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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.
*/
#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kStorage
#define DEBUG_LOG_LEVEL 4
#include "mongo/platform/basic.h"
#include "mongo/db/storage/flow_control.h"
#include <algorithm>
#include <fmt/format.h>
#include <limits>
#include "mongo/db/concurrency/flow_control_ticketholder.h"
#include "mongo/db/concurrency/lock_manager_defs.h"
#include "mongo/db/repl/member_data.h"
#include "mongo/db/repl/replication_coordinator.h"
#include "mongo/db/server_options.h"
#include "mongo/db/storage/flow_control_parameters_gen.h"
#include "mongo/util/background.h"
#include "mongo/util/fail_point_service.h"
#include "mongo/util/log.h"
namespace mongo {
MONGO_FAIL_POINT_DEFINE(flowControlTicketOverride);
namespace {
const auto getFlowControl = ServiceContext::declareDecoration<std::unique_ptr<FlowControl>>();
int multiplyWithOverflowCheck(double term1, double term2, int maxValue) {
if (term1 == 0.0 || term2 == 0.0) {
// Early return to avoid any divide by zero errors.
return 0;
}
if (static_cast<double>(std::numeric_limits<int>::max()) / term2 < term1) {
// Multiplying term1 and term2 would overflow, return maxValue.
return maxValue;
}
double ret = term1 * term2;
if (ret >= maxValue) {
return maxValue;
}
return static_cast<int>(ret);
}
std::uint64_t getLagMillis(Date_t myLastApplied, Date_t lastCommitted) {
if (!myLastApplied.isFormattable() || !lastCommitted.isFormattable()) {
return 0;
}
return static_cast<std::uint64_t>(durationCount<Milliseconds>(myLastApplied - lastCommitted));
}
std::uint64_t getThresholdLagMillis() {
return static_cast<std::uint64_t>(1000.0 * gFlowControlThresholdLagPercentage.load() *
gFlowControlTargetLagSeconds.load());
}
Timestamp getMedianAppliedTimestamp(const std::vector<repl::MemberData>& sortedMemberData) {
if (sortedMemberData.size() == 0) {
return Timestamp::min();
}
const int sustainerIdx = sortedMemberData.size() / 2;
return sortedMemberData[sustainerIdx].getLastAppliedOpTime().getTimestamp();
}
/**
* Sanity checks whether the successive queries of topology data are comparable for doing a flow
* control calculation. In particular, the number of members must be the same and the median
* applier's timestamp must not go backwards.
*/
bool sustainerAdvanced(const std::vector<repl::MemberData>& prevMemberData,
const std::vector<repl::MemberData>& currMemberData) {
if (currMemberData.size() == 0 || currMemberData.size() != prevMemberData.size()) {
warning() << "Flow control detected a change in topology. PrevMemberSize: "
<< prevMemberData.size() << " CurrMemberSize: " << currMemberData.size();
return false;
}
auto currSustainerAppliedTs = getMedianAppliedTimestamp(currMemberData);
auto prevSustainerAppliedTs = getMedianAppliedTimestamp(prevMemberData);
if (currSustainerAppliedTs < prevSustainerAppliedTs) {
warning() << "Flow control's sustainer time decreased. PrevSustainer: "
<< prevSustainerAppliedTs << " CurrSustainer: " << currSustainerAppliedTs;
return false;
}
return true;
}
} // namespace
FlowControl::FlowControl(repl::ReplicationCoordinator* replCoord)
: ServerStatusSection("flowControl"),
_replCoord(replCoord),
_lastTimeSustainerAdvanced(Date_t::now()) {}
FlowControl::FlowControl(ServiceContext* service, repl::ReplicationCoordinator* replCoord)
: ServerStatusSection("flowControl"),
_replCoord(replCoord),
_lastTimeSustainerAdvanced(Date_t::now()) {
// Initialize _lastTargetTicketsPermitted to maximum tickets to make sure flow control doesn't
// cause a slow start on start up.
