernelRadius + threadIdx.y;
#pragma unroll
for(int i = -kernelRadius; i<=kernelRadius; i++)
for(int j=-kernelRadius; j<=kernelRadius; j++)
sum += data[x+i][y+j] * gpu_D[i+kernelRadius][j+kernelRadius];
result[gLoc] = sum;
}

The kernel only receives the surface matrix and
the result where to store the convolved image. The kernel isn’t provided
because it has been put into a special memory called “**constant** memory” which is read-only by kernels, pre-fetched and
highly optimized to let all threads read from a specific location with minimum
latency. The downside is that this kind of memory is extremely limited (in the
order of 64Kb) so should be used wisely. Declaring our 3x3 kernel as constant
memory grants us a significant speed advantage

__device__ __constant__ float gpu_D[laplacianD][laplacianD]; // Laplacian 2D kernel

The image below helps to determine how threads
load from the surface matrix in the global memory the data and store them into faster
on-chip shared memory before actually using them in the convolution operation.
The purple 3x3 square is the kernel window and the central element is the value
we are actually pivoting on. The grid is a 172x172 blocks 3x3 threads each one;
each block of 3x3 threads have four stages to complete before entering the
convolution loop: load the upper left apron and image data into shared memory
(the upper left red square from the kernel purple window), load the upper right
area (red square), load the lower left area (red square) and load the lower
right area (idem). Since shared memory is only available to the threads in a
block, each block loads its own shared area. Notice that we chose to let every
thread read something from global memory to maximize coalescence, but we are
not actually going to use every single element. The image shows a yellow area
and a gray area: the yellow data is actually going to be used in the
convolution operation for each element in the purple kernel square (it
comprises aprons and data) while the gray area isn’t going to be used by any
convolution performed by the block we are considering.

After filling each block’s shared memory array,
the CUDA threads get synchronized to minimize their divergence. Then the
execution of the convolution algorithm is performed: shared data is multiplied
against constant kernel data resulting in a highly optimized operation.

The *#pragma
unroll* directive instructs the compiler to unroll (where possible) the loop
to reduce cycle control overhead and improve performances. A small example of
loop unrolling: the following loop

for(int i=0;i<1000;i++)

a[i] =
b[i] + c[i];

might be optimized by unrolling it

for(int i=0;i<1000;i+=2)

{

a[i] =
b[i] + c[i];

a[i+1] =
b[i+1] + c[i+1];

}

so that the control instructions are executed
less and the overall loop improves its performances. It is to be noticed that
almost every optimization in CUDA code needs to be carefully and thoroughly
tested because a different architecture and different program control flows
might produce different results (as well as different compiler optimizations
that, unfortunately, cannot be always trusted).

Also notice that the IMUL macro is used in the
code which is defined as

#define
IMUL(a,b) __mul24(a,b)

On devices of CUDA compute capability 1.x,
32-bit integer multiplication is implemented using multiple instructions as it
is not natively supported. 24-bit integer multiplication is natively supported
via the __[u]mul24 intrinsic. However on devices of compute capability 2.0,
however, 32-bit integer multiplication is natively supported, but 24-bit
integer multiplication is not. __[u]mul24 is therefore implemented using
multiple instructions and should not be used. So if you are planning to use the
code on 2.x devices, make sure to redefine the macro directive.

A typical code which could call the kernel we
just wrote could be

sMatrix convolutionGPU_i(sMatrix& A, sMatrix& B)
{
unsigned int A_bytes = A.rows*A.columns*sizeof(float);
sMatrix result(A.rows, A.columns);
float *cpu_result = (float*)malloc(A_bytes);
cudaError_t chk;
chk = cudaMemcpy(m_gpuData->gpu_matrixA, A.values, A_bytes, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT TRANSFER MEMORY TO GPU");
return result;
}
dim3 blocks(172,172);
dim3 threads(3,3);
convolutionGPU<<<blocks,threads>>>(m_gpuData->gpu_matrixA, m_gpuData->gpu_matrixResult);
chk = cudaMemcpy(cpu_result, m_gpuData->gpu_matrixResult, A_bytes, cudaMemcpyDeviceToHost);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT TRANSFER MEMORY FROM GPU");
return result;
}
free(result.values);
result.values = cpu_result;
return result;
}

obviously CUDA memory should be *cudaMalloc*-ated at the beginning of our
program and freed only when the GPU work is complete.

