Change StopWatch interface

Summary: So that we can avoid calling NowSecs() in MakeRoomForWrite twice

Test Plan: make all check

Reviewers: yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D20529

db/db_impl.cc

@@ -4156,9 +4156,8 @@ Status DBImpl::MakeRoomForWrite(
       mutex_.Unlock();
       uint64_t delayed;
       {
-        StopWatch sw(env_, stats_, STALL_L0_SLOWDOWN_COUNT, true);
+        StopWatch sw(env_, stats_, STALL_L0_SLOWDOWN_COUNT, &delayed);
         env_->SleepForMicroseconds(slowdown);
-        delayed = sw.ElapsedMicros();
       }
       RecordTick(stats_, STALL_L0_SLOWDOWN_MICROS, delayed);
       allow_delay = false;  // Do not delay a single write more than once
@@ -4184,13 +4183,12 @@ Status DBImpl::MakeRoomForWrite(
       }
       uint64_t stall;
       {
-        StopWatch sw(env_, stats_, STALL_MEMTABLE_COMPACTION_COUNT, true);
+        StopWatch sw(env_, stats_, STALL_MEMTABLE_COMPACTION_COUNT, &stall);
         if (!has_timeout) {
           bg_cv_.Wait();
         } else {
           bg_cv_.TimedWait(expiration_time);
         }
-        stall = sw.ElapsedMicros();
       }
       RecordTick(stats_, STALL_MEMTABLE_COMPACTION_MICROS, stall);
       cfd->internal_stats()->AddCFStats(
@@ -4201,13 +4199,12 @@ Status DBImpl::MakeRoomForWrite(
           cfd->GetName().c_str());
       uint64_t stall;
       {
-        StopWatch sw(env_, stats_, STALL_L0_NUM_FILES_COUNT, true);
+        StopWatch sw(env_, stats_, STALL_L0_NUM_FILES_COUNT, &stall);
         if (!has_timeout) {
           bg_cv_.Wait();
         } else {
           bg_cv_.TimedWait(expiration_time);
         }
-        stall = sw.ElapsedMicros();
       }
       RecordTick(stats_, STALL_L0_NUM_FILES_MICROS, stall);
       cfd->internal_stats()->AddCFStats(
@@ -4219,9 +4216,8 @@ Status DBImpl::MakeRoomForWrite(
       mutex_.Unlock();
       uint64_t delayed;
       {
-        StopWatch sw(env_, stats_, HARD_RATE_LIMIT_DELAY_COUNT, true);
+        StopWatch sw(env_, stats_, HARD_RATE_LIMIT_DELAY_COUNT, &delayed);
         env_->SleepForMicroseconds(1000);
-        delayed = sw.ElapsedMicros();
       }
       // Make sure the following value doesn't round to zero.
       uint64_t rate_limit = std::max((delayed / 1000), (uint64_t) 1);
@@ -4241,16 +4237,16 @@ Status DBImpl::MakeRoomForWrite(
       // TODO: add statistics
       uint64_t slowdown = SlowdownAmount(score, cfd->options()->soft_rate_limit,
                                          cfd->options()->hard_rate_limit);
+      uint64_t elapsed = 0;
       mutex_.Unlock();
       {
-        StopWatch sw(env_, stats_, SOFT_RATE_LIMIT_DELAY_COUNT, true);
+        StopWatch sw(env_, stats_, SOFT_RATE_LIMIT_DELAY_COUNT, &elapsed);
         env_->SleepForMicroseconds(slowdown);
-        slowdown = sw.ElapsedMicros();
         rate_limit_delay_millis += slowdown;
       }
       allow_soft_rate_limit_delay = false;
       mutex_.Lock();
-      cfd->internal_stats()->RecordLevelNSlowdown(max_level, slowdown, true);
+      cfd->internal_stats()->RecordLevelNSlowdown(max_level, elapsed, true);
     } else {
       unique_ptr<WritableFile> lfile;
       log::Writer* new_log = nullptr;

util/stop_watch.h

@@ -9,25 +9,31 @@
 
 namespace rocksdb {
 // Auto-scoped.
-// Records the statistic into the corresponding histogram.
+// Records the measure time into the corresponding histogram if statistics
+// is not nullptr. It is also saved into *elapsed if the pointer is not nullptr.
 class StopWatch {
  public:
   StopWatch(Env * const env, Statistics* statistics,
-            const uint32_t hist_type, bool force_enable = false)
+            const uint32_t hist_type,
+            uint64_t* elapsed = nullptr)
     : env_(env),
       statistics_(statistics),
       hist_type_(hist_type),
-      enabled_(statistics && statistics->HistEnabledForType(hist_type)),
-      start_time_(enabled_ || force_enable ? env->NowMicros() : 0) {
+      elapsed_(elapsed),
+      stats_enabled_(statistics && statistics->HistEnabledForType(hist_type)),
+      start_time_((stats_enabled_ || elapsed != nullptr) ?
+                  env->NowMicros() : 0) {
   }
 
-  uint64_t ElapsedMicros() const {
-    return env_->NowMicros() - start_time_;
-  }
 
   ~StopWatch() {
-    if (enabled_) {
-      statistics_->measureTime(hist_type_, ElapsedMicros());
+    if (elapsed_) {
+      *elapsed_ = env_->NowMicros() - start_time_;
+    }
+    if (stats_enabled_) {
+      statistics_->measureTime(hist_type_,
+          (elapsed_ != nullptr) ? *elapsed_ :
+                                  (env_->NowMicros() - start_time_));
     }
   }
 
@@ -35,7 +41,8 @@ class StopWatch {
   Env* const env_;
   Statistics* statistics_;
   const uint32_t hist_type_;
-  bool enabled_;
+  uint64_t* elapsed_;
+  bool stats_enabled_;
   const uint64_t start_time_;
 };