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You might not need to make them virtual.
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Just learning OpenMP.
So what is , if any , the difference between "processor" / CPU or "thread"?
They seems to be used interchangeably.
And do I have to specify # of processors / threads to be actually used by the app?
I suppose I can also select which part of code will be "multi x", but that is irrelevant for now.
int iCPU = 0;
iCPU = omp_get_num_procs();
cout << "# of CPU's " << dec << +iCPU << endl;
omp_set_num_threads(iCPU);
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Initially for the application will set the number of threads = nofCores, as 1 thread per core will get you the best performance.
From there your performance goes down for both other options
Sometime you don't want all the cores doing stuff so you control it and you will run across things like
#pragma omp parallel num_threads(3)
{
... some code for 3 cores
}
Then you get the other option multiple threads per core ... that involves context switching so again performance loss.
So it starts that way on initial easy samples but it won't stay that way for very long 
In vino veritas
modified 9-Feb-19 4:08am.
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Processors or CPUs mean the same thing. They are the hardware devices (or parts of a chip) that execute the instructions in a program. Processes or Threads, are activities that can run inside a processor. A thread is simply a discrete set of instructions that are executed by the CPU. Some applications run quite happily with a single thread. Others require multiple threads in order to perform different parts of the program in parallel. However, using multiple threads is not always the most efficient way to run an application, since a single CPU (or 'core' in a modern chip) can only run one thread at a time. So if you have a four-core chip you can run four threads in parallel with reasonable efficiency. With a single core it would probably not be very efficient owing to operating system overhead to manage switching between threads. Unless you really need threads, and fully understand the concept and practicalities of them, they are best avoided.
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So the example I used retrieves # of processors / CPU hardware and sets the # of threads per each processor / CPU equal to # of processors / CPU hardware.
Does not make much sense.
The way I read the example - it really does not tell me how many processors / CPU hardware will be utilized.
Perhaps I need to read the OpenMP doc.
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I do not know exactly what you are trying to achieve here. Maybe the documentation makes it clear.
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My objective is for the app to utilize ALL hardware processors / CPU.
From posts so far I feel I do not need multiple threads running in each CPU, one will do for now.
I am just trying to understand the relations between processors and threads.
The following code leads to believe that the threads are NOT related to # of processors.
int iCPU = 0;
iCPU = omp_get_num_procs();
cout << "# of CPU's " << dec << +iCPU << endl;
gets "4" which is number of CPU's
int iMAXThread = 0;
iMAXThread = omp_get_max_threads();
cout << "# of iMAXThread " << dec << +iMAXThread << endl;
also gets "4" - apparently default of "one thread to CPU ?"
// Now set the number of threads
omp_set_num_threads(10);
iMAXThread = omp_get_max_threads();
returns "10" per "system ?"
cout << "# of iMAXThread " << dec << +iMAXThread << endl;
exit(1);
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I'd recommend you google for "openmp tutorial C++" and work through one or more of them. I don't know anything about OpenMP, except that its a MultiProcessing (i.e. threading) extension to C/C++. I do know that multi-threading is easy to get wrong. Added to which, having multiple threads means that you get out-of-order (to the human brain, anyway) execution, which can lead to unexpected results e.g. (the following code comes from one of the tutorials ... don't ask me anything further about it!)
<div class="op">Quote:</div>$ cat example.c
#include <omp.h>
#include <stdio.h>
int main()
{
#pragma omp parallel
{
int ID = omp_get_thread_num();
printf("hello(%d)", ID);
printf("world(%d)\n", ID);
}
return 0;
}
$ gcc -fopenmp example.c -o example
$ ./example
hello(1)world(1)
hello(0)world(0)
hello(3)world(3)
hello(2)world(2)
$ ./example
hello(3)world(3)
hello(0)world(0)
hello(1)world(1)
hello(2)world(2)
$ ./example
hello(3)world(3)
hello(2)hello(1)world(1)
hello(0)world(0)
world(2)
That's about as simple as a MP program gets, and you can see that
a) the threads didn't run in the same order every time, and
b) in the last run thread(2) was interrupted by thread(1) and thread(0) before it completed.
Proceed with caution!
modified 9-Feb-19 14:37pm.
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I'd also like to point out that with threads, debugging becomes more challenging, too!
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I really appreciate all contributions to this thread so far.
As anything "new" to me I do not have any specific / planned way to proceed with my learning , especially to cover some of the less obvious / less visible aspects.
I do however maintain that learning new technology is not always "from bottom up" , it is a mosaic / puzzle whose pieces by itself do not make much sense.
Let me restate my "objective" in using OpenMP - utilize ALL CPU's available in the application.
I have deliberately NOT specified WHAT the app does. My objective should be descriptive enough.
But to attempt to refine the "puzzle" , and for those curious, I do not foresee any future applications critical in sequence of how the app is executed.
I still believe that utilizing all hardware CPU's will eventually benefit the app, but honestly I am not interested in proving / disproving that in any specific way.
As far as OpenMP "tutorials" - it is very mature technology and most of the tutorials reflect that. And some of them are just repetitious in the cautioning the reader in "difficulties " of debugging multi threaded applications.
Cheers
Vaclav
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OpenMP works out how many CPU core you have at runtime.
It isn't a fixed answer and there may be cores unavailable to you and there isn't a thing you can do about it.
This call returns the number of cores available to you at time of call
int omp_get_num_procs();
Inside Windows or Linux certain threads may have a core locked with core affinity and you can sit there and whistle dixie the OS is never going to give OpenMP that core.
OpenMP is not the OS and is not in control of core scheduling it asks and locks the maximum the O/S will allow.
