C / C++ / MFC

30-Mar-04 15:08

Program Code:

#include<stdio.h>
#include<time.h>

#define N 1024
int i,j,k;
float slice[N][N];

void main()
{
time_t start,end;
float s;
start=time(NULL);

for(k=0;k<100;k++)
{
for(j=0;j<n;j++)
="" for(i="0;i<N;i++)" {=""
<code=""> slice[i][j]=(float)(slice[i][j]+0.01);
slice[j][i]=(float)(slice[j][i]+0.01);
}
printf("%d\n",k);
}

end=time(NULL);
s=difftime(end,start);
printf(" The total time is %f:",s);
}

Questions or problems(Compiled by Visual C++ 6.0):
1.If N equals 1022,1023,1025 or 1026, its run time is about 13 seconds, else if N=1024 that will be about 56 seconds. that is, the speed is very different.
2. "slice[j][i]=(float)(slice[j][i]+0.01)" is executed over two times faster than "slice[i][j]=(float)(slice[i][j]+0.01);". You can have a try by cutting off one of these sentences.
3. An exception will happen if "int i,j,k; float slice[N][N];" is moved into function main() that says "test.exe has encountered a problem and needs to close. We are sorry for the inconvenience.". In addition, my program is named by "test.cpp"

Have you known these problems?
and could you give me an explanation
and how to avoid these bad results
please

Any is appreciated!
Thanks!

Maxwell Chen30-Mar-04 16:24

30-Mar-04 16:24

1) With VC++6, and with VC++7(2002), I saw the same situation.

2) Regarding to
FlyingDancer wrote:
float slice[N][N];" is moved into function main()

please try this, since you use .cpp extension. It does not crash.

// #include<stdio.h>
#include <iostream>
#include <time.h>

// #define N 1023
// float slice[N][N];
void main()
{
	int i,j,k;
	time_t start,end;
	float s;
	const int N = 1024;
	float (*slice)[N] = new float[N][N];

	start=time(NULL);
	for(k=0;k<100;k++)
	{
		for(j=0;j<N;j++)
			for(i=0;i<N;i++)
			{
				slice[i][j]=(float)(slice[i][j]+0.01F);
				slice[j][i]=(float)(slice[j][i]+0.01F);
			}
			printf("%d\n",k);
	}
	end=time(NULL);
	s=difftime(end,start);
	printf("   The total time is %f:",s);
	delete[] slice;
}

3) Regarding to 1024 taking that long time, I dunno. I guess that it may be the x86 instructions...

Maxwell Chen

FlyingDancer30-Mar-04 18:45

30-Mar-04 18:45

Yeah
It can work well, not considering its run speed

Your way is to adopt an array pointer, that seems a little different
but what has been solved regarding to a big array slice[N][N] by adopting a array pointer?

This is a difficult problem, really. Is it related with OS, compiling way? Or maybe it is a memory allocating problem...

How do you think about this?

ohadp30-Mar-04 19:16

ohadp

30-Mar-04 19:16

it crashes because if not allocated dynamically this array is allocated on the stack which probably can't hold 1024*1024*4-bytes...

FlyingDancer30-Mar-04 21:39

30-Mar-04 21:39

Problem: "slice[j][i]=(float)(slice[j][i]+0.01F);" is executed faster than "slice[i][j]=(float)(slice[i][j]+0.01F);"

I think it should be answered from two aspects

1. In VC, A two-dimension array is stored according to its row first, then its col,...
2. Virtual memory technology. Paging and Swaping
In this problem every row of that array couldn't get enough free space
so when accessing to any data of another row a swapping action will happen

therefore one runs faster than the other

Am I right?