However, as we stated before, converting a
sequentially-designed program into a parallel one isn’t an easy task and often
requires more than just a plain function-to-function conversion (it depends on
the application). In our case substituting just a CPU-convolution function with
a GPU-convolution function won’t work. In fact even though we distributed our
workload in a better way from the CPU version (see the images below for a
CPU-GPU exclusive time percentage), **we
actually slowed down the whole simulation**.

The reason is simple: our *kernel()* function is called whenever a draw event is dispatched, so
it needs to be called very often. Although the CUDA kernel is faster than the
CPU convolution function and although GPU memory bandwidths are higher than
CPU’s, transferring from (possibly paged-out) host memory to global device
memory back and forth just kills our simulation performances. Applications
which would benefit more from a CUDA approach usually perform a single-shot
heavily-computational kernel workload and then transfer back the results. Real
time applications might benefit from a concurrent kernels approach, but a 2.x
capability device would be required.

In order to actually accelerate our simulation,
a greater code revision is required.

Another more subtle thing to take into account
when working with GPU code is CPU optimizations: take a look at the following
asm codes for the CPU version of the line

un =
(*data.u)*data.a1 + (*data.u0)*data.a2 +
convolutionCPU((*data.u),(*data.D))*data.c1;

000000013F2321EF mov r8,qword ptr [m_simulationData+20h (13F234920h)]
000000013F2321F6 mov rdx,qword ptr [m_simulationData+10h (13F234910h)]
000000013F2321FD lea rcx,[rbp+2Fh]
000000013F232201 call convolutionCPU (13F231EC0h)
000000013F232206 nop
000000013F232207 movss xmm2,dword ptr [m_simulationData+8 (13F234908h)]
000000013F23220F lea rdx,[rbp+1Fh]
000000013F232213 mov rcx,rax
000000013F232216 call sMatrix::operator* (13F2314E0h)
000000013F23221B mov rdi,rax
000000013F23221E movss xmm2,dword ptr [m_simulationData+4 (13F234904h)]
000000013F232226 lea rdx,[rbp+0Fh]
000000013F23222A mov rcx,qword ptr [m_simulationData+18h (13F234918h)]
000000013F232231 call sMatrix::operator* (13F2314E0h)
000000013F232236 mov rbx,rax
000000013F232239 movss xmm2,dword ptr [m_simulationData (13F234900h)]
000000013F232241 lea rdx,[rbp-1]
000000013F232245 mov rcx,qword ptr [m_simulationData+10h (13F234910h)]
000000013F23224C call sMatrix::operator* (13F2314E0h)
000000013F232251 nop
000000013F232252 mov r8,rbx
000000013F232255 lea rdx,[rbp-11h]
000000013F232259 mov rcx,rax
000000013F23225C call sMatrix::operator+ (13F2315B0h)
000000013F232261 nop
000000013F232262 mov r8,rdi
000000013F232265 lea rdx,[rbp-21h]
000000013F232269 mov rcx,rax
000000013F23226C call sMatrix::operator+ (13F2315B0h)
000000013F232271 nop
000000013F232272 cmp dword ptr [rax],1F4h
000000013F232278 jne kernel+33Fh (13F2324CFh)
000000013F23227E cmp dword ptr [rax+4],1F4h
000000013F232285 jne kernel+33Fh (13F2324CFh)
000000013F23228B mov r8d,0F4240h
000000013F232291 mov rdx,qword ptr [rax+8]
000000013F232295 mov rcx,r12
000000013F232298 call memcpy (13F232DDEh)
000000013F23229D nop
000000013F23229E mov rcx,qword ptr [rbp-19h]
000000013F2322A2 call qword ptr [__imp_operator delete (13F233090h)]
000000013F2322A8 nop
000000013F2322A9 mov rcx,qword ptr [rbp-9]
000000013F2322AD call qword ptr [__imp_operator delete (13F233090h)]
000000013F2322B3 nop
000000013F2322B4 mov rcx,qword ptr [rbp+7]
000000013F2322B8 call qword ptr [__imp_operator delete (13F233090h)]
000000013F2322BE nop
000000013F2322BF mov rcx,qword ptr [rbp+17h]
000000013F2322C3 call qword ptr [__imp_operator delete (13F233090h)]
000000013F2322C9 nop
000000013F2322CA mov rcx,qword ptr [rbp+27h]
000000013F2322CE call qword ptr [__imp_operator delete (13F233090h)]
000000013F2322D4 nop
000000013F2322D5 mov rcx,qword ptr [rbp+37h]
000000013F2322D9 call qword ptr [__imp_operator delete (13F233090h)]

and now
take a look at the GPU version of the line

un =
(*data.u)*data.a1 + (*data.u0)*data.a2 + convolutionGPU_i
((*data.u),(*data.D))*data.c1;

