ceph/src/rocksdb/third-party/folly/folly/synchronization/detail/Sleeper.h

   1 //  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 /*
   9  * @author Keith Adams <kma@fb.com>
  10  * @author Jordan DeLong <delong.j@fb.com>
  11  */
  12
  13 #include <cstdint>
  14 #include <thread>
  15
  16 #include <folly/portability/Asm.h>
  17
  18 namespace folly {
  19
  20 //////////////////////////////////////////////////////////////////////
  21
  22 namespace detail {
  23
  24 /*
  25  * A helper object for the contended case. Starts off with eager
  26  * spinning, and falls back to sleeping for small quantums.
  27  */
  28 class Sleeper {
  29   static const uint32_t kMaxActiveSpin = 4000;
  30
  31   uint32_t spinCount;
  32
  33  public:
  34   Sleeper() noexcept : spinCount(0) {}
  35
  36   static void sleep() noexcept {
  37     /*
  38      * Always sleep 0.5ms, assuming this will make the kernel put
  39      * us down for whatever its minimum timer resolution is (in
  40      * linux this varies by kernel version from 1ms to 10ms).
  41      */
  42     std::this_thread::sleep_for(std::chrono::microseconds{500});
  43   }
  44
  45   void wait() noexcept {
  46     if (spinCount < kMaxActiveSpin) {
  47       ++spinCount;
  48       asm_volatile_pause();
  49     } else {
  50       sleep();
  51     }
  52   }
  53 };
  54
  55 } // namespace detail
  56 } // namespace folly
  57