一、MongoDB 慢日志基础配置：Profiling 操作指南
mongodb对慢日志怎么做的呢？一般教程会这样指导：
1.1 查看当前 Profiling 状态
cpp
> use admin
switched to db admin
> db.getProfilingStatus();
{ "was" : 0, "slowms" : 100, "sampleRate" : 1 }1.2 MongoDB Profiling 核心级别（0/1/2 级）说明
MongoDB 的 Profiling 有was三个级别：0 ：关闭 Profiling（默认）；1 ：记录执行时间超过 slowms 阈值的操作；2：记录所有操作（谨慎使用，可能产生大量日志）。
1.3 启动参数配置 Profiling
启动脚本增加--slowms 50 --profile 1参数
bash
>mongod --dbpath C:\me\studyhome\mongodb\mongo-r4.0.7\data -vvvvv --slowms 50 --profile 11.4 Profiling 配置后的验证方法
再次验证Profiling 级别
二、MongoDB 慢日志底层实现机制拆解
2.1 会话状态流转与命令传递流程
mongo/transport/service_state_machine.cpp已经分析startSession创建ASIOSession过程，并且验证connection是否超过限制ASIOSession和connection是循环接受客户端命令，状态转变流程是：State::Created 》 State::Source 》State::SourceWait 》 State::Process 》 State::SinkWait 》 State::Source 》State::SourceWait。
State::Process将命令发送给mongodb数据库，数据库执行命令find。
2.2 handleRequest 核心方法执行逻辑（service_entry_point_common.cpp）
session接受到客户端发送的命令Msg，把接受到的Msg信息发送给mongodb数据库执行DbResponse dbresponse = ServiceEntryPointMongod->handleRequest(）
/mongo/db/service_entry_point_common.cpp中的handleRequest方法代码：
cpp
DbResponse ServiceEntryPointCommon::handleRequest(OperationContext* opCtx,
const Message& m,
const Hooks& behaviors) {
...
CurOp& currentOp = *CurOp::get(opCtx);
{
stdx::lock_guard<Client> lk(*opCtx->getClient());
// Commands handling code will reset this if the operation is a command
// which is logically a basic CRUD operation like query, insert, etc.
currentOp.setNetworkOp_inlock(op);
currentOp.setLogicalOp_inlock(networkOpToLogicalOp(op));
}
OpDebug& debug = currentOp.debug();
boost::optional<long long> slowMsOverride;
bool forceLog = false;
DbResponse dbresponse;
if (op == dbMsg || (op == dbQuery && isCommand)) {
dbresponse = receivedCommands(opCtx, m, behaviors);
} else if (op == dbQuery) {
invariant(!isCommand);
opCtx->markKillOnClientDisconnect();
dbresponse = receivedQuery(opCtx, nsString, c, m, behaviors);
} else if (op == dbGetMore) {
dbresponse = receivedGetMore(opCtx, m, currentOp, &forceLog);
} else {
...
if (LastError::get(opCtx->getClient()).hadNotMasterError()) {
notMasterLegacyUnackWrites.increment();
uasserted(ErrorCodes::NotMaster,
str::stream()
<< "Not-master error while processing '" << networkOpToString(op)
<< "' operation on '" << nsString << "' namespace via legacy "
<< "fire-and-forget command execution.");
}
}
const bool shouldSample = currentOp.completeAndLogOperation(
opCtx, MONGO_LOG_DEFAULT_COMPONENT, dbresponse.response.size(), slowMsOverride, forceLog);
Top::get(opCtx->getServiceContext())
.incrementGlobalLatencyStats(
opCtx,
durationCount<Microseconds>(currentOp.elapsedTimeExcludingPauses()),
currentOp.getReadWriteType());
if (currentOp.shouldDBProfile(shouldSample)) {
// Performance profiling is on
if (opCtx->lockState()->isReadLocked()) {
LOG(1) << "note: not profiling because recursive read lock";
} else if (c.isInDirectClient()) {
LOG(1) << "note: not profiling because we are in DBDirectClient";
} else if (behaviors.lockedForWriting()) {
// TODO SERVER-26825: Fix race condition where fsyncLock is acquired post
// lockedForWriting() call but prior to profile collection lock acquisition.
LOG(1) << "note: not profiling because doing fsync+lock";
} else if (storageGlobalParams.readOnly) {
LOG(1) << "note: not profiling because server is read-only";
} else {
invariant(!opCtx->lockState()->inAWriteUnitOfWork());
profile(opCtx, op);
}
}
recordCurOpMetrics(opCtx);
return dbresponse;
}CurOp& currentOp = *CurOp::get(opCtx);记录开始时间，获取当前操作对应curop ；
dbresponse = receivedCommands(opCtx, m, behaviors);执行请求命令
const bool shouldSample = currentOp.completeAndLogOperation(opCtx, MONGO_LOG_DEFAULT_COMPONENT, dbresponse.response.size(), slowMsOverride, forceLog);处理时间差，根据参数判断是否需要记录日志。
profile(opCtx, op)将慢日志记录到db.system.profile中。
2.3 completeAndLogOperation 方法：耗时计算与日志判断
completeAndLogOperation核心方法如下：
cpp
bool CurOp::completeAndLogOperation(OperationContext* opCtx,
logger::LogComponent component,
boost::optional<size_t> responseLength,
boost::optional<long long> slowMsOverride,
bool forceLog) {
// Log the operation if it is eligible according to the current slowMS and sampleRate settings.
const bool shouldLogOp = (forceLog || shouldLog(component, logger::LogSeverity::Debug(1)));
const long long slowMs = slowMsOverride.value_or(serverGlobalParams.slowMS);
LOG(1) << "conca responseLength=" <<responseLength;
LOG(1) << "conca serverGlobalParams.slowMS=" <<serverGlobalParams.slowMS;
LOG(1) << "conca slowMsOverride=" <<slowMsOverride;
const auto client = opCtx->getClient();
// Record the size of the response returned to the client, if applicable.
if (responseLength) {
_debug.responseLength = *responseLength;
}
// Obtain the total execution time of this operation.
