| // -*- C++ -*- |
| //===----------------------------------------------------------------------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| #ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H |
| #define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H |
| |
| #include <__config> |
| #include <chrono> |
| |
| #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) |
| #pragma GCC system_header |
| #endif |
| |
| _LIBCPP_BEGIN_NAMESPACE_STD |
| |
| static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64; |
| |
| // Polls a thread for a condition given by a predicate, and backs off based on a backoff policy |
| // before polling again. |
| // |
| // - __f is the "test function" that should return true if polling succeeded, and false if it failed. |
| // |
| // - __bf is the "backoff policy", which is called with the duration since we started polling. It should |
| // return false in order to resume polling, and true if polling should stop entirely for some reason. |
| // In general, backoff policies sleep for some time before returning control to the polling loop. |
| // |
| // - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded, |
| // the polling loop will return false to report a timeout. |
| template<class _Fn, class _BFn> |
| _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI |
| bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) { |
| auto const __start = chrono::high_resolution_clock::now(); |
| for (int __count = 0;;) { |
| if (__f()) |
| return true; // _Fn completion means success |
| if (__count < __libcpp_polling_count) { |
| __count += 1; |
| continue; |
| } |
| chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start; |
| if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed) |
| return false; // timeout failure |
| if (__bf(__elapsed)) |
| return false; // _BFn completion means failure |
| } |
| } |
| |
| // A trivial backoff policy that always immediately returns the control to |
| // the polling loop. |
| // |
| // This is not very well-behaved since it will cause the polling loop to spin, |
| // so this should most likely only be used on single-threaded systems where there |
| // are no other threads to compete with. |
| struct __spinning_backoff_policy { |
| _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR |
| bool operator()(chrono::nanoseconds const&) const { |
| return false; |
| } |
| }; |
| |
| _LIBCPP_END_NAMESPACE_STD |
| |
| #endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H |