FlowControlTicketholder::set(service, std::make_unique<FlowControlTicketholder>(_kMaxTickets));
service->getPeriodicRunner()->scheduleJob(
{"FlowControlRefresher",
[this](Client* client) {
FlowControlTicketholder::get(client->getServiceContext())->refreshTo(getNumTickets());
},
Seconds(1)});
}
FlowControl* FlowControl::get(ServiceContext* service) {
return getFlowControl(service).get();
}
FlowControl* FlowControl::get(ServiceContext& service) {
return getFlowControl(service).get();
}
FlowControl* FlowControl::get(OperationContext* ctx) {
return get(ctx->getClient()->getServiceContext());
}
void FlowControl::set(ServiceContext* service, std::unique_ptr<FlowControl> flowControl) {
auto& globalFlow = getFlowControl(service);
globalFlow = std::move(flowControl);
}
/**
* Returns -1.0 if there are not enough samples.
*/
double FlowControl::_getLocksPerOp() {
// Primaries sample the number of operations it has applied alongside how many global lock
// acquisitions (in MODE_IX) it took to process those operations. This method looks at the two
// most recent samples and returns the ratio of global lock acquisitions to operations processed
// for the current client workload.
Sample backTwo;
Sample backOne;
std::size_t numSamples;
{
stdx::lock_guard<stdx::mutex> lk(_sampledOpsMutex);
numSamples = _sampledOpsApplied.size();
if (numSamples >= 2) {
backTwo = _sampledOpsApplied[numSamples - 2];
backOne = _sampledOpsApplied[numSamples - 1];
} else {
_lastLocksPerOp.store(0.0);
return -1.0;
}
}
auto ret = (double)(std::get<2>(backOne) - std::get<2>(backTwo)) /
(double)(std::get<1>(backOne) - std::get<1>(backTwo));
_lastLocksPerOp.store(ret);
return ret;
}
BSONObj FlowControl::generateSection(OperationContext* opCtx,
const BSONElement& configElement) const {
BSONObjBuilder bob;
// Most of these values are only computed and meaningful when flow control is enabled.
bob.append("enabled", gFlowControlEnabled.load());
bob.append("targetRateLimit", _lastTargetTicketsPermitted.load());
bob.append("timeAcquiringMicros",
FlowControlTicketholder::get(opCtx)->totalTimeAcquiringMicros());
bob.append("locksPerOp", _lastLocksPerOp.load());
bob.append("sustainerRate", _lastSustainerAppliedCount.load());
bob.append("isLagged", _isLagged.load());
bob.append("isLaggedCount", _isLaggedCount.load());
bob.append("isLaggedTimeMicros", _isLaggedTimeMicros.load());
return bob.obj();
}
/**
* Advance the `_*MemberData` fields and sort the new data by the element's last applied optime.
*/
void FlowControl::_updateTopologyData() {
_prevMemberData = _currMemberData;
_currMemberData = _replCoord->getMemberData();
// Sort MemberData with the 0th index being the node with the lowest applied optime.
