[RocksDB] move stats counting outside of mutex protected region for DB::Get()

Summary:
As title. This is possible as tickers are atomic now.
db_bench on high qps in-memory muti-thread random get workload, showed ~5% throughput improvement.

Test Plan: make check; db_bench; db_stress

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12555

db/db_impl.cc

@@ -2209,7 +2209,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
 
   StopWatch sw(env_, options_.statistics, DB_GET);
   SequenceNumber snapshot;
-  MutexLock l(&mutex_);
+  mutex_.Lock();
   if (options.snapshot != nullptr) {
     snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
   } else {
@@ -2254,6 +2254,9 @@ Status DBImpl::GetImpl(const ReadOptions& options,
   mem->Unref();
   imm.UnrefAll();
   current->Unref();
+  mutex_.Unlock();
+
+  // Note, tickers are atomic now - no lock protection needed any more.
   RecordTick(options_.statistics, NUMBER_KEYS_READ);
   RecordTick(options_.statistics, BYTES_READ, value->size());
   return s;
@@ -2265,7 +2268,7 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
 
   StopWatch sw(env_, options_.statistics, DB_MULTIGET);
   SequenceNumber snapshot;
-  MutexLock l(&mutex_);
+  mutex_.Lock();
   if (options.snapshot != nullptr) {
     snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
   } else {
@@ -2329,6 +2332,8 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
   mem->Unref();
   imm.UnrefAll();
   current->Unref();
+  mutex_.Unlock();
+
   RecordTick(options_.statistics, NUMBER_MULTIGET_CALLS);
   RecordTick(options_.statistics, NUMBER_MULTIGET_KEYS_READ, numKeys);
   RecordTick(options_.statistics, NUMBER_MULTIGET_BYTES_READ, bytesRead);
@@ -2413,22 +2418,28 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
   uint64_t last_sequence = versions_->LastSequence();
   Writer* last_writer = &w;
   if (status.ok() && my_batch != nullptr) {  // nullptr batch is for compactions
+    // TODO: BuildBatchGroup physically concatenate/copy all write batches into
+    // a new one. Mem copy is done with the lock held. Ideally, we only need
+    // the lock to obtain the last_writer and the references to all batches.
+    // Creation (copy) of the merged batch could have been done outside of the
+    // lock protected region.
     WriteBatch* updates = BuildBatchGroup(&last_writer);
-    const SequenceNumber current_sequence = last_sequence + 1;
-    WriteBatchInternal::SetSequence(updates, current_sequence);
-    int my_batch_count = WriteBatchInternal::Count(updates);
-    last_sequence += my_batch_count;
-    // Record statistics
-    RecordTick(options_.statistics, NUMBER_KEYS_WRITTEN, my_batch_count);
-    RecordTick(options_.statistics,
-               BYTES_WRITTEN,
-               WriteBatchInternal::ByteSize(updates));
+
     // Add to log and apply to memtable.  We can release the lock
     // during this phase since &w is currently responsible for logging
     // and protects against concurrent loggers and concurrent writes
     // into mem_.
     {
       mutex_.Unlock();
+      const SequenceNumber current_sequence = last_sequence + 1;
+      WriteBatchInternal::SetSequence(updates, current_sequence);
+      int my_batch_count = WriteBatchInternal::Count(updates);
+      last_sequence += my_batch_count;
+      // Record statistics
+      RecordTick(options_.statistics, NUMBER_KEYS_WRITTEN, my_batch_count);
+      RecordTick(options_.statistics,
+                 BYTES_WRITTEN,
+                 WriteBatchInternal::ByteSize(updates));
       if (options.disableWAL) {
         flush_on_destroy_ = true;
       }