000000013F7E23A3 mov rax,qword ptr [data]
000000013F7E23AB movss xmm0,dword ptr [rax+8]
000000013F7E23B0 movss dword ptr [rsp+0A8h],xmm0
000000013F7E23B9 mov rax,qword ptr [data]
000000013F7E23C1 mov r8,qword ptr [rax+20h]
000000013F7E23C5 mov rax,qword ptr [data]
000000013F7E23CD mov rdx,qword ptr [rax+10h]
000000013F7E23D1 lea rcx,[rsp+70h]
000000013F7E23D6 call convolutionGPU_i (13F7E1F20h)
000000013F7E23DB mov qword ptr [rsp+0B0h],rax
000000013F7E23E3 mov rax,qword ptr [rsp+0B0h]
000000013F7E23EB mov qword ptr [rsp+0B8h],rax
000000013F7E23F3 movss xmm0,dword ptr [rsp+0A8h]
000000013F7E23FC movaps xmm2,xmm0
000000013F7E23FF lea rdx,[rsp+80h]
000000013F7E2407 mov rcx,qword ptr [rsp+0B8h]
000000013F7E240F call sMatrix::operator* (13F7E2B20h)
000000013F7E2414 mov qword ptr [rsp+0C0h],rax
000000013F7E241C mov rax,qword ptr [rsp+0C0h]
000000013F7E2424 mov qword ptr [rsp+0C8h],rax
000000013F7E242C mov rax,qword ptr [data]
000000013F7E2434 movss xmm0,dword ptr [rax+4]
000000013F7E2439 movaps xmm2,xmm0
000000013F7E243C lea rdx,[rsp+50h]
000000013F7E2441 mov rax,qword ptr [data]
000000013F7E2449 mov rcx,qword ptr [rax+18h]
000000013F7E244D call sMatrix::operator* (13F7E2B20h)
000000013F7E2452 mov qword ptr [rsp+0D0h],rax
000000013F7E245A mov rax,qword ptr [rsp+0D0h]
000000013F7E2462 mov qword ptr [rsp+0D8h],rax
000000013F7E246A mov rax,qword ptr [data]
000000013F7E2472 movss xmm2,dword ptr [rax]
000000013F7E2476 lea rdx,[rsp+40h]
000000013F7E247B mov rax,qword ptr [data]
000000013F7E2483 mov rcx,qword ptr [rax+10h]
000000013F7E2487 call sMatrix::operator* (13F7E2B20h)
000000013F7E248C mov qword ptr [rsp+0E0h],rax
000000013F7E2494 mov rax,qword ptr [rsp+0E0h]
000000013F7E249C mov qword ptr [rsp+0E8h],rax
000000013F7E24A4 mov r8,qword ptr [rsp+0D8h]
000000013F7E24AC lea rdx,[rsp+60h]
000000013F7E24B1 mov rcx,qword ptr [rsp+0E8h]
000000013F7E24B9 call sMatrix::operator+ (13F7E2BF0h)
000000013F7E24BE mov qword ptr [rsp+0F0h],rax
000000013F7E24C6 mov rax,qword ptr [rsp+0F0h]
000000013F7E24CE mov qword ptr [rsp+0F8h],rax
000000013F7E24D6 mov r8,qword ptr [rsp+0C8h]
000000013F7E24DE lea rdx,[rsp+90h]
000000013F7E24E6 mov rcx,qword ptr [rsp+0F8h]
000000013F7E24EE call sMatrix::operator+ (13F7E2BF0h)
000000013F7E24F3 mov qword ptr [rsp+100h],rax
000000013F7E24FB mov rax,qword ptr [rsp+100h]
000000013F7E2503 mov qword ptr [rsp+108h],rax
000000013F7E250B mov rdx,qword ptr [rsp+108h]
000000013F7E2513 lea rcx,[un]
000000013F7E2518 call sMatrix::operator= (13F7E2A90h)
000000013F7E251D nop
000000013F7E251E lea rcx,[rsp+90h]
000000013F7E2526 call sMatrix::~sMatrix (13F7E2970h)
000000013F7E252B nop
000000013F7E252C lea rcx,[rsp+60h]
000000013F7E2531 call sMatrix::~sMatrix (13F7E2970h)
000000013F7E2536 nop
000000013F7E2537 lea rcx,[rsp+40h]
000000013F7E253C call sMatrix::~sMatrix (13F7E2970h)
000000013F7E2541 nop
000000013F7E2542 lea rcx,[rsp+50h]
000000013F7E2547 call sMatrix::~sMatrix (13F7E2970h)
000000013F7E254C nop
000000013F7E254D lea rcx,[rsp+80h]
000000013F7E2555 call sMatrix::~sMatrix (13F7E2970h)
000000013F7E255A nop
000000013F7E255B lea rcx,[rsp+70h]
000000013F7E2560 call sMatrix::~sMatrix(13F7E2970h)

the code, although the data
involved are practically the same, looks much more bloated up (there are even
nonsense operations, look at address 000000013F7E23DB). Probably letting the CPU
finish the calculation after the GPU has done its work is not a good idea.