std::sort(_currMemberData.begin(),
_currMemberData.end(),
[](const repl::MemberData& left, const repl::MemberData& right) -> bool {
return left.getLastAppliedOpTime() < right.getLastAppliedOpTime();
});
}
int FlowControl::_calculateNewTicketsForLag(const std::vector<repl::MemberData>& prevMemberData,
const std::vector<repl::MemberData>& currMemberData,
std::int64_t locksUsedLastPeriod,
double locksPerOp,
std::uint64_t lagMillis,
std::uint64_t thresholdLagMillis) {
invariant(lagMillis >= thresholdLagMillis);
using namespace fmt::literals;
const auto currSustainerAppliedTs = getMedianAppliedTimestamp(currMemberData);
const auto prevSustainerAppliedTs = getMedianAppliedTimestamp(prevMemberData);
invariant(prevSustainerAppliedTs <= currSustainerAppliedTs,
"PrevSustainer: {} CurrSustainer: {}"_format(prevSustainerAppliedTs.toString(),
currSustainerAppliedTs.toString()));
const std::int64_t sustainerAppliedCount =
_approximateOpsBetween(prevSustainerAppliedTs, currSustainerAppliedTs);
LOG(DEBUG_LOG_LEVEL) << " PrevApplied: " << prevSustainerAppliedTs
<< " CurrApplied: " << currSustainerAppliedTs
<< " NumSustainerApplied: " << sustainerAppliedCount;
if (sustainerAppliedCount > 0) {
_lastTimeSustainerAdvanced = Date_t::now();
} else {
auto warnThresholdSeconds = gFlowControlWarnThresholdSeconds.load();
const auto now = Date_t::now();
if (warnThresholdSeconds > 0 &&
now - _lastTimeSustainerAdvanced >= Seconds(warnThresholdSeconds)) {
warning() << "Flow control is engaged and the sustainer point is not moving. Please "
"check the health of all secondaries.";
// Log once every `warnThresholdSeconds` seconds.
_lastTimeSustainerAdvanced = now;
}
}
_lastSustainerAppliedCount.store(static_cast<int>(sustainerAppliedCount));
if (sustainerAppliedCount == -1) {
// We don't know how many ops the sustainer applied. Hand out less tickets than were
// used in the last period.
return std::min(static_cast<int>(locksUsedLastPeriod / 2.0), _kMaxTickets);
}
// Given a "sustainer rate", this function wants to calculate what fraction the primary should
// accept writes at to allow secondaries to catch up.
//
// When the commit point lag is similar to the threshold, the function will output an exponent
// close to 0 resulting in a coefficient close to 1. In this state, the primary will accept
// writes roughly on pace with the sustainer rate.
//
// As another example, as the commit point lag increases to say, 2x the threshold, the exponent
// will be close to 1. In this case the primary will accept writes at roughly the
// `gFlowControlDecayConstant` (original default of 0.5).
auto exponent = static_cast<double>(lagMillis - thresholdLagMillis) /
static_cast<double>(thresholdLagMillis);
invariant(exponent >= 0.0);
const double reduce = pow(gFlowControlDecayConstant.load(), exponent);
// The fudge factor, by default is 0.95. Keeping this value close to one reduces oscillations in
// an environment where secondaries consistently process operations slower than the primary.
double sustainerAppliedPenalty =
sustainerAppliedCount * reduce * gFlowControlFudgeFactor.load();
LOG(DEBUG_LOG_LEVEL) << "Sustainer: " << sustainerAppliedCount << " LagMillis: " << lagMillis
<< " Threshold lag: " << thresholdLagMillis << " Exponent: " << exponent
<< " Reduce: " << reduce << " Penalty: " << sustainerAppliedPenalty;
return multiplyWithOverflowCheck(locksPerOp, sustainerAppliedPenalty, _kMaxTickets);
}
int FlowControl::getNumTickets() {
// Flow Control is only enabled on nodes that can accept writes.
const bool canAcceptWrites = _replCoord->canAcceptNonLocalWrites();
MONGO_FAIL_POINT_BLOCK(flowControlTicketOverride, failpointObj) {
int numTickets = failpointObj.getData().getIntField("numTickets");
if (numTickets > 0 && canAcceptWrites) {
return numTickets;
}
}
// Flow Control is only enabled if FCV is 4.2.
const bool isFCV42 =
(serverGlobalParams.featureCompatibility.isVersionInitialized() &&
serverGlobalParams.featureCompatibility.getVersion() ==
ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo42);
// It's important to update the topology on each iteration.
