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 | |
29 | namespace folly { |
30 | |
31 | ////////////////////////////////////////////////////////////////////// |
32 | |
33 | namespace 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 | */ |
39 | class 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 | |