Maxwell Chen30-Mar-04 22:20

30-Mar-04 22:20

Maxwell Chen

Paul Ranson31-Mar-04 1:57

Paul Ranson

31-Mar-04 1:57

1. I don't know, other than it's likely to be a virtual memory pathology.

2. This is your code,

<br />
// v1<br />
for ( j = 0; j< N; j++ )<br />
{<br />
        for ( i = 0; i < N; i++ )<br />
	{<br />
                slice [i][j] = (float)(slice[i][j] + 0.01 ) ;<br />
        }<br />
}<br />
// which is equivalent to<br />
for ( j = 0; j< N; j++ )<br />
{<br />
        for ( i = 0; i < N; i++ )<br />
	{<br />
                float * pf = slice + ( i * N ) + j ;<br />
                *pf += 0.01 ;<br />
        }<br />
}<br />
<br />
// v2<br />
for ( j = 0; j< N; j++ )<br />
{<br />
        for ( i = 0; i < N; i++ )<br />
	{<br />
                slice [j][i] = (float)(slice[j][i] + 0.01 ) ;<br />
        }<br />
}<br />
// which is equivalent to<br />
for ( j = 0; j< N; j++ )<br />
{<br />
        float * pf = slice + (j * N) ;<br />
        for ( i = 0; i < N; i++ )<br />
	{<br />
                *pf += 0.01 ;<br />
                ++pf ;<br />
        }<br />
}<br />

IOW in the first example you are asking the CPU to do an extra multiplication each time around the inner loop. The optimiser may be able to turn it into an addition (if that's faster...), but it's still extra work.

More subtley the second example accesses memory consecutively, so the data is much more likely to be in the CPU cache, whereas the first accesses every N * sizeof ( float ) bytes which means the next value will never be in the cache, accessing main memory means waiting about, accessing the cache puts that off, and since the cache is read and written to main memory in relatively large chunks you will get an entire 'cache line' of modified values going to main memory in the same time as it takes to write one.

Anyway it would be worth examining the generated machine code for each example to see what the optimiser actually does, and perhaps play with the options.

3. The default stack size for Win32 is 1MB. You are asking to allocate 4MB (sizeof ( float ) == 4 ) so the only way is to exit with an exception. You can adjust this in the linker, or with EditBin, but for a data structure of this nature either declaring it statically as in your example or allocation on the heap as in Maxwell's is appropriate.

Paul

FlyingDancer31-Mar-04 13:30

31-Mar-04 13:30

Great!
Full and clear!!
Thank you very much!!!
Laugh | :laugh:

C to Visual C++.Net

gyrogearloose30-Mar-04 15:04

gyrogearloose

30-Mar-04 15:04

I'd like to convert the C programs found in "Controling the world With Your PC" to Visual C++.Net programs. I have the dll for Win2000,XP and need sort of a starting point to the prodess. Ideas, tips? An exsample even ? Smile | :)

I did old QB & a little pascal. Got VB.Net and VC++.Net on the computer.

Thanks

Gyrogearloose

Christian Graus30-Mar-04 15:14

Christian Graus

30-Mar-04 15:14

Any C program should run in VC++, including VC++.NET. What problems are you having ? The most likely one is that a project you create will have precompiled headers turned on, and you'll need to turn them off, or create a stdafx.h file and include it everywhere.

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

Maxwell Chen30-Mar-04 15:37

30-Mar-04 15:37

Christian Graus wrote:
Any C program should run in VC++, including VC++.NET

Not C99 (ISO/IEC 9899:1999) programs.

Poke tongue | ;-P

Maxwell Chen

Christian Graus30-Mar-04 15:38

Christian Graus

30-Mar-04 15:38

Oh, yeah. I forgot about that. D'Oh! | :doh:

Christian

I have drunk the cool-aid and found it wan and bitter. - Chris Maunder

Maxwell Chen30-Mar-04 15:46

30-Mar-04 15:46

Maxwell Chen

gyrogearloose31-Mar-04 13:23

gyrogearloose

31-Mar-04 13:23

Well, yes that sounds like that will be the next problem and answer. If I try to use (open project) from the start page, it only shows exstentions for the VB.Net that I have on the computer with the C++. It doesn't show C++ exstentions at all. If however I click on a file like duolphase.C , it will display it in visual studio but there are no controls avalable to edit the program. I mean no tabs, no controls at all.
I tryed to set options-environment but found nothing refering to the start page that made any difference. The only thing I can figure is I'll have to type it in as a new project and go from there. Like the header file thing you mentioned.
Wander what would happen if I rename xxxxxx.C to xxxxxx.cs ?? Smile | :)

Tryed posting in the VS forum but no answer as yet.

In case your wandering, yes there is a comercial app in the works and a gui-servo&stepper motor app for public domain.
Thanks for the help.

Gyrogearloose