_updateTopologyData();
const repl::OpTimeAndWallTime myLastApplied = _replCoord->getMyLastAppliedOpTimeAndWallTime();
const repl::OpTimeAndWallTime lastCommitted = _replCoord->getLastCommittedOpTimeAndWallTime();
const double locksPerOp = _getLocksPerOp();
const std::int64_t locksUsedLastPeriod = _getLocksUsedLastPeriod();
if (serverGlobalParams.enableMajorityReadConcern == false ||
gFlowControlEnabled.load() == false || isFCV42 == false || canAcceptWrites == false ||
locksPerOp < 0.0) {
_trimSamples(std::min(lastCommitted.opTime.getTimestamp(),
getMedianAppliedTimestamp(_prevMemberData)));
return _kMaxTickets;
}
int ret = 0;
const auto thresholdLagMillis = getThresholdLagMillis();
const bool isHealthy =
getLagMillis(myLastApplied.wallTime, lastCommitted.wallTime) < thresholdLagMillis ||
// _approximateOpsBetween will return -1 if the input timestamps are in the same "bucket".
// This is an indication that there are very few ops between the two timestamps.
//
// Don't let the no-op writer on idle systems fool the sophisticated "is the replica set
// lagged" classifier.
_approximateOpsBetween(lastCommitted.opTime.getTimestamp(),
myLastApplied.opTime.getTimestamp()) == -1;
if (isHealthy) {
// The add/multiply technique is used to ensure ticket allocation can ramp up quickly,
// particularly if there were very few tickets to begin with.
ret = multiplyWithOverflowCheck(_lastTargetTicketsPermitted.load() +
gFlowControlTicketAdderConstant.load(),
gFlowControlTicketMultiplierConstant.load(),
_kMaxTickets);
_lastTimeSustainerAdvanced = Date_t::now();
if (_isLagged.load()) {
_isLagged.store(false);
auto waitTime = curTimeMicros64() - _startWaitTime;
_isLaggedTimeMicros.fetchAndAddRelaxed(waitTime);
}
} else if (sustainerAdvanced(_prevMemberData, _currMemberData)) {
// Expected case where flow control has meaningful data from the last period to make a new
// calculation.
ret =
_calculateNewTicketsForLag(_prevMemberData,
_currMemberData,
locksUsedLastPeriod,
locksPerOp,
getLagMillis(myLastApplied.wallTime, lastCommitted.wallTime),
thresholdLagMillis);
if (!_isLagged.load()) {
_isLagged.store(true);
_isLaggedCount.fetchAndAddRelaxed(1);
_startWaitTime = curTimeMicros64();
}
} else {
// Unexpected case where consecutive readings from the topology state don't meet some basic
// expectations.
ret = _lastTargetTicketsPermitted.load();
_lastTimeSustainerAdvanced = Date_t::now();
// Since this case does not give conclusive evidence that isLagged could have meaningfully
// transitioned from true to false, it does not make sense to update the _isLagged*
// variables here.
}
ret = std::max(ret, gFlowControlMinTicketsPerSecond.load());
LOG(DEBUG_LOG_LEVEL) << "Are lagged? " << (_isLagged.load() ? "true" : "false")
<< " Curr lag millis: "
<< getLagMillis(myLastApplied.wallTime, lastCommitted.wallTime)
<< " OpsLagged: "
<< _approximateOpsBetween(lastCommitted.opTime.getTimestamp(),
myLastApplied.opTime.getTimestamp())
<< " Granting: " << ret
<< " Last granted: " << _lastTargetTicketsPermitted.load()
<< " Last sustainer applied: " << _lastSustainerAppliedCount.load()
<< " Acquisitions since last check: " << locksUsedLastPeriod
<< " Locks per op: " << _lastLocksPerOp.load()
<< " Count of lagged periods: " << _isLaggedCount.load()
<< " Total duration of lagged periods: " << _isLaggedTimeMicros.load();
_lastTargetTicketsPermitted.store(ret);
_trimSamples(