Since there are other functions
which can be parallelized (like the *matrix2bitmap()*
function), we need to move as much workload as possible on the device.

First we need to allocate memory
on the device at the beginning of the program and deallocate it when finished.
Small data like our algorithm parameters can be stored into constant memory to
boost performances, while matrices large data is more suited into global memory
(constant memory size is very limited).

void initializeGPUData()
{
float dt = (float)0.05;
float c = 1;
float dx = 1;
float k = (float)0.002;
float da = (float)0.07;
sMatrix u0(DIM,DIM);
for(int i=0; i<DIM; i++)
{
for(int j=0; j<DIM; j++)
{
u0(i,j) = 0.0f;
}
}
CPUsMatrix2Bitmap(u0, renderImg);
float kdt=k*dt;
float c1=pow(dt,2)*pow(c,2)/pow(dx,2);
const int dim = 4*dropletRadius+1;
sMatrix xd(dim, dim);
sMatrix yd(dim, dim);
for(int i=0; i<dim; i++)
{
for(int j=-2*dropletRadius; j<=2*dropletRadius; j++)
{
xd(i,j+2*dropletRadius) = j;
yd(j+2*dropletRadius,i) = j;
}
}
float m_Zd[dim][dim];
for(int i=0; i<dim; i++)
{
for(int j=0; j<dim; j++)
{
m_Zd[i][j] = -da*exp(-pow(xd(i,j)/dropletRadius,2)-pow(yd(i,j)/dropletRadius,2));
}
}
unsigned int UU0_bytes = DIM*DIM*sizeof(float);
cudaError_t chk;
chk = cudaMalloc((void**)&m_gpuData.gpu_u, UU0_bytes);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMalloc((void**)&m_gpuData.gpu_u0, UU0_bytes);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMalloc((void**)&m_gpuData.ris0, UU0_bytes);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMalloc((void**)&m_gpuData.ris1, UU0_bytes);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMalloc((void**)&m_gpuData.ris2, UU0_bytes);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMalloc((void**)&m_gpuData.gpu_ptr, DIM*DIM*4);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE GPU MEMORY");
return;
}
chk = cudaMemcpy(m_gpuData.gpu_u0, u0.values, UU0_bytes, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT TRANSFER MEMORY TO GPU");
return;
}
chk = cudaMemcpy(m_gpuData.gpu_u, u0.values, UU0_bytes, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT TRANSFER MEMORY TO GPU");
return;
}
float m_D[3][3];
m_D[0][0] = 0.0f; m_D[1][0] = 1.0f; m_D[2][0]=0.0f;
m_D[0][1] = 1.0f; m_D[1][1] = -4.0f; m_D[2][1]=1.0f;
m_D[0][2] = 0.0f; m_D[1][2] = 1.0f; m_D[2][2]=0.0f;
chk = cudaMemcpyToSymbol((const char*)gpu_D, m_D, 9*sizeof(float), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
const float a1 = (2-kdt);
chk = cudaMemcpyToSymbol((const char*)&gpu_a1, &a1, sizeof(float), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
const float a2 = (kdt-1);
chk = cudaMemcpyToSymbol((const char*)&gpu_a2, &a2, sizeof(float), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
chk = cudaMemcpyToSymbol((const char*)&gpu_c1, &c1, sizeof(float), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
const int ddim = dim;
chk = cudaMemcpyToSymbol((const char*)&gpu_Ddim, &ddim, sizeof(int), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
const int droplet_dsz = dropletRadius;
chk = cudaMemcpyToSymbol((const char*)&gpu_dsz, &droplet_dsz, sizeof(int), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
chk = cudaMemcpyToSymbol((constchar*)&gpu_Zd, &m_Zd, sizeof(float)*dim*dim, 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
chk = cudaMemcpyToSymbol((const char*)&gpu_pp_step, &pp_step, sizeof(float), 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
chk = cudaMemcpyToSymbol((const char*)&gpu_m_colorMap, &m_colorMap, sizeof(unsigned char)*COLOR_NUM*3, 0, cudaMemcpyHostToDevice);
if(chk != cudaSuccess)
{
printf("\nCONSTANT MEMORY TRANSFER FAILED");
return;
}
}
void deinitializeGPUData()
{
cudaFree(m_gpuData.gpu_u);
cudaFree(m_gpuData.gpu_u0);
cudaFree(m_gpuData.gpu_ptr);
cudaFree(m_gpuData.ris0);
cudaFree(m_gpuData.ris1);
cudaFree(m_gpuData.ris2);
}

