StopWatch not to get time if it is created for statistics and it is disabled

Summary: Currently, even if statistics is not enabled, StopWatch only for the stats still gets the time of the day, which is wasteful. This patch adds a new option to StopWatch to disable this get in this case.

Test Plan: make all check

Reviewers: dhruba, haobo, igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14703

Conflicts:
	db/db_impl.cc

db/db_impl.cc

@@ -2080,11 +2080,11 @@ Status DBImpl::FinishCompactionOutputFile(CompactionState* compact,
   if (s.ok() && !options_.disableDataSync) {
     if (options_.use_fsync) {
       StopWatch sw(env_, options_.statistics.get(),
-                   COMPACTION_OUTFILE_SYNC_MICROS);
+                   COMPACTION_OUTFILE_SYNC_MICROS, false);
       s = compact->outfile->Fsync();
     } else {
       StopWatch sw(env_, options_.statistics.get(),
-                   COMPACTION_OUTFILE_SYNC_MICROS);
+                   COMPACTION_OUTFILE_SYNC_MICROS, false);
       s = compact->outfile->Sync();
     }
   }
@@ -2717,7 +2717,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
                        bool* value_found) {
   Status s;
 
-  StopWatch sw(env_, options_.statistics.get(), DB_GET);
+  StopWatch sw(env_, options_.statistics.get(), DB_GET, false);
   StopWatchNano snapshot_timer(env_, false);
   StartPerfTimer(&snapshot_timer);
   SequenceNumber snapshot;
@@ -2798,7 +2798,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
 std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
                                      const std::vector<Slice>& keys,
                                      std::vector<std::string>* values) {
-  StopWatch sw(env_, options_.statistics.get(), DB_MULTIGET);
+  StopWatch sw(env_, options_.statistics.get(), DB_MULTIGET, false);
   StopWatchNano snapshot_timer(env_, false);
   StartPerfTimer(&snapshot_timer);
 
@@ -2958,7 +2958,7 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
   w.disableWAL = options.disableWAL;
   w.done = false;
 
-  StopWatch sw(env_, options_.statistics.get(), DB_WRITE);
+  StopWatch sw(env_, options_.statistics.get(), DB_WRITE, false);
   mutex_.Lock();
   writers_.push_back(&w);
   while (!w.done && &w != writers_.front()) {

util/stop_watch.h

@@ -15,9 +15,10 @@ class StopWatch {
   explicit StopWatch(
     Env * const env,
     Statistics* statistics = nullptr,
-    const Histograms histogram_name = DB_GET) :
+    const Histograms histogram_name = DB_GET,
+    bool auto_start = true) :
       env_(env),
-      start_time_(env->NowMicros()),
+      start_time_((!auto_start && !statistics) ? 0 : env->NowMicros()),
       statistics_(statistics),
       histogram_name_(histogram_name) {}