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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.
*/
#pragma once
#include "mongo/platform/atomic_proxy.h"
#include "mongo/platform/atomic_word.h"
namespace mongo {
/**
* This is a collection of parameters that affect how much snapshot history the storage engine will
* maintain to support point-in-time transactions (read or write). This is referred to as the
* snapshot window.
*/
struct SnapshotWindowParams {
// maxTargetSnapshotHistoryWindowInSeconds (startup & runtime server paramter, range 0+).
//
// Dictates the maximum lag in seconds oldest_timestamp should be behind stable_timestamp.
// targetSnapshotHistoryWindowInSeconds below is the actual active lag setting target.
//
// Note that the window size can become greater than this if an ongoing operation is holding an
// older snapshot open.
AtomicWord<int> maxTargetSnapshotHistoryWindowInSeconds{5};
// targetSnapshotHistoryWindowInSeconds (not a server parameter, range 0+).
//
// Dictates the target lag in seconds oldest_timestamp should be set behind stable_timestamp.
// Should only be set in the range [0, maxTargetSnapshotHistoryWindowInSeconds].
//
// Note that this is the history window we attempt to maintain, but our current system state may
// not always reflect it: the window can only change as more writes come in, so it can take time
// for the actual window size to catch up with a change. This value guides actions whenever the
// system goes to update the oldest_timestamp value.
AtomicWord<int> targetSnapshotHistoryWindowInSeconds{
maxTargetSnapshotHistoryWindowInSeconds.load()};
// cachePressureThreshold (startup & runtime server paramter, range [0, 100]).
//
// Dictates what percentage of cache in use is considered too high. This setting helps preempt
// storage cache pressure immobilizing the system. Attempts to increase
// targetSnapshotHistoryWindowInSeconds will be ignored when the cache pressure reaches this
// threshold. Additionally, a periodic task will decrease targetSnapshotHistoryWindowInSeconds
// when cache pressure exceeds the threshold.
AtomicWord<int> cachePressureThreshold{50};
// snapshotWindowMultiplicativeDecrease (startup & runtime server paramter, range (0,1)).
//
// Controls by what multiplier the target snapshot history window setting is decreased when
// cache pressure becomes too high, per the cachePressureThreshold setting.
AtomicDouble snapshotWindowMultiplicativeDecrease{0.75};
// snapshotWindowAdditiveIncreaseSeconds (startup & runtime server paramter, range 1+).
//
// Controls by how much the target snapshot history window setting is increased when cache
// pressure is OK, per cachePressureThreshold, and we need to service older snapshots for global
// point-in-time reads.
AtomicWord<int> snapshotWindowAdditiveIncreaseSeconds{2};
// minMillisBetweenSnapshotWindowInc (startup & runtime server paramter, range 0+).
// minMillisBetweenSnapshotWindowDec (startup & runtime server paramter, range 0+).
//
// Controls how often attempting to increase/decrease the target snapshot window will have an
// effect. Multiple callers within minMillisBetweenSnapshotWindowInc will have the same effect
// as one. This protects the system because it takes time for the target snapshot window to
// affect the actual storage engine snapshot window. The stable timestamp must move forward for
// the window between it and oldest timestamp to grow or shrink.
AtomicWord<int> minMillisBetweenSnapshotWindowInc{500};
AtomicWord<int> minMillisBetweenSnapshotWindowDec{500};
// checkCachePressurePeriodSeconds (startup & runtime server paramter, range 1+)
//
// Controls the period of the task that checks for cache pressure and decreases
// targetSnapshotHistoryWindowInSeconds if the pressure is above cachePressureThreshold. The
// target window size setting must not be decreased too fast because time must be allowed for
// the storage engine to attempt to act on the new setting.
AtomicWord<int> checkCachePressurePeriodSeconds{5};
};
extern SnapshotWindowParams snapshotWindowParams;
} // namespace mongo
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