After initializing the GPU memory the
openGLRenderer can be started as usual to call the *kernel()* function in order to obtain a valid render-able surface
image matrix. But there’s a difference now, right in the openGLRenderer
constructor

openGLRenderer::openGLRenderer(void)
{
cublasStatus_t status = cublasInit();
if (status != CUBLAS_STATUS_SUCCESS)
{
printf("\nCRITICAL: CUBLAS LIBRARY FAILED TO LOAD");
return;
}
cudaError_t chk = cudaHostAlloc((void**)&renderImg, DIM*DIM*4*sizeof(char), cudaHostAllocDefault);
if(chk != cudaSuccess)
{
printf("\nCRITICAL: CANNOT ALLOCATE PAGELOCKED MEMORY");
return ;
}
}

First
we decided to use CUBLAS library to perform matrix addition for two reasons:

- our row-major data on the device is ready to be used by the CUBLAS functions yet (cublasMalloc is just a wrapper around the cudaMalloc)
- CUBLAS library is extremely optimized for large matrices operations; our matrices aren’t that big but this could help extending the architecture for a future version

Using our *sMatrix* wrapper is no more an efficient
choice and we need to get rid of it while working on the device, although we
can still use it for the initialization stage.

The second fundamental thing that we need to
notice in the openGLRenderer constructor is that we allocated host-side memory
(the memory that will contain the data to be rendered) with *cudaHostAlloc* instead of the classic *malloc*. As the documentation states, allocating
memory with such a function grants that the CUDA driver will track the virtual
memory ranges allocated with this function and accelerate calls to function
like *cudaMemCpy*. Host memory
allocated with *cudaHostAlloc* is often
referred as “pinned memory”, and cannot be paged-out (and because of that
allocating excessive amounts of it may degrade system performance since it
reduces the amount of memory available to the system for paging). This
expedient will grant additional speed in memory transfers between device and
host.

We are now ready to take a peek at the revised *kernel()* function

void kernel(unsigned char *ptr)
{
dim3 blocks(172,172);
dim3 threads(3,3);
convolutionGPU<<<blocks,threads>>>(m_gpuData.gpu_u, m_gpuData.ris0);
multiplyEachElementby_c1<<<blocks,threads>>>(m_gpuData.ris0, m_gpuData.ris1);
multiplyEachElementby_a1<<<blocks,threads>>>(m_gpuData.gpu_u, m_gpuData.ris0);
multiplyEachElementby_a2<<<blocks,threads>>>(m_gpuData.gpu_u0, m_gpuData.ris2);
cublasSaxpy(DIM*DIM, 1.0f, m_gpuData.ris0, 1, m_gpuData.ris2, 1);
cublasSaxpy(DIM*DIM, 1.0f, m_gpuData.ris2, 1, m_gpuData.ris1, 1);
cudaMemcpy(m_gpuData.gpu_u0, m_gpuData.gpu_u, DIM*DIM*sizeof(float), cudaMemcpyDeviceToDevice);
cudaMemcpy(m_gpuData.gpu_u, m_gpuData.ris1, DIM*DIM*sizeof(float), cudaMemcpyDeviceToDevice);
gpuMatrix2Bitmap<<<blocks,threads>>>(m_gpuData.gpu_u, m_gpuData.gpu_ptr);
cudaMemcpy(ptr, m_gpuData.gpu_ptr, DIM*DIM*4, cudaMemcpyDeviceToHost);
if(first_droplet == 1)
{
first_droplet = 0;
int x0d= DIM / 2;
int y0d= DIM / 2;
cudaMemcpy(m_gpuData.ris0, m_gpuData.gpu_u, DIM*DIM*sizeof(float), cudaMemcpyDeviceToDevice);
addDropletToU<<<blocks,threads>>>(m_gpuData.ris0, x0d,y0d, m_gpuData.gpu_u);
}
while(dropletQueueCount >0)
{
dropletQueueCount--;
int y0d = DIM - dropletQueue[dropletQueueCount].my;
cudaMemcpy(m_gpuData.ris0, m_gpuData.gpu_u, DIM*DIM*sizeof(float), cudaMemcpyDeviceToDevice);
addDropletToU<<<blocks,threads>>>(m_gpuData.ris0, dropletQueue[dropletQueueCount].mx,y0d, m_gpuData.gpu_u);
}
cudaThreadSynchronize();
}