So if you aren't getting all the cores you need to look at the O/S level.
google something like "Affinity control outside OpenMP"
In vino veritas
modified 9-Feb-19 22:59pm.
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Thanks Leon.
Found this and my first impression is that simple "Hello world" MAY be better way to get more familiar with OpenMP. Just "adding OpenMP" to my existing code is not going to tell me if it is doing anything useful until I add some controls / measurements / trace.
That is contrary to my initial statement that I am not interested in analyzing OpenP processes, but to do that I need more experience with OpenMP.
Error[^]
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I would suggest you need a better understand of threads, and some practical experience of using them. Without understanding the basics, I doubt that OpenMP is going to bring you any benefit.
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No, threads and processors are not related in any way. Processors (CPUs) are hardware, threads are pieces of code that run in a CPU. An application can create as many threads as it likes regardless of how many processors exist in the system that it runs on. In most cases you do not need to know how many CPUs are available, since your application should be based on its business/technical design rather than the hardware it runs on. Finally, do not assume that you can control how many processors your application uses; the operating system is in control of resources, and allocates them as necessary.
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Need c coding for the below scenario
Case 1: Input: string 1: -(a+b+c)
String 2: -a-b-c
Output: True
Case 2: Input: string 1: -(a-b)
String 2: -a-b
Output: False
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Post what you have tried and try to ask specific question(s). You're not likely to find someone that will just write it for you.
"the debugger doesn't tell me anything because this code compiles just fine" - random QA comment
"Facebook is where you tell lies to your friends. Twitter is where you tell the truth to strangers." - chriselst
"I don't drink any more... then again, I don't drink any less." - Mike Mullikins uncle
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Hint count the minus signs in your expanded string
In vino veritas
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This is just curiosity , not a question.
I code in C++ , using IDE.
It "builds" the executable using "make" without me messing with "automatically generated" "make" by IDE. Accessing the "make " sometime warns that is it so generated by IDE and not to modify it.
Most of the time I follow instructions, so I let IDE to do its stuff.
I have noticed that some code posts here include manually building the "make".
Why if run of the mill IDE does it "automatically" ?
I am sure there are occasions when IDE generated make could use some tweaking, but in general IDE does the job well.
Off soap box.
Cheers
Vaclav
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It is the same as the compiler. You can let the IDE set the options and generate the command, or you can run it yourself in a command window. Programs such as cl (the C/C++ compiler), make etc., have existed for years, long before the IDEs were developed. The IDEs just try to make life easier for you. I have a Visual Studio project that does not run the standard way so I had to create my own Makefile to build it. You are free to do that with any of your projects if you want greater control over how they are built.
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Vaclav_ wrote: I have noticed that some code posts here include manually building the "make".
Why if run of the mill IDE does it "automatically" ?
Your IDE probably creates the Makefile from some sort of project database, perhaps a .proj file. Lets assume that I create a library that you would like to use, but I use a different IDE than you, which uses Project.DB files that are completely different than the .proj files your IDE uses. If my project creates a Makefile that can be used from the command line, then you don't have to figure out how to import and build the library using your IDE, or how to install and use the IDE I use. Additionally, your IDE might clobber the Makefile distributed with my project, in which case you're going to have to go try to make sure you've got all the dependencies right, which might take a lot of time.
In the land of UNIX like OS (Linux, BSD, Darwin, etc), make is almost ubiquitous. If the OS doesn't ship with make, someone will probably have a version of make (e.g. gnu-make) available. That means make is, more or less, the de facto build tool.
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The answer to that is historic most of the original C compilers were around with DOS and Unix all we had was a command line. So on the command line you used to have to type these extremely long strings and so everyone wrote DOS batch files or unix shell scripts. Come 1976 and make was created by Stuart Feldman and it was released to public domain.
Make never really took off with DOS users as by 1983 Windows 1.0 was released and a small company called Borland began with Pascal and later C as a text GUI on the windows platform.
Borland - Wikipedia[^]
Microsoft saw the success of Borland as a threat (they used DOS command line in house) and essentially began to mimic Borland at least on the GUI (the compiler remained a dos executable) and thus was born the first microsoft compiler tools.
With no big bucks for GUI compilers being spent on linux make became a staple thing for the linux OS. Even most embedded compilers tended to stay as DOS or Windows with a couple of exceptions. So generally you could pick Windows versus Linux developers by did they know there way around make which developed it's own language.
More recently GUI compiler IDE's started to pop up in linux and so they will import and work with Make files. Windows Visual Studio now can even work with make files as it is targetting linux developers although in all honesty they would like them to move to CMake.
There is also a very subtle difference between windows and linux repositories because of the history that many linux programmers fail to notice. Most windows code has the header files for the C code files in the same directory, on most linux repositories you will find all the headers in an include directory (they seem to view it is good practice) and the source files elsewhere. The reason for the difference is Windows compilers have a specific search order which is same directory as C file, library directory and then user defined include directories. Under windows compiling #include <some.h> is very different to #include "some.h" (the first means the C system directory the second means the user defined directory) on linux repositories that difference always seems to get lost (all the code is pubic domain none is company copyrighted) they randomly they use either -l or -L and fix it in the makefile. Under Windows you start messing around with the system directories you will get in a world of hurt (as much is precompiled copyrighted) and usually only SDK's do that.
In vino veritas
modified 7-Feb-19 23:34pm.
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leon de boer wrote: all the code is pubic domain
I now have a most strange image in my mind ... 
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Quote: I have noticed that some code posts here include manually building the "make". Why if run of the mill IDE does it "automatically" ?
Possibly because I am not using an IDE.
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