1/*
2 * Copyright 2015-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19/*
20 * @author Keith Adams <kma@fb.com>
21 * @author Jordan DeLong <delong.j@fb.com>
22 */
23
24#include <cstdint>
25
26#include <folly/portability/Asm.h>
27#include <folly/portability/Time.h>
28
29namespace folly {
30
31//////////////////////////////////////////////////////////////////////
32
33namespace detail {
34
35/*
36 * A helper object for the contended case. Starts off with eager
37 * spinning, and falls back to sleeping for small quantums.
38 */
39class Sleeper {
40 static const uint32_t kMaxActiveSpin = 4000;
41
42 uint32_t spinCount;
43
44 public:
45 Sleeper() noexcept : spinCount(0) {}
46
47 static void sleep() noexcept {
48 /*
49 * Always sleep 0.5ms, assuming this will make the kernel put
50 * us down for whatever its minimum timer resolution is (in
51 * linux this varies by kernel version from 1ms to 10ms).
52 */
53 struct timespec ts = {0, 500000};
54 nanosleep(&ts, nullptr);
55 }
56
57 void wait() noexcept {
58 if (spinCount < kMaxActiveSpin) {
59 ++spinCount;
60 asm_volatile_pause();
61 } else {
62 sleep();
63 }
64 }
65};
66
67} // namespace detail
68} // namespace folly
69