libcxx/include/__thread/poll_with_backoff.h

   1 // -*- C++ -*-
   2 //===----------------------------------------------------------------------===//
   3 //
   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
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
  11 #define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
  12
  13 #include <__chrono/duration.h>
  14 #include <__chrono/high_resolution_clock.h>
  15 #include <__config>
  16
  17 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  18 #  pragma GCC system_header
  19 #endif
  20
  21 _LIBCPP_BEGIN_NAMESPACE_STD
  22
  23 static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
  24
  25 // Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
  26 // before polling again.
  27 //
  28 // - __poll is the "test function" that should return true if polling succeeded, and false if it failed.
  29 //
  30 // - __backoff is the "backoff policy", which is called with the duration since we started polling. It should
  31 //   return false in order to resume polling, and true if polling should stop entirely for some reason.
  32 //   In general, backoff policies sleep for some time before returning control to the polling loop.
  33 //
  34 // - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
  35 //   the polling loop will return false to report a timeout.
  36 template <class _Poll, class _Backoff>
  37 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __libcpp_thread_poll_with_backoff(
  38     _Poll&& __poll, _Backoff&& __backoff, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
  39   auto const __start = chrono::high_resolution_clock::now();
  40   for (int __count = 0;;) {
  41     if (__poll())
  42       return true; // __poll completion means success
  43     if (__count < __libcpp_polling_count) {
  44       __count += 1;
  45       continue;
  46     }
  47     chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
  48     if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
  49       return false; // timeout failure
  50     if (__backoff(__elapsed))
  51       return false; // __backoff completion means failure
  52   }
  53 }
  54
  55 // A trivial backoff policy that always immediately returns the control to
  56 // the polling loop.
  57 //
  58 // This is not very well-behaved since it will cause the polling loop to spin,
  59 // so this should most likely only be used on single-threaded systems where there
  60 // are no other threads to compete with.
  61 struct __spinning_backoff_policy {
  62   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator()(chrono::nanoseconds const&) const { return false; }
  63 };
  64
  65 _LIBCPP_END_NAMESPACE_STD
  66
  67 #endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H