2 //===----------------------------------------------------------------------===//
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
8 //===----------------------------------------------------------------------===//
10 #ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
11 #define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
13 #include <__availability>
14 #include <__chrono/duration.h>
15 #include <__chrono/high_resolution_clock.h>
18 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
19 # pragma GCC system_header
22 _LIBCPP_BEGIN_NAMESPACE_STD
24 static _LIBCPP_CONSTEXPR
const int __libcpp_polling_count
= 64;
26 // Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
27 // before polling again.
29 // - __f is the "test function" that should return true if polling succeeded, and false if it failed.
31 // - __bf is the "backoff policy", which is called with the duration since we started polling. It should
32 // return false in order to resume polling, and true if polling should stop entirely for some reason.
33 // In general, backoff policies sleep for some time before returning control to the polling loop.
35 // - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
36 // the polling loop will return false to report a timeout.
37 template<class _Fn
, class _BFn
>
38 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
39 bool __libcpp_thread_poll_with_backoff(_Fn
&& __f
, _BFn
&& __bf
, chrono::nanoseconds __max_elapsed
= chrono::nanoseconds::zero()) {
40 auto const __start
= chrono::high_resolution_clock::now();
41 for (int __count
= 0;;) {
43 return true; // _Fn completion means success
44 if (__count
< __libcpp_polling_count
) {
48 chrono::nanoseconds
const __elapsed
= chrono::high_resolution_clock::now() - __start
;
49 if (__max_elapsed
!= chrono::nanoseconds::zero() && __max_elapsed
< __elapsed
)
50 return false; // timeout failure
52 return false; // _BFn completion means failure
56 // A trivial backoff policy that always immediately returns the control to
59 // This is not very well-behaved since it will cause the polling loop to spin,
60 // so this should most likely only be used on single-threaded systems where there
61 // are no other threads to compete with.
62 struct __spinning_backoff_policy
{
63 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
64 bool operator()(chrono::nanoseconds
const&) const {
69 _LIBCPP_END_NAMESPACE_STD
71 #endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H