The line

un = (*data.u)*data.a1 + (*data.u0)*data.a2
+ convolution((*data.u),(*data.D))*data.c1;

has completely been superseded by multiple
kernel calls which respectively operate a convolution operation, multiply
matrix data by algorithm constants and perform a matrix-matrix addition via
CUBLAS. Everything is performed in the device including the point-to-RGBvalue
mapping (which is a highly parallelizable operation since must be performed for
every value in the surface image matrix). Stepping forward in time is also
accomplished with device methods. Eventually the data is copied back to the
page-locked pinned host memory and droplets waiting in the queue are added for
the next iteration to the *u* surface
simulation data matrix.

The CUDA kernels called by the *kernel()* function are the following

__global__ void convolutionGPU(float *A, float *result)
{
__shared__ float data[laplacianD*2][laplacianD*2];
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
const int x0 = threadIdx.x + IMUL(blockIdx.x,blockDim.x);
const int y0 = threadIdx.y + IMUL(blockIdx.y,blockDim.y);
int x,y;
x = x0 - kernelRadius;
y = y0 - kernelRadius;
if(x < 0 || y < 0)
data[threadIdx.x][threadIdx.y] = 0.0f;
else
data[threadIdx.x][threadIdx.y] = A[ gLoc - kernelRadius - IMUL(DIM,kernelRadius)];
x = x0 + kernelRadius + 1;
y = y0 - kernelRadius;
if(x >= DIM || y < 0)
data[threadIdx.x + blockDim.x][threadIdx.y] = 0.0f;
else
data[threadIdx.x + blockDim.x][threadIdx.y] = A[ gLoc + kernelRadius+1 - IMUL(DIM,kernelRadius)];
x = x0 - kernelRadius;
y = y0 + kernelRadius+1;
if(x < 0 || y >= DIM)
data[threadIdx.x][threadIdx.y + blockDim.y] = 0.0f;
else
data[threadIdx.x][threadIdx.y + blockDim.y] = A[ gLoc - kernelRadius + IMUL(DIM,(kernelRadius+1))];
x = x0 + kernelRadius+1;
y = y0 + kernelRadius+1;
if(x >= DIM || y >= DIM)
data[threadIdx.x + blockDim.x][threadIdx.y + blockDim.y] = 0.0f;
else
data[threadIdx.x + blockDim.x][threadIdx.y + blockDim.y] = A[ gLoc + kernelRadius+1 + IMUL(DIM,(kernelRadius+1))];
__syncthreads();
float sum = 0;
x = kernelRadius + threadIdx.x;
y = kernelRadius + threadIdx.y;
#pragma unroll
for(int i = -kernelRadius; i<=kernelRadius; i++)
for(int j=-kernelRadius; j<=kernelRadius; j++)
sum += data[x+i][y+j] * gpu_D[i+kernelRadius][j+kernelRadius];
result[gLoc] = sum;
}
__global__ void multiplyEachElementby_c1(float *matrix, float *result)
{
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
result[gLoc] = matrix[gLoc]*gpu_c1;
}
__global__ void multiplyEachElementby_a1(float *matrix, float *result)
{
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
result[gLoc] = matrix[gLoc]*gpu_a1;
}
__global__ void multiplyEachElementby_a2(float *matrix, float *result)
{
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
result[gLoc] = matrix[gLoc]*gpu_a2;
}
__global__ void gpuMatrix2Bitmap(float *matrix, BYTE *bitmap)
{
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
int cvalue = (int)((matrix[gLoc] + 1.0f)/gpu_pp_step);
if(cvalue < 0)
cvalue = 0;
else if(cvalue >= COLOR_NUM)
cvalue = COLOR_NUM-1;
bitmap[gLoc*4] = gpu_m_colorMap[cvalue][0];
bitmap[gLoc*4 + 1] = gpu_m_colorMap[cvalue][1];
bitmap[gLoc*4 + 2] = gpu_m_colorMap[cvalue][2];
bitmap[gLoc*4 + 3] = 0xFF;
}
__global__ void addDropletToU(float *matrix, int x0d, int y0d, float *result)
{
const int gLoc = threadIdx.x + IMUL(blockIdx.x,blockDim.x) + IMUL(threadIdx.y,DIM) + IMUL(blockIdx.y,blockDim.y)*DIM;
const int x0 = threadIdx.x + IMUL(blockIdx.x,blockDim.x);
const int y0 = threadIdx.y + IMUL(blockIdx.y,blockDim.y);
if (x0 >= x0d-gpu_dsz*2 && y0 >= y0d-gpu_dsz*2 && x0 <= x0d+gpu_dsz*2 && y0 <= y0d+gpu_dsz*2)
{
result[gLoc] = matrix[gLoc] + gpu_Zd[x0 -(x0d-gpu_dsz*2)][y0 - (y0d-gpu_dsz*2)];
}
else
result[gLoc] = matrix[gLoc];
}

