faux-folly/folly/detail/Sleeper.h

/*
 * Copyright 2015 Facebook, Inc.
 *
 * 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.
 */

#pragma once

/*
 * @author Keith Adams <kma@fb.com>
 * @author Jordan DeLong <delong.j@fb.com>
 */

#include <cinttypes>
#include <ctime>

#include <folly/Portability.h>

namespace folly {

//////////////////////////////////////////////////////////////////////

namespace detail {

  /*
   * A helper object for the contended case. Starts off with eager
   * spinning, and falls back to sleeping for small quantums.
   */
  class Sleeper {
    static const uint32_t kMaxActiveSpin = 4000;

    uint32_t spinCount;

  public:
    Sleeper() : spinCount(0) {}

    void wait() {
      if (spinCount < kMaxActiveSpin) {
        ++spinCount;
        asm_volatile_pause();
      } else {
        /*
         * Always sleep 0.5ms, assuming this will make the kernel put
         * us down for whatever its minimum timer resolution is (in
         * linux this varies by kernel version from 1ms to 10ms).
         */
        struct timespec ts = { 0, 500000 };
        nanosleep(&ts, nullptr);
      }
    }
  };

}
}