[RocksDB] Simplify StopWatch implementation

Summary:
Make stop watch a simple implementation, instead of subclass of a virtual class
Allocate stop watches off the stack instead of heap.
Code is more terse now.

Test Plan: make all check, db_bench with --statistics=1

Reviewers: haobo, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10809

db/db_impl.cc

@@ -1930,9 +1930,7 @@ Status DBImpl::Get(const ReadOptions& options,
                    std::string* value) {
   Status s;
 
-  std::unique_ptr<StopWatch> sw = stats::StartStopWatch(env_,
-                                                        options_.statistics,
-                                                        DB_GET);
+  StopWatch sw(env_, options_.statistics, DB_GET);
   SequenceNumber snapshot;
   MutexLock l(&mutex_);
   if (options.snapshot != nullptr) {
@@ -2025,9 +2023,7 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
   w.disableWAL = options.disableWAL;
   w.done = false;
 
-  std::unique_ptr<StopWatch> sw = stats::StartStopWatch(env_,
-                                                        options_.statistics,
-                                                        DB_WRITE);
+  StopWatch sw(env_, options_.statistics, DB_WRITE);
   MutexLock l(&mutex_);
   writers_.push_back(&w);
   while (!w.done && &w != writers_.front()) {

util/stop_watch.h

@@ -3,41 +3,31 @@
 
 #include "leveldb/env.h"
 #include "leveldb/statistics.h"
-#include <iostream>
-namespace leveldb {
 
+namespace leveldb {
+// Auto-scoped.
+// Records the statistic into the corresponding histogram.
 class StopWatch {
  public:
-  virtual uint64_t ElapsedMicros() = 0;
-  virtual ~StopWatch() {}
-};
+  StopWatch(
+    Env * const env,
+    std::shared_ptr<Statistics> statistics,
+    const Histograms histogram_name) :
+      env_(env),
+      start_time_(env->NowMicros()),
+      statistics_(statistics),
+      histogram_name_(histogram_name) {}
 
-class DoNothingStopWatch : public StopWatch {
- public:
-  virtual uint64_t ElapsedMicros() {
-    return 0;
-  }
-};
 
-// Auto-scoped.
-// Records the statistic into the corresponding histogram.
-class ScopedRecordingStopWatch : public StopWatch {
- public:
-  ScopedRecordingStopWatch(Env * const env,
-                           std::shared_ptr<Statistics> statistics,
-                           const Histograms histogram_name) :
-                            env_(env),
-                            start_time_(env->NowMicros()),
-                            statistics_(statistics),
-                            histogram_name_(histogram_name) {}
 
-  virtual uint64_t ElapsedMicros() {
+  uint64_t ElapsedMicros() {
     return env_->NowMicros() - start_time_;
   }
 
-  virtual ~ScopedRecordingStopWatch() {
-    uint64_t elapsed_time = env_->NowMicros() - start_time_;
-    statistics_->measureTime(histogram_name_, elapsed_time);
+  ~StopWatch() {
+    if (statistics_) {
+      statistics_->measureTime(histogram_name_, ElapsedMicros());
+    }
   }
 
  private:
@@ -47,22 +37,5 @@ class ScopedRecordingStopWatch : public StopWatch {
   const Histograms histogram_name_;
 
 };
-
-namespace stats {
-// Helper method
-std::unique_ptr<StopWatch> StartStopWatch(Env * const env,
-                                          std::shared_ptr<Statistics> stats,
-                                          Histograms histogram_name) {
-  assert(env);
-  if (stats) {
-    return std::unique_ptr<StopWatch>(new ScopedRecordingStopWatch(
-                                           env,
-                                           stats,
-                                           histogram_name));
-  } else {
-    return std::unique_ptr<StopWatch>(new DoNothingStopWatch());
-  }
-};
-} // namespace stats
 } // namespace leveldb
 #endif // STORAGE_LEVELDB_UTIL_STOP_WATCH_H_