Notice that we preferred to “hardcode” the
constant values usages with different kernels rather than introducing
divergence with a conditional branch. The only kernel that increases thread
divergence is the *addDropletToU* since
only a few threads are actually performing the Gaussian packet-starting routine
(see the theoric algorithm described a few paragraphs ago), but this isn’t a
problem due to its low calling frequency.

The timing measurements and performance
comparisons have been performed on the following system

Intel 2
quad cpu Q9650 @ 3.00 Ghz

6 GB ram

64 bit OS

NVIDIA GeForce GTX 285 (1GB DDR3 @ 1476 Mhz, 240 CUDA
cores)

The CUDA version we used to compile the projects is 4.2, if you have problems make sure to install the right version or change it as described in the readme file.

To benchmark the CUDA kernel
execution we used the cudaEventCreate / cudaEventRecord / cudaEventSynchronize /
cudaEventElapsedTime functions shipped with every CUDA version, while for the
CPU version we used two Windows platform-dependent APIs:
QueryPerformanceFrequency and QueryPerformanceCounter.

We
split the benchmark into four stages: a startup stage where the only droplet in
the image is the default one, a second stage when both the CPU and the GPU
version stabilized themselves, a third one where we add 60-70 droplets to the
rendering queue and a final one when the application is left running for 15-20
minutes. We saw that in every test the CPU performed worse than the GPU version
which could rely on a large grid of threads ready to split up an upcoming
significant workload and provide a fixed rendering time. On the other hand in
the long term period, although the GPU still did better, the CPU version showed
a small performance increment, perhaps thanks to the caching mechanisms.

Notice that an application operating on larger
data would surely have taken a greater advantage from a massive parallelization
approach. Our wave PDE simulation is quite simple indeed and did not require a
significant workload thus reducing the performance gain that could have been
achieved.

Once and for all: there’s not a general rule to
convert a sequentially-designed algorithm to a parallel one and each case must
be evaluated in its own context and architecture. Also using CUDA could provide
a great advantage in scientific simulations, but one should not consider the
GPU as a substitute of the CPU but rather as a algebraic coprocessor that can
rely on massive data parallelization. Combining CPU sequential code parts with
GPU parallel code parts is the key to succeed.

The last, but not the least, section of this
paper provide a checklist of best-practices and errors to avoid when writing
CUDA kernels in order to get the maximum from your GPU-accelerated application

- Minimize host <-> device transfers, especially device -> host transfers which are slower, also if that could mean running on the device kernels that would not have been slower on the CPU.
- Use pinned memory (pagelocked) on the host side to exploit bandwidth advantages. Be careful not to abuse it or you’ll slow your entire system down.
- cudaMemcpy is a blocking function, if possible use it asynchronously with pinned memory + a CUDA stream in order to overlap transfers with kernel executions.
- If your graphic card is integrated, zero-copy memory (that is: pinned memory allocated with cudaHostAllocMapped flag) always grants an advantage. If not, there is no certainty since the memory is not cached by the GPU.
- Always check your device compute capabilities (use cudaGetDeviceProperties), if < 2.0 you cannot use more than 512 threads per block (65535 blocks maximum).
- Check your graphic card specifications: when launching a AxB block grid kernel, each SM (streaming multiprocessor) can serve a part of them. If your card has a maximum of 1024 threads per SM you should size your blocks in order to fill as many of them as possible but not too many (otherwise you would get scheduling latencies). Every warp is usually 32 thread (although this is not a fixed value and is architecture dependent) and is the minimum scheduling unit on each SM (only one warp is executed at any time and all threads in a warp execute the same instruction - SIMD), so you should consider the following example: on a GT200 card you need to perform a matrix multiplication. Should you use 8x8, 16x16 or 32x32 threads per block?

