Very slow rendering

I'm experiencing very slow rendering on my Debian 8 system (SDL2 version 2.0.4). Here's an example:

Code:

#define _GNU_SOURCE 

#include <stdio.h>
#include <SDL.h>

#include <time.h>

int main(void)
{
    SDL_version version;
    SDL_GetVersion(&version);
    printf("%d.%d.%d\n", (int) version.major, (int) version.minor, (int) version.patch);

    SDL_Init(SDL_INIT_VIDEO);

    SDL_Window *window = SDL_CreateWindow( "An SDL2 window",
        SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
        640, 480,
        SDL_WINDOW_SHOWN);

    SDL_Renderer *renderer = SDL_CreateRenderer(window,
        -1, SDL_RENDERER_ACCELERATED);

    SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);

    for (int i = 0; i < 60; i++) {
        SDL_RenderClear(renderer);

        struct timespec start_get_time, end_get_time;
        Uint32 start_get_ticks, end_get_ticks;

        start_get_ticks = SDL_GetTicks();
        clock_gettime(CLOCK_MONOTONIC, &start_get_time);

        SDL_RenderPresent(renderer); /* TOO SLOW */

        clock_gettime(CLOCK_MONOTONIC, &end_get_time);
        end_get_ticks = SDL_GetTicks();

        long nsecs = end_get_time.tv_nsec - start_get_time.tv_nsec;

        printf("nsecs: %ld\n", nsecs);
        printf("msecs: %u\n", (unsigned) end_get_ticks - start_get_ticks);
    }

    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}



and here's some of the output:

Code:

2.0.4
nsecs: 407449
msecs: 0
nsecs: 306949
msecs: 1
nsecs: 483087
msecs: 0
...
nsecs: 30375208
msecs: 31
nsecs: 4968098
msecs: 5
nsecs: 28803868
msecs: 29
nsecs: 5190540
msecs: 5
nsecs: 25599449
msecs: 26
nsecs: 28874268
msecs: 30
nsecs: 5098142
msecs: 5
nsecs: 27725881
msecs: 28
nsecs: 29498292
msecs: 29
nsecs: 5148706
msecs: 5
nsecs: 25935731
msecs: 26
nsecs: 1477544
msecs: 1



is that... normal? i assume not? in the real applications, these slowdowns are a lot more consistent. 12-25 milliseconds per call to RenderPresent. strace shows the following:

Code:

socket(PF_LOCAL, SOCK_STREAM|SOCK_CLOEXEC, 0) = 3
connect(3, {sa_family=AF_LOCAL, sun_path=@"/tmp/.X11-unix/X0"}, 20) = 0  
...
recvmsg(3, 0x7ffd9801e220, 0)  = -1 EAGAIN (Resource temporarily unavailable)
recvmsg(3, 0x7ffd9801e220, 0)  = -1 EAGAIN (Resource temporarily unavailable)



Again, there is a lot more "Resource unavailable" in the real program. Any advice?

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