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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program 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
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#include "mongo/platform/basic.h"
#include "mongo/util/background_thread_clock_source.h"
#include <chrono>
#include <memory>
#include <thread>
#include "mongo/stdx/thread.h"
#include "mongo/util/concurrency/idle_thread_block.h"
#include "mongo/util/time_support.h"
#define MONGO_LOGV2_DEFAULT_COMPONENT ::mongo::logv2::LogComponent::kControl
namespace mongo {
BackgroundThreadClockSource::BackgroundThreadClockSource(std::unique_ptr<ClockSource> clockSource,
Milliseconds granularity)
: _clockSource(std::move(clockSource)), _state(kTimerPaused), _granularity(granularity) {
_startTimerThread();
_tracksSystemClock = true;
}
BackgroundThreadClockSource::~BackgroundThreadClockSource() {
{
stdx::unique_lock<stdx::mutex> lock(_mutex);
_inShutdown = true;
_condition.notify_one();
}
_timer.join();
}
Milliseconds BackgroundThreadClockSource::getPrecision() {
return _granularity;
}
Date_t BackgroundThreadClockSource::now() {
// Since this is called very frequently by many threads, the common case should not write to
// shared memory.
//
// If we read ReaderHasRead, we have at least the last time from a previous reader, or the
// background thread.
if (MONGO_unlikely(_state.load() != kReaderHasRead)) { // acquire
_updateClockAndWakeTimerIfNeeded();
}
return Date_t::lastNow();
}
void BackgroundThreadClockSource::_updateClock() {
// We capture the lastUpdate time now to ensure that we sleep for the right target granularity,
// even if it takes a while for the background thread to wake up.
_lastUpdate = _clockSource->now();
// Updates Date_t::lastNow by calling Date_t::now()
Date_t::now();
}
// This will be called at most once per _granularity per thread. In common cases it will only be
// called by a single thread per _granularity.
void BackgroundThreadClockSource::_updateClockAndWakeTimerIfNeeded() {
uint8_t expected = kTimerWillPause;
_state.compareAndSwap(&expected, kReaderHasRead);
// Try to go from TimerWillPause to ReaderHasRead.
if (expected != kTimerPaused) {
// There are three possible states _state could have been in before this cas:
//
// kTimerWillPause - In this case, we've transitioned to kReaderHasRead, telling the timer
// it still has recent readers and should continue to loop. In that case,
// we have no more to do, return.
//
// kReaderHasRead - Another thread had already performed the kTimerWillPause ->
// kReaderHasRead transition, we have no work to do, return.
//
// kTimerPaused - The timer was paused, so we have to wake it up. Don't return and attempt
// to wake the timer thread.
//
// For the first two cases, we can be sure we've acquired an up to date notion of time (from
// the timer thread or a reader that has woken calling updateClock()), from either
// succeeding or failing the cas above.
return;
}
// We may be in timer paused
stdx::lock_guard<stdx::mutex> lock(_mutex);
// See if we still observe paused, after taking a lock. This prevents multiple threads from
// racing to update the clock.
if (_state.load() != kTimerPaused) {
// If not, someone else has taken care of it
return;
}
// If we were still in pause, there are a couple of tasks we have to do:
//
// 1. update the clock
// 2. store kReaderHasRead
// 3. wake the background thread
//
// It's important that we do them in that order, so that the background thread sleeps
// exactly granularity from now, and so that readers that observe kReaderHasRead pick up the
// updated time. Failing to keep that order may cause them to observe what may be a very
// stale read (if the background timer was a sleep for an extended period).
_updateClock();
_state.store(kReaderHasRead); // release
_condition.notify_one();
}
size_t BackgroundThreadClockSource::timesPausedForTest() {
stdx::lock_guard<stdx::mutex> lk(_mutex);
return _timesPaused;
}
void BackgroundThreadClockSource::_startTimerThread() {
// Start the background thread that repeatedly sleeps for the specified duration of milliseconds
// and wakes up to store the current time.
_timer = stdx::thread([&]() {
setThreadName("BackgroundThreadClockSource");
stdx::unique_lock<stdx::mutex> lock(_mutex);
_started = true;
_condition.notify_one();
while (!_inShutdown) {
// update the clock every pass
_updateClock();
// Always transition to will pause on every run.
auto old = _state.swap(kTimerWillPause); // release
// There are 3 possible states _state could have been in:
//
// kTimerWillPause - We slept until the next tick without a reader. We should pause
//
// kReaderHasRead - A reader has read since our last sleep. We should sleep again
//
// kTimerPaused - We were asleep and spuriously woke or we just started (we start in
// kTimerPaused)
//
// If we do pause our wake up will indicate:
// 1. That we've had a reader and it's time to tick again
// 2. That we're in shutdown, where we'll early return from the next condvar wait
// 3. Experiencing a spurious wake, which may make us tick an extra time, but no more
// than once per granularity
if (old != kReaderHasRead) {
// Stop running if nothing has read the time since we last updated the time.
_state.store(kTimerPaused);
_timesPaused++;
// We don't care about spurious wake ups here, at worst we'll update the clock an
// extra time.
MONGO_IDLE_THREAD_BLOCK;
_condition.wait(lock);
}
MONGO_IDLE_THREAD_BLOCK;
_clockSource->waitForConditionUntil(
_condition, lock, _lastUpdate + _granularity, [this] { return _inShutdown; });
}
});
// Wait for the thread to start. This prevents other threads from calling now() until the timer
// thread is at its first wait() call. While the code would work without this, it makes startup
// more predictable and therefore easier to test.
stdx::unique_lock<stdx::mutex> lock(_mutex);
_condition.wait(lock, [this] { return _started; });
}
} // namespace mongo
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