For 8X8 blocks, we have 64 threads per block. Since each SM can take up to 1024 threads, there are 16 Blocks (1024/64). However, each SM can only take up to 8 blocks. Hence only 512 (64*8) threads will go into each SM -> SM execution resources are under-utilized; fewer wraps to schedule around long latency operations

For 16X16 blocks , we have 256 threads per Block. Since each SM can take up to 1024 threads, it can take up to 4 Blocks (1024/256) and the 8 blocks limit isn’t hit -> Full thread capacity in each SM and maximal number of warps for scheduling around long-latency operations (1024/32= 32 wraps).

For 32X32 blocks, we have 1024 threads per block -> Not even one can fit into an SM! (there’s a 512 threads per block limitation).

- Check the SM registers limit per block and divide them by the thread number: you’ll get the maximum register number you can use in a thread. Exceeding it by just one per thread will cause less warp to be scheduled and decreased performance.
- Check the shared memory per block and make sure you don’t exceed that value. Exceeding it will cause less warp to be scheduled and decreased performance.
- Always check the thread number to be inferior than the maximum threads value supported by your device.
- Use reduction and locality techniques wherever applicable.
- If possible, never split your kernel code into conditional branches (if-then-else), different paths would cause more executions for the same warp and the following overhead.
- Use reduction techniques with divergence minimization when possible (that is, try to perform a reduction with warps performing a coalesced reading per cycle as described in chapter 6 of Kirk and Hwu book)
- Coalescence is achieved by forcing hardware reading consecutive data. If each thread in a warp access consecutive memory coalescence is significantly increased (half of the threads in a warp should access global memory at the same time), that’s why with large matrices (row-major) reading by columns is better than rows readings. Coalescence can be increased with locality (i.e. threads cooperating in loading data needed by other threads); kernels should perform coalesced readings with locality purposes in order to maximize memory throughputs. Storing values into shared memory (when possible) is a good practice too.
- Make sure you don’t have unused threads/blocks, by design or because of your code. As said before graphic cards have limits like maximum threads on SM and maximum blocks per SM. Designing your grid without consulting your card specifications is highly discouraged.
- As stated before adding more registers than the card maximum registers limit is a recipe for performance loss. Anyway adding a register may also cause instructions to be added, that is: more time to parallelize transfers or to schedule warps and better performances. Again: there’s no general rule, you should abide by the best practises when designing your code and then experiment by yourself.
- Data prefetching means preloading data you don’t actually need at the moment to gain performance in a future operation (closely related to locality). Combining data prefetching in matrix tiles can solve many long-latency memory access problems.
- Unrolling loops is preferable when applicable (as if the loop is small). Ideally loop unrolling should be automatically done by the compiler, but checking to be sure of that is always a better choice.
- Reduce thread granularity with rectangular tiles (Chapter 6 Kirk and Hwu book) when working with matrices to avoid multiple row/columns readings from global memory by different blocks.
- Textures are cached memory, if used properly they are significantly faster than global memory, that is: texture are better suited for 2D spatial locality accesses (e.g. multidimensional arrays) and might perform better in some specific cases (again, this is a case-by-case rule).
- Try to mantain at least 25% of the overall registers occupied, otherwise access latency could not be hidden by other warps’ computations.
- The number of threads per block should always be a multiple of warp size to favor coalesced readings.
- Generally if a SM supports more than just one block, more than 64 threads per block should be used.
- The starting values to begin experimenting with kernel grids are between 128 and 256 threads per block.
- High latency problems might be solved by using more blocks with less threads instead of just one block with a lot of threads per SM. This is important expecially for kernels which often call __syncthreads().
- If the kernel fails, use cudaGetLastError() to check the error value, you might have used too many registers / too much shared or constant memory / too many threads.
- CUDA functions and hardware perform best with float data type. Its use is highly encouraged.
- Integer division and modulus (%) are expensive operators, replace them with bitwise operations whenever possible. If n is a power of 2,

- Avoid automatic data conversion from double to float if possible.
- Mathematical functions with a __ preceeding them are hardware implemented, they’re faster but less accurate. Use them if precision is not a critical goal (e.g. __sinf(x) rather than sinf(x)).
- Use signed integers rather than unsigned integers in loops because some compilers optimize signed integers better (overflows with signed integers cause undefined behavior, then compilers might aggressively optimize them).
- Remember that floating point math is not associative because of round-off errors. Massively parallel results might differ because of this.
- 1) If
your device supports concurrent kernel executions (see
*concurrentKernels* device property) you might be able to gain
additional performance by running kernels in parallel. Check your device
specifications and the CUDA programming guides.

This concludes our article. The
goal was an exploration of GPGPU applications capabilities to improve
scientific simulations and programs which aim to large data manipulations.