std::min(lastCommitted.opTime.getTimestamp(), getMedianAppliedTimestamp(_prevMemberData)));
return ret;
}
std::int64_t FlowControl::_approximateOpsBetween(Timestamp prevTs, Timestamp currTs) {
std::int64_t prevApplied = -1;
std::int64_t currApplied = -1;
stdx::lock_guard<stdx::mutex> lk(_sampledOpsMutex);
for (auto&& sample : _sampledOpsApplied) {
if (prevApplied == -1 && prevTs.asULL() <= std::get<0>(sample)) {
prevApplied = std::get<1>(sample);
}
if (currApplied == -1 && currTs.asULL() <= std::get<0>(sample)) {
currApplied = std::get<1>(sample);
break;
}
}
if (prevApplied != -1 && currApplied == -1) {
currApplied = std::get<1>(_sampledOpsApplied[_sampledOpsApplied.size() - 1]);
}
if (prevApplied != -1 && currApplied != -1) {
return currApplied - prevApplied;
}
return -1;
}
void FlowControl::sample(Timestamp timestamp, std::uint64_t opsApplied) {
if (serverGlobalParams.enableMajorityReadConcern == false) {
return;
}
stdx::lock_guard<stdx::mutex> lk(_sampledOpsMutex);
_numOpsSinceStartup += opsApplied;
if (_numOpsSinceStartup - _lastSample <
static_cast<std::size_t>(gFlowControlSamplePeriod.load())) {
// Naively sample once every 1000 or so operations.
return;
}
if (_sampledOpsApplied.size() > 0 &&
static_cast<std::uint64_t>(timestamp.asULL()) <= std::get<0>(_sampledOpsApplied.back())) {
// The optime generator mutex is no longer held, these timestamps can come in out of order.
return;
}
SingleThreadedLockStats stats;
reportGlobalLockingStats(&stats);
_lastSample = _numOpsSinceStartup;
const auto lockAcquisitions = stats.get(resourceIdGlobal, LockMode::MODE_IX).numAcquisitions;
LOG(DEBUG_LOG_LEVEL) << "Sampling. Time: " << timestamp << " Applied: " << _numOpsSinceStartup
<< " LockAcquisitions: " << lockAcquisitions;
if (_sampledOpsApplied.size() <
static_cast<std::deque<Sample>::size_type>(gFlowControlMaxSamples)) {
_sampledOpsApplied.emplace_back(
static_cast<std::uint64_t>(timestamp.asULL()), _numOpsSinceStartup, lockAcquisitions);
} else {
// At ~24 bytes per sample, 1 million samples is ~24MB of memory. Instead of growing
// proportionally to replication lag, FlowControl opts to lose resolution (the number of
// operations between recorded samples increases). Hitting the sample limit implies there's
// replication lag. When there's replication lag, the oldest values are actively being used
// to compute the number of tickets to allocate. FlowControl intentionally prioritizes the
// oldest entries as those are, by definition, the most valuable when there is lag. Instead,
// we choose to lose resolution at the newest value.
_sampledOpsApplied[_sampledOpsApplied.size() - 1] = {
static_cast<std::uint64_t>(timestamp.asULL()), _numOpsSinceStartup, lockAcquisitions};
}
}
void FlowControl::_trimSamples(const Timestamp trimTo) {
int numTrimmed = 0;
stdx::lock_guard<stdx::mutex> lk(_sampledOpsMutex);
// Always leave at least two samples for calculating `locksPerOp`.
while (_sampledOpsApplied.size() > 2 &&
std::get<0>(_sampledOpsApplied.front()) < trimTo.asULL()) {
_sampledOpsApplied.pop_front();
++numTrimmed;
}
LOG(DEBUG_LOG_LEVEL) << "Trimmed samples. Num: " << numTrimmed;
}
int64_t FlowControl::_getLocksUsedLastPeriod() {
SingleThreadedLockStats stats;
reportGlobalLockingStats(&stats);
int64_t counter = stats.get(resourceIdGlobal, LockMode::MODE_IX).numAcquisitions;
int64_t ret = counter - _lastPollLockAcquisitions;
_lastPollLockAcquisitions = counter;
return ret;
}
} // namespace mongo
|