[gcc r10-9029] libstdc++: Optimise std::future::wait_for
Jonathan Wakely
redi@gcc.gnu.org
Mon Nov 16 21:15:15 GMT 2020
https://gcc.gnu.org/g:c9f528dd1a14aacadec4638e7ee8ecff69fa0ee5
commit r10-9029-gc9f528dd1a14aacadec4638e7ee8ecff69fa0ee5
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Thu Nov 12 21:25:14 2020 +0000
libstdc++: Optimise std::future::wait_for
To poll a std::future to see if it's ready you have to call one of the
timed waiting functions. The most obvious way is wait_for(0s) but this
was previously very inefficient because it would turn the relative
timeout to an absolute one by calling system_clock::now(). When the
relative timeout is zero (or less) we're obviously going to get a time
that has already passed, but the overhead of obtaining the current time
can be dozens of microseconds.
This patch makes future::wait_for avoid waiting at all when the relative
timeout is zero or less, to avoid the unnecessary overhead of getting
the current time. This makes polling with wait_for(0s) take only a few
cycles instead of dozens of milliseconds.
libstdc++-v3/ChangeLog:
* include/std/future (future::wait_for): Do not wait for
durations less than or equal to zero.
* testsuite/30_threads/future/members/poll.cc: New test.
(cherry picked from commit 93fc47746815ea9dac413322fcade2931f757e7f)
Diff:
---
libstdc++-v3/include/std/future | 14 ++-
.../testsuite/30_threads/future/members/poll.cc | 106 +++++++++++++++++++++
2 files changed, 117 insertions(+), 3 deletions(-)
diff --git a/libstdc++-v3/include/std/future b/libstdc++-v3/include/std/future
index 97506a27e37..09f4af6fe7c 100644
--- a/libstdc++-v3/include/std/future
+++ b/libstdc++-v3/include/std/future
@@ -346,10 +346,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// to synchronize with the thread that made it ready.
if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
return future_status::ready;
+
if (_M_is_deferred_future())
return future_status::deferred;
- if (_M_status._M_load_when_equal_for(_Status::__ready,
- memory_order_acquire, __rel))
+
+ // Don't wait unless the relative time is greater than zero.
+ if (__rel > __rel.zero()
+ && _M_status._M_load_when_equal_for(_Status::__ready,
+ memory_order_acquire,
+ __rel))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2100. timed waiting functions must also join
@@ -378,10 +383,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
// to synchronize with the thread that made it ready.
if (_M_status._M_load(memory_order_acquire) == _Status::__ready)
return future_status::ready;
+
if (_M_is_deferred_future())
return future_status::deferred;
+
if (_M_status._M_load_when_equal_until(_Status::__ready,
- memory_order_acquire, __abs))
+ memory_order_acquire,
+ __abs))
{
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// 2100. timed waiting functions must also join
diff --git a/libstdc++-v3/testsuite/30_threads/future/members/poll.cc b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
new file mode 100644
index 00000000000..1f54ce7cc3f
--- /dev/null
+++ b/libstdc++-v3/testsuite/30_threads/future/members/poll.cc
@@ -0,0 +1,106 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-O3" }
+// { dg-do run { target c++11 } }
+// { dg-additional-options "-pthread" { target pthread } }
+// { dg-require-gthreads "" }
+
+#include <future>
+#include <chrono>
+#include <iostream>
+#include <testsuite_hooks.h>
+
+const int iterations = 200;
+
+using namespace std;
+
+template<typename Duration>
+double
+print(const char* desc, Duration dur)
+{
+ auto ns = chrono::duration_cast<chrono::nanoseconds>(dur).count();
+ double d = double(ns) / iterations;
+ cout << desc << ": " << ns << "ns for " << iterations
+ << " calls, avg " << d << "ns per call\n";
+ return d;
+}
+
+int main()
+{
+ promise<int> p;
+ future<int> f = p.get_future();
+
+ auto start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_for(chrono::seconds(0));
+ auto stop = chrono::high_resolution_clock::now();
+ double wait_for_0 = print("wait_for(0s)", stop - start);
+
+ start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_until(chrono::system_clock::time_point::min());
+ stop = chrono::high_resolution_clock::now();
+ double wait_until_sys_min __attribute__((unused))
+ = print("wait_until(system_clock minimum)", stop - start);
+
+ start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_until(chrono::steady_clock::time_point::min());
+ stop = chrono::high_resolution_clock::now();
+ double wait_until_steady_min __attribute__((unused))
+ = print("wait_until(steady_clock minimum)", stop - start);
+
+ start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_until(chrono::system_clock::time_point());
+ stop = chrono::high_resolution_clock::now();
+ double wait_until_sys_epoch __attribute__((unused))
+ = print("wait_until(system_clock epoch)", stop - start);
+
+ start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_until(chrono::steady_clock::time_point());
+ stop = chrono::high_resolution_clock::now();
+ double wait_until_steady_epoch __attribute__((unused))
+ = print("wait_until(steady_clock epoch", stop - start);
+
+ p.set_value(1);
+
+ start = chrono::high_resolution_clock::now();
+ for(int i = 0; i < iterations; i++)
+ f.wait_for(chrono::seconds(0));
+ stop = chrono::high_resolution_clock::now();
+ double ready = print("wait_for when ready", stop - start);
+
+ // Polling before ready with wait_for(0s) should be almost as fast as
+ // after the result is ready.
+ VERIFY( wait_for_0 < (ready * 10) );
+
+ // polling before ready using wait_until(min) should not be terribly slow.
+ VERIFY( wait_until_sys_min < (ready * 100) );
+ // Converting from steady clock to system clock adds overhead before GCC 11.
+ VERIFY( wait_until_steady_min < (ready * 500) );
+
+ // The following two tests fail with GCC 11, see
+ // https://gcc.gnu.org/pipermail/libstdc++/2020-November/051422.html
+
+ // polling before ready using wait_until(epoch) should not be terribly slow.
+ VERIFY( wait_until_sys_epoch < (ready * 100) );
+ // Converting from steady clock to system clock adds overhead before GCC 11.
+ VERIFY( wait_until_steady_epoch < (ready * 500) );
+}
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