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/**
* Copyright (C) 2022-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.
*/
#pragma once
#include <queue>
#include "mongo/db/operation_context.h"
#include "mongo/db/service_context.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/util/concurrency/admission_context.h"
#include "mongo/util/concurrency/mutex.h"
#include "mongo/util/concurrency/ticket_queues.h"
#include "mongo/util/concurrency/ticketholder.h"
#include "mongo/util/hierarchical_acquisition.h"
#include "mongo/util/time_support.h"
namespace mongo {
class Ticket;
/**
* A ticketholder implementation that centralises all ticket acquisition/releases. Waiters will get
* placed in a specific internal queue according to some logic. Releasers will wake up a waiter
* from a group chosen according to some logic.
*/
class PriorityTicketHolder : public TicketHolderWithQueueingStats {
public:
explicit PriorityTicketHolder(int numTickets,
int lowPriorityBypassThreshold,
ServiceContext* serviceContext);
~PriorityTicketHolder() override{};
int available() const override final {
return _ticketsAvailable.load();
};
int queued() const override final {
return _enqueuedElements.loadRelaxed();
}
bool recordImmediateTicketStatistics() noexcept override final {
return true;
};
/**
* Number of times low priority operations are expedited for ticket admission over normal
* priority operations.
*/
std::int64_t expedited() const {
return _expeditedLowPriorityAdmissions.loadRelaxed();
}
/**
* Returns the number of times the low priority queue is bypassed in favor of dequeuing from the
* normal priority queue when a ticket becomes available.
*/
std::int64_t bypassed() const {
return _lowPriorityBypassCount.loadRelaxed();
};
void updateLowPriorityAdmissionBypassThreshold(const int& newBypassThreshold);
private:
enum class QueueType : unsigned int {
kLowPriority = 0,
kNormalPriority = 1,
// Exclusively used for statistics tracking. This queue should never have any processes
// 'queued'.
kImmediatePriority = 2,
QueueTypeSize = 3
};
boost::optional<Ticket> _tryAcquireImpl(AdmissionContext* admCtx) override final;
boost::optional<Ticket> _waitForTicketUntilImpl(OperationContext* opCtx,
AdmissionContext* admCtx,
Date_t until,
WaitMode waitMode) override final;
void _releaseToTicketPoolImpl(AdmissionContext* admCtx) noexcept override final;
void _resize(int newSize, int oldSize) noexcept override final;
QueueStats& _getQueueStatsToUse(const AdmissionContext* admCtx) noexcept override final;
void _appendImplStats(BSONObjBuilder& b) const override final;
bool _tryAcquireTicket();
/**
* Wakes up a waiting thread (if it exists) in order for it to attempt to obtain a ticket.
* Implementors MUST wake at least one waiting thread if at least one thread is pending to be
* woken between all the queues. In other words, attemptToDequeue on each non-empty Queue must
* be called until either it returns true at least once or has been called on all queues.
*
* Care must be taken to ensure that only CPU-bound work is performed here and it doesn't block.
*
* When called the following invariants will be held:
* - The number of items in each queue will not change during the execution
* - No other thread will proceed to wait during the execution of the method
*/
void _dequeueWaitingThread(const ticket_queues::SharedLockGuard& sharedQueueLock);
/**
* Returns whether there are higher priority threads pending to get a ticket in front of the
* given queue type and not enough tickets for all of them.
*/
bool _hasToWaitForHigherPriority(const ticket_queues::UniqueLockGuard& lk, QueueType queueType);
unsigned int _enumToInt(QueueType queueType) {
return static_cast<unsigned int>(queueType);
}
unsigned int _enumToInt(QueueType queueType) const {
return static_cast<unsigned int>(queueType);
}
ticket_queues::Queue& _getQueue(QueueType queueType) {
return _queues[_enumToInt(queueType)];
}
QueueType _getQueueType(const AdmissionContext* admCtx) {
auto priority = admCtx->getPriority();
switch (priority) {
case AdmissionContext::Priority::kLow:
return QueueType::kLowPriority;
case AdmissionContext::Priority::kNormal:
return QueueType::kNormalPriority;
case AdmissionContext::Priority::kImmediate:
return QueueType::kImmediatePriority;
default:
MONGO_UNREACHABLE;
}
}
ticket_queues::QueueMutex _queueMutex;
std::array<ticket_queues::Queue, static_cast<unsigned int>(QueueType::QueueTypeSize)> _queues;
std::array<QueueStats, static_cast<unsigned int>(QueueType::QueueTypeSize)> _stats;
/**
* Limits the number times the low priority queue is non-empty and bypassed in favor of the
* normal priority queue for the next ticket admission.
*
* Updates must be done under the ticket_queues::UniqueLockGuard.
*/
int _lowPriorityBypassThreshold;
/**
* Counts the number of times normal operations are dequeued over operations queued in the low
* priority queue.
*/
AtomicWord<std::uint64_t> _lowPriorityBypassCount{0};
/**
* Number of times ticket admission is expedited for low priority operations.
*/
AtomicWord<std::int64_t> _expeditedLowPriorityAdmissions{0};
AtomicWord<int> _ticketsAvailable;
AtomicWord<int> _enqueuedElements;
ServiceContext* _serviceContext;
};
} // namespace mongo
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