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Avoid acquiring SyncPoint mutex when it is disabled (#3991) 

Summary:
In `db_stress` profile the vast majority of CPU time is spent acquiring the `SyncPoint` mutex. I mistakenly assumed #3939 had fixed this mutex contention problem by disabling `SyncPoint` processing. But actually the lock was still being acquired just to check whether processing is enabled. We can avoid that overhead by using an atomic to track whether it's enabled.
Closes https://github.com/facebook/rocksdb/pull/3991

Differential Revision: D8393825

Pulled By: ajkr

fbshipit-source-id: 5bc4e3c722ee7304e7a9c2439998c456b05a6897
This commit is contained in:
Andrew Kryczka 2018-06-13 13:03:09 -07:00 committed by Facebook Github Bot
parent d82f1421b4
commit a720401877
2 changed files with 4 additions and 4 deletions
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2
util/sync_point_impl.cc
View File

@ -89,11 +89,11 @@ void SyncPoint::Data::ClearAllCallBacks() {
}
void SyncPoint::Data::Process(const std::string& point, void* cb_arg) {
std::unique_lock<std::mutex> lock(mutex_);
if (!enabled_) {
return;
}
std::unique_lock<std::mutex> lock(mutex_);
auto thread_id = std::this_thread::get_id();
auto marker_iter = markers_.find(point);

6
util/sync_point_impl.h
View File

@ -6,6 +6,7 @@
#include "util/sync_point.h"
#include <assert.h>
#include <atomic>
#include <condition_variable>
#include <functional>
#include <mutex>
@ -22,6 +23,7 @@
#ifndef NDEBUG
namespace rocksdb {
struct SyncPoint::Data {
Data() : enabled_(false) {}
// Enable proper deletion by subclasses
virtual ~Data() {}
// successor/predecessor map loaded from LoadDependency
@ -35,7 +37,7 @@ struct SyncPoint::Data {
std::condition_variable cv_;
// sync points that have been passed through
std::unordered_set<std::string> cleared_points_;
bool enabled_ = false;
std::atomic<bool> enabled_;
int num_callbacks_running_ = 0;
void LoadDependency(const std::vector<SyncPointPair>& dependencies);
@ -51,11 +53,9 @@ struct SyncPoint::Data {
void ClearCallBack(const std::string& point);
void ClearAllCallBacks();
void EnableProcessing() {
std::lock_guard<std::mutex> lock(mutex_);
enabled_ = true;
}
void DisableProcessing() {
std::lock_guard<std::mutex> lock(mutex_);
enabled_ = false;
}
void ClearTrace() {