context_switch_latency.c

#define _GNU_SOURCE

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sched.h>
#include <pthread.h>
#include <sys/eventfd.h>

int to_thread, from_thread;
volatile int state;  // 0 = warmup.  1 = benchmark.  2 = exit.

int use_xstate = 0;
int thread_cpu;

void maybe_use_xstate(void)
{
  if (use_xstate)
    asm volatile ("pxor %%xmm0, %%xmm0" : : : "xmm0");
}

void *threadproc(void *x)
{
  cpu_set_t cpuset;
  CPU_ZERO(&cpuset);
  CPU_SET(thread_cpu, &cpuset);
  if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
    printf("[WARN]\tsched_setaffinity failed\n");

  while(1)
    {
      uint64_t buf;

      if (read(to_thread, &buf, 8) != 8)
	abort();

      int s = state;
      if (s == 2)
	return 0;
      else if (1 || s == 0)
	maybe_use_xstate();

      buf = 1;
      if (write(from_thread, &buf, 8) != 8)
	abort();
    }
}

void bounce()
{
  uint64_t buf = 1;
  if (write(to_thread, &buf, 8) != 8)
    abort();
  
  if (read(from_thread, &buf, 8) != 8)
    abort();
}

void killit()
{
  uint64_t buf = 1;
  state = 2;
  if (write(to_thread, &buf, 8) != 8)
    abort();
}

int main(int argc, char **argv)
{
  struct timespec start, end;
  pthread_t thread;
  uint64_t i, iters = 100000;
  uint64_t ns;
  int main_cpu;

  if (argc != 3) {
    printf("Usage: %s <0|1> <same|different>\n\nSet the parameter to 1 to use xstate\nUse 'same' for same CPU or 'different' for cross-CPU\n",
	   argv[0]);
    return 1;
  } else {
    if (!strcmp(argv[1], "1"))
      use_xstate = 1;
    else if (strcmp(argv[1], "0"))
      abort();

    if (!strcmp(argv[2], "same")) {
      main_cpu = 0;
      thread_cpu = 0;
    } else if (!strcmp(argv[2], "different")) {
      main_cpu = 0;
      thread_cpu = 1;
    } else {
      abort();
    }
  }

  printf("use_xstate = %d\n", use_xstate);

  to_thread = eventfd(0, 0);
  if (to_thread < 0)
    abort();
  from_thread = eventfd(0, 0);
  if (from_thread < 0)
    abort();
  
  if (pthread_create(&thread, 0, threadproc, 0) != 0)
    abort();

  cpu_set_t cpuset;
  CPU_ZERO(&cpuset);
  CPU_SET(main_cpu, &cpuset);
  if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
    printf("[WARN]\tsched_setaffinity failed\n");

  /* Warm up (and burn the xstate heuristic) */
  for(i = 0; i < 10000; i++) {
    bounce();
    maybe_use_xstate();
  }

  /* Clear out the xstate hack. */
  state = 1;
  bounce();

  clock_gettime(CLOCK_MONOTONIC, &start);

  for (i = 0; i < iters; i++) {
    bounce();
    maybe_use_xstate();
  }

  clock_gettime(CLOCK_MONOTONIC, &end);

  killit();

  ns = 1000000000ULL * (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec);

  printf("%llu iters at %.1f ns/switch\n",
	 (unsigned long long)iters, (double)ns / (2*iters));

  pthread_join(thread, 0);

  return 0;
}