benchmarks/semaphore_benchmark.cpp

/*
 * Copyright (C) 2014 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include <pthread.h>
#include <semaphore.h>
#include <stdatomic.h>
#include <stdio.h>
#include <stdlib.h>

#include <benchmark/benchmark.h>

static void BM_semaphore_sem_getvalue(benchmark::State& state) {
  sem_t semaphore;
  sem_init(&semaphore, 1, 1);

  while (state.KeepRunning()) {
    int dummy;
    sem_getvalue(&semaphore, &dummy);
  }
}
BENCHMARK(BM_semaphore_sem_getvalue);

static void BM_semaphore_sem_wait_sem_post(benchmark::State& state) {
  sem_t semaphore;
  sem_init(&semaphore, 1, 1);

  while (state.KeepRunning()) {
    sem_wait(&semaphore);
    sem_post(&semaphore);
  }
}
BENCHMARK(BM_semaphore_sem_wait_sem_post);

// This test reports the overhead of the underlying futex wake syscall on
// the producer. It does not report the overhead from issuing the wake to the
// point where the posted consumer thread wakes up. It suffers from
// clock_gettime syscall overhead. Lock the CPU speed for consistent results
// as we may not reach >50% cpu utilization.
//
// We will run a background thread that catches the sem_post wakeup and
// loops immediately returning back to sleep in sem_wait for the next one. This
// thread is run with policy SCHED_OTHER (normal policy), a middle policy.
//
// The primary thread will run at SCHED_IDLE (lowest priority policy) when
// monitoring the background thread to detect when it hits sem_wait sleep. It
// will do so with no clock running. Once we are ready, we will switch to
// SCHED_FIFO (highest priority policy) to time the act of running sem_post
// with the benchmark clock running. This ensures nothing else in the system
// can preempt our timed activity, including the background thread. We are
// also protected with the scheduling policy of letting a process hit a
// resource limit rather than get hit with a context switch.
//
// The background thread will start executing either on another CPU, or
// after we back down from SCHED_FIFO, but certainly not in the context of
// the timing of the sem_post.

static atomic_int BM_semaphore_sem_post_running;

static void* BM_semaphore_sem_post_start_thread(void* arg) {
  sem_t* semaphore = reinterpret_cast<sem_t*>(arg);
  while ((BM_semaphore_sem_post_running > 0) && !sem_wait(semaphore)) {
  }
  BM_semaphore_sem_post_running = -1;
  return NULL;
}

class SemaphoreFixture : public benchmark::Fixture {
 public:
  void SetUp(const benchmark::State&) {
    sem_init(&semaphore, 0, 0);

    pthread_attr_t attr;
    pthread_attr_init(&attr);

    memset(&param, 0, sizeof(param));
    pthread_attr_setschedparam(&attr, &param);
    pthread_attr_setschedpolicy(&attr, SCHED_OTHER);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
    pthread_t pthread;
    pthread_create(&pthread, &attr, BM_semaphore_sem_post_start_thread, &semaphore);
    pthread_attr_destroy(&attr);

    sched_setscheduler(0, SCHED_IDLE, &param);

    BM_semaphore_sem_post_running = 1;
    setup = true;
  }

  ~SemaphoreFixture() {
    if (setup) {
      // Only do this if the test was actually run.
      sched_setscheduler(0, SCHED_OTHER, &param);

      if (BM_semaphore_sem_post_running > 0) {
        BM_semaphore_sem_post_running = 0;
      }
      do {
        sem_post(&semaphore);
        sched_yield();
      } while (BM_semaphore_sem_post_running != -1);
    }
  }

  sem_t semaphore;
  sched_param param;
  bool setup = false;
};

BENCHMARK_F(SemaphoreFixture, semaphore_sem_post)(benchmark::State& state) {
  while (state.KeepRunning()) {
    state.PauseTiming();

    int trys = 3, dummy = 0;
    do {
      if (BM_semaphore_sem_post_running < 0) {
        sched_setscheduler(0, SCHED_OTHER, &param);
        fprintf(stderr, "BM_semaphore_sem_post: start_thread died unexpectedly\n");
        abort();
      }
      sched_yield();
      sem_getvalue(&semaphore, &dummy);
      if (dummy < 0) {  // POSIX.1-2001 possibility 1
        break;
      }
      if (dummy == 0) { // POSIX.1-2001 possibility 2
        --trys;
      }
    } while (trys);

    param.sched_priority = 1;
    sched_setscheduler(0, SCHED_FIFO, &param);

    state.ResumeTiming();
    sem_post(&semaphore);

    param.sched_priority = 0;
    sched_setscheduler(0, SCHED_IDLE, &param);
  }
}