[RocksDB] Minimize Mutex protected code section in the critical path

Summary: rocksdb uses a single global lock to protect in memory metadata. We should minimize the mutex protected code section to increase the effective parallelism of the program. See https://our.intern.facebook.com/intern/tasks/?t=2218928

Test Plan:
make check
db_bench

Reviewers: dhruba, heyongqiang

CC: zshao, leveldb

Differential Revision: https://reviews.facebook.net/D9705

db/db_impl.cc

@@ -1876,11 +1876,12 @@ Status DBImpl::Get(const ReadOptions& options,
                    const Slice& key,
                    std::string* value) {
   Status s;
-  MutexLock l(&mutex_);
+
   std::unique_ptr<StopWatch> sw = stats::StartStopWatch(env_,
                                                         options_.statistics,
                                                         DB_GET);
   SequenceNumber snapshot;
+  MutexLock l(&mutex_);
   if (options.snapshot != nullptr) {
     snapshot = reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_;
   } else {
@@ -1894,12 +1895,11 @@ Status DBImpl::Get(const ReadOptions& options,
   imm.RefAll();
   current->Ref();
 
-  bool have_stat_update = false;
-  Version::GetStats stats;
-
   // Unlock while reading from files and memtables
-  {
+
   mutex_.Unlock();
+  bool have_stat_update = false;
+  Version::GetStats stats;
   // First look in the memtable, then in the immutable memtable (if any).
   LookupKey lkey(key, snapshot);
   if (mem->Get(lkey, value, &s)) {
@@ -1911,7 +1911,6 @@ Status DBImpl::Get(const ReadOptions& options,
     have_stat_update = true;
   }
   mutex_.Lock();
-  }
 
   if (!options_.disable_seek_compaction &&
       have_stat_update && current->UpdateStats(stats)) {
@@ -1961,10 +1960,10 @@ Status DBImpl::Write(const WriteOptions& options, WriteBatch* my_batch) {
   w.disableWAL = options.disableWAL;
   w.done = false;
 
-  MutexLock l(&mutex_);
   std::unique_ptr<StopWatch> sw = stats::StartStopWatch(env_,
                                                         options_.statistics,
                                                         DB_WRITE);
+  MutexLock l(&mutex_);
   writers_.push_back(&w);
   while (!w.done && &w != writers_.front()) {
     w.cv.Wait();