Cpu Affinity Masks (Putting Threads on Different Cpus)

CPU Affinity Masks (Putting Threads on different CPUs)

You're trying to set the affinity of threads that you did not initialize.

Edit: Ok, let me give you some more info:

Don't mix thread handles (the thing you store in the pthread_t variable) and what they represent (a thread of execution that runs somewhere). What you were trying to do is to set a property of a thread before it starts, with an API that requires the thread object. As it happens pthread_create creates the object and starts the execution at the same time, so trying to use pthread_setaffinity_np is not the right way to go (this is useful if you want to change the affinity of a currently running thread).

But... pthread_create has an attribute parameter (you're passing NULL to it). This is storing the information of how you want the thread to be created.

Affinity is one of the attributes you can set through that parameter. See the man-page documentation for pthread_attr_init and pthread_attr_setaffinity_np for how exactly

How can I set a thread's CPU affinity in Qt5?

Usually that sort of things is done by extracting the native thread handle and then doing whatever system specific stuff necessary, as no accepted cross-platform API exists for low level thread management.

Indeed, if we inspect the source for the qthread_unix.cpp we will see the following:

Qt::HANDLE QThread::currentThreadId() Q_DECL_NOTHROW
{
    // requires a C cast here otherwise we run into trouble on AIX
    return to_HANDLE(pthread_self());
}

And in qthread_win.cpp the implementation will differ in the expected way:

Qt::HANDLE QThread::currentThreadId() Q_DECL_NOTHROW
{
    return reinterpret_cast<Qt::HANDLE>(quintptr(GetCurrentThreadId()));
}

So, it is responsibility of the application code to do the proper low level actions pertaining to each platform it is expected to run on.

why doesn't the System Monitor show correct CPU affinity?

You have no answers, I will give you what I can:some partial help

Assuming you checked the return values from pthread_setaffinity_np:

How you assign your cpuset is very important, create it in the main thread. For what you want. It will propagate to successive threads. Did you check return codes?

The cpuset you actually get will be the intersection of hardware available cpus and the cpuset you define.
min.h in the code below is a generic build include file. You have to define _GNU_SOURCE - please note the comment on the last line of the code. CPUSET and CPUSETSIZE are macros. I think I define them somewhere else, I do not remember. They may be in a standard header.

#define _GNU_SOURCE
#include "min.h"
#include <pthread.h>


int
main(int argc, char **argv)
{
    int s, j;
    cpu_set_t cpuset;
    pthread_t tid=pthread_self();

    // Set affinity mask to include CPUs 0 & 1

    CPU_ZERO(&cpuset);
    for (j = 0; j < 2; j++)
        CPU_SET(j, &cpuset);

    s = pthread_setaffinity_np(tid, sizeof(cpu_set_t), &cpuset);
    if (s != 0)
    {
        fprintf(stderr, "%d ", s);
        perror(" pthread_setaffinity_np");
        exit(1);
     }
     // lets see what we really have in the actual affinity mask assigned our thread

    s = pthread_getaffinity_np(tid, sizeof(cpu_set_t), &cpuset);
    if (s != 0)
    {
        fprintf(stderr, "%d ", s);
        perror(" pthread_setaffinity_np");
        exit(1);
     }

    printf("my cpuset has:\n");
    for (j = 0; j < CPU_SETSIZE; j++)
        if (CPU_ISSET(j, &cpuset))
            printf("    CPU %d\n", j);
    // @Andres note: any pthread_create call from here on creates a thread with the identical
    // cpuset - you do not have to call it in every thread.
    return 0;
}

How can I find which processor each thread is running on?

Use GetCurrentProcessorNumber() ( http://msdn.microsoft.com/en-us/library/windows/desktop/ms683181(v=vs.85).aspx )

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