Basic Linux and Windows Thread Prioritisation for worker threads

[hmelder]

This PR implements basic thread prioritisation on Linux and Windows and fixes a bug in dispatch_get_global_queue where the wrong QOS type is used.

Note that this only sets the initial thread priority by translating QoS to thread priorities. Relative priorities are ignored for now. Later QoS overrides and other priority adjustments have no effect.

See the comments for implementation details.

Result on Windows

#include <stdio.h>
#include <windows.h>
#include <dispatch/dispatch.h>

int main (int arg,char **argv)
{
    void (^myBlock) (void) = ^{
            PWSTR desc;
            HANDLE current = GetCurrentThread();
            int prio = GetThreadPriority(current);
            GetThreadDescription(current, &desc);
            wprintf(L"priority %d descr %ls\n", prio, desc);
            LocalFree(desc);
    };

    dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_queue_t utilityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_queue_t defaultQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_queue_t highQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    dispatch_async(backgroundQueue, myBlock);
    dispatch_async(utilityQueue, myBlock);
    dispatch_async(defaultQueue, myBlock);
    dispatch_async(highQueue, myBlock);

    dispatch_main();

}

hugo@YellowBox MSYS /tmp
# ./a.exe
priority 0 descr com.apple.root.default-qos
priority -1 descr com.apple.root.utility-qos
priority -4 descr com.apple.root.background-qos
priority 1 descr com.apple.root.user-initiated-qos

Result on Linux

#include <stdio.h>
#include <dispatch/dispatch.h>
#include <pthread.h>
#include <sys/resource.h>

int main (int arg,char **argv)
{
    void (^myBlock) (void) = ^{
	    int prio = getpriority(PRIO_PROCESS, 0);
	    printf("block with priority %d \n", prio);
    };

    dispatch_queue_t backgroundQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_queue_t utilityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_queue_t defaultQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_queue_t highQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

    dispatch_async(backgroundQueue, myBlock);
    dispatch_async(utilityQueue, myBlock);
    dispatch_async(defaultQueue, myBlock);
    dispatch_async(highQueue, myBlock);

    dispatch_main();
}

block with priority 2
block with priority 0
block with priority 10
block with priority -2

[hmelder]

We can probably move all platform-dependent thread prioritisation logic into a new function like _dispatch_set_priority() (analog to _dispatch_get_priority()). This function sets the TSD slot __TSD_THREAD_QOS_CLASS on Darwin, setpriority on Linux, and SetThreadPriority on Windows.

[triplef]

@compnerd @lxbndr would be great to get your thoughts on this. 🙏