_end = curTimeMicros64();
_debug.executionTimeMicros = durationCount<Microseconds>(elapsedTimeExcludingPauses());
LOG(1) << "conca _debug.executionTimeMicros=" <<_debug.executionTimeMicros;
const bool shouldSample =
client->getPrng().nextCanonicalDouble() < serverGlobalParams.sampleRate;
LOG(1) << "conca serverGlobalParams.sampleRate=" <<serverGlobalParams.sampleRate;
LOG(1) << "conca shouldSample=" <<shouldSample;
if (shouldLogOp || (shouldSample && _debug.executionTimeMicros > slowMs * 1000LL)) {
auto lockerInfo = opCtx->lockState()->getLockerInfo(_lockStatsBase);
if (_debug.storageStats == nullptr && opCtx->lockState()->wasGlobalLockTaken() &&
opCtx->getServiceContext()->getStorageEngine()) {
、.
try {
Lock::GlobalLock lk(opCtx,
MODE_IS,
Date_t::now() + Milliseconds(500),
Lock::InterruptBehavior::kLeaveUnlocked);
if (lk.isLocked()) {
_debug.storageStats = opCtx->recoveryUnit()->getOperationStatistics();
} else {
warning(component) << "Unable to gather storage statistics for a slow "
"operation due to lock aquire timeout";
}
} catch (const ExceptionForCat<ErrorCategory::Interruption>&) {
warning(component) << "Unable to gather storage statistics for a slow "
"operation due to interrupt";
}
}
// Gets the time spent blocked on prepare conflicts.
auto prepareConflictDurationMicros =
PrepareConflictTracker::get(opCtx).getPrepareConflictDuration();
_debug.prepareConflictDurationMillis =
duration_cast<Milliseconds>(prepareConflictDurationMicros);
log(component) << _debug.report(opCtx, (lockerInfo ? &lockerInfo->stats : nullptr));
}
// Return 'true' if this operation should also be added to the profiler.
return shouldDBProfile(shouldSample);
}
bool shouldDBProfile(bool shouldSample = true) {
// Profile level 2 should override any sample rate or slowms settings.
if (_dbprofile >= 2)
return true;
if (!shouldSample || _dbprofile <= 0)
return false;
return elapsedTimeExcludingPauses() >= Milliseconds{serverGlobalParams.slowMS};
}serverGlobalParams.slowMS默认值是100，前面mongod启动脚本设置--slowms 50
serverGlobalParams.sampleRate默认值是1.0
2.4 shouldDBProfile：慢日志记录的核心判断逻辑
shouldDBProfile判断是否记录慢日志，profile>=2记录，profile=0不记录，profile=1 & slowms判断是否记录慢日志，前面mongod启动脚本设置--profile 1，所以这个find命令耗时超过50ms，记录到慢日志中。
/mongo/db/server_options.h源代码：
cpp
int defaultProfile = 0; // --profile
int slowMS = 100; // --time in ms that is "slow"
double sampleRate = 1.0; // --samplerate rate at which to sample slow queries
int defaultLocalThresholdMillis = 15; // --localThreshold in ms to consider a node local
bool moveParanoia = false; // for move chunk paranoia三、MongoDB 慢日志运行验证与结果分析
运行过程关键日志打印结果：
cpp
2025-05-14T21:18:29.292+0800 D1 - [conn1] conca responseLength=240
2025-05-14T21:18:29.292+0800 D1 - [conn1] conca serverGlobalParams.slowMS=50
2025-05-14T21:18:29.292+0800 D1 - [conn1] conca slowMsOverride=(nothing)
2025-05-14T21:18:29.292+0800 D1 - [conn1] conca _debug.executionTimeMicros=56413
2025-05-14T21:18:29.293+0800 D1 - [conn1] conca serverGlobalParams.sampleRate=1
2025-05-14T21:18:29.293+0800 D1 - [conn1] conca shouldSample=1
2025-05-14T21:18:29.293+0800 I COMMAND [conn1] command db.user appName: "MongoDB Shell" command: find { find: "user", filter: { age: 28.0 }, sort: { wages: 1.0 }, lsid: { id: UUID("7f72749d-bdaf-4b1c-9ad4-b15ab44b055c") }, $db: "db" } planSummary: IXSCAN { age: 1 } keysExamined:2 docsExamined:2 hasSortStage:1 fromMultiPlanner:1 cursorExhausted:1 numYields:3 nreturned:2 queryHash:A13CF6D8 planCacheKey:FD90BAA0 reslen:240 locks:{ ReplicationStateTransition: { acquireCount: { w: 4 } }, Global: { acquireCount: { r: 4 } }, Database: { acquireCount: { r: 4 } }, Collection: { acquireCount: { r: 4 } }, Mutex: { acquireCount: { r: 1 } } } storage:{ data: { bytesRead: 944, timeReadingMicros: 957 } } protocol:op_msg 56ms
2025-05-14T21:18:29.293+0800 I - [conn1] Creating profile collection: db.system.profile
2025-05-14T21:18:29.293+0800 I STORAGE [conn1] createCollection: db.system.profile with generated UUID: 05224cf7-a845-41b2-b71d-7e3d899c5757 and options: { capped: true, size: 1048576 }