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Setting thread priority in Linux with Boost

Posted by: admin November 30, 2017 Leave a comment

Questions:

The Boost Libraries don’t seem to have a device for setting a thread’s priority. Would this be the best code to use on Linux or is there a better method?

boost::thread myThread( MyFunction() );

struct sched_param param;
param.sched_priority = 90;
pthread_attr_setschedparam( myThread.native_handle(), SCHED_RR, &param);

I don’t have alot of Linux programming experience.

Answers:

That’s the basic template for how I would do it but after searching around I found next to no code examples so I guess the verdict is out on whether it is best or not.

The problem is that boost::thread does not have a constructor that allows pthead attributes to be passed in at thread creation so you have to make changes after the thread starts. The only other way I know to get around that is through the process/thread schedule inheritance. Unless directed otherwise, new threads will inherit the schedule/priority of their creator so you could change the current thread before creating worker threads and then change back if you want. Seems awkward but it is an alternative.

Here’s a hack of an example that hopefully demostrates both. You may need to change policy and priority as appropriate and run as root.

Be careful with the way you set the priority. Various restrictions apply.

#include <iostream>
#include <boost/thread/thread.hpp>
#include <unistd.h>
#include <sched.h>
#include <cstdio>


void* threadfunc()
{
    sleep(5);
}

void displayAndChange(boost::thread& daThread)
{
    int retcode;
    int policy;

    pthread_t threadID = (pthread_t) daThread.native_handle();

    struct sched_param param;

    if ((retcode = pthread_getschedparam(threadID, &policy, &param)) != 0)
    {
        errno = retcode;
        perror("pthread_getschedparam");
        exit(EXIT_FAILURE);
    }

    std::cout << "INHERITED: ";
    std::cout << "policy=" << ((policy == SCHED_FIFO)  ? "SCHED_FIFO" :
                               (policy == SCHED_RR)    ? "SCHED_RR" :
                               (policy == SCHED_OTHER) ? "SCHED_OTHER" :
                                                         "???")
              << ", priority=" << param.sched_priority << std::endl;


    policy = SCHED_FIFO;
    param.sched_priority = 4;

    if ((retcode = pthread_setschedparam(threadID, policy, &param)) != 0)
    {
        errno = retcode;
        perror("pthread_setschedparam");
        exit(EXIT_FAILURE);
    }

    std::cout << "  CHANGED: ";
    std::cout << "policy=" << ((policy == SCHED_FIFO)  ? "SCHED_FIFO" :
                               (policy == SCHED_RR)    ? "SCHED_RR" :
                               (policy == SCHED_OTHER) ? "SCHED_OTHER" :
                                                          "???")
              << ", priority=" << param.sched_priority << std::endl;
}


int main(int argc, char* argv[])
{
    int policy, res;

    struct sched_param param;

    if ((policy = sched_getscheduler(getpid())) == -1)
    {
        perror("sched_getscheduler");
        exit(EXIT_FAILURE);
    }

    if ((res = sched_getparam(getpid(), &param)) == -1)
    {
        perror("sched_getparam");
        exit(EXIT_FAILURE);
    }

    std::cout << " ORIGINAL: ";
    std::cout << "policy=" << ((policy == SCHED_FIFO)  ? "SCHED_FIFO" :
                               (policy == SCHED_RR)    ? "SCHED_RR" :
                               (policy == SCHED_OTHER) ? "SCHED_OTHER" :
                                                          "???")
              << ", priority=" << param.sched_priority << std::endl;


    policy = SCHED_RR;
    param.sched_priority = 2;

    if ((res = sched_setscheduler(getpid(), policy, &param)) == -1)
    {
        perror("sched_setscheduler");
        exit(EXIT_FAILURE);
    }

    boost::thread t1(&threadfunc);

    displayAndChange(t1);

    t1.join();

    return 0;
}

Questions:
Answers:

You can have a look at this library (written by a student on mine, not released yet, look at the svn repository):
https://sourceforge.net/projects/threadutility/

Basically, he wrote a wrapper of the boost::thread that allows to specify and set threads’ priorities in a portable way, by selecting the right internal implementation depending on the platform (currently Linux or Windows). In Linux, the code uses the sched_setscheduler() syscall.

Questions:
Answers:

I don’t think Linux really has thread priorities – in most kernels you can use ‘Nice’ levels, but that is probably about it (Which would simplify the scheduler) – however, not all linux systems will respect that! (Depends on the scheduler).

Questions:
Answers:

boost::thread does have the ability to take in pthread attributes before pthread_create() is called. It provides the type boost::thread::attributes, which itself can only be used to set the stack size (on systems that support it), but it also presents a .get_native_handle() method, which returns a pthread_attr_t, on which you can set your desired attributes. You can then simply call make_thread() with that boost::thread::attributes object as an argument. See the second and third code boxes in this section: http://www.boost.org/doc/libs/1_53_0/doc/html/thread/thread_management.html#thread.thread_management.tutorial.attributes