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

Question

I have search for questions/answers on CPU affinity and read the results but I am still cannot get my threads to nail up to a single CPU.

I am working on an application that will be run on a dedicated linux box so I am not concerned about other processes, only my own. This app currently spawns off one pthread and then the main thread enters a while loop to process control messages using POSIX msg queues. This while loop blocks waiting for a control msg to come in and then processes it. So the main thread is very simple and non-critical. My code is working very well as I can send this app messages and it will process them just fine. All control messages are very small in size and are use to just control the functionality of the application, that is, only a few control messages are ever send/received.

Before I enter this while loop, I use sched_getaffinity() to log all of the CPUs available. Then I use sched_setaffinity() to set this process to a single CPU. Then I call sched_getaffinity() again to check if it is set to run on only one CPU and it is indeed correct.

The single pthread that was spawned off does a similar thing. The first thing I do in the newly created pthread is call pthread_getaffinity_np() and check the available CPUs, then call pthread_setaffinity_np() to set it to a different CPU then call pthread_getaffinity_np() to check if it is set as desired and it is indeed correct.

This is what is confusing. When I run the app and view the CPU History in System Monitor, I see no difference from when I run the app without all of this set affinity stuff. The scheduler still runs a couple of seconds in each of the 4 CPUs on this quad core box. So it appears that the scheduler is ignoring my affinity settings.

Am I wrong in expecting to see some proof that the main thread and the pthread are actually running in their own single CPU?? or have I forgotten to do something more to get this to work as I intend?

Thanks,

-Andres

jim mcnamara jim mcnamara · Accepted Answer · 2013-12-14T03:22:35

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;
}

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

1 Answers