Instrument memtable seeks

Summary: As title

Test Plan: compiles

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D34191

db/memtable.cc

@@ -223,6 +223,8 @@ class MemTableIterator: public Iterator {
 
   virtual bool Valid() const override { return valid_; }
   virtual void Seek(const Slice& k) override {
+    PERF_TIMER_GUARD(seek_on_memtable_time);
+    PERF_COUNTER_ADD(seek_on_memtable_count, 1);
     if (bloom_ != nullptr &&
         !bloom_->MayContain(prefix_extractor_->Transform(ExtractUserKey(k)))) {
       valid_ = false;

include/rocksdb/perf_context.h

@@ -51,6 +51,10 @@ struct PerfContext {
   // total time spent after Get() finds a key
   uint64_t get_post_process_time;
   uint64_t get_from_output_files_time; // total time reading from output files
+  // total time spent on seeking memtable
+  uint64_t seek_on_memtable_time;
+  // number of seeks issued on memtable
+  uint64_t seek_on_memtable_count;
   // total time spent on seeking child iters
   uint64_t seek_child_seek_time;
   // number of seek issued in child iterators

util/perf_context.cc

@@ -44,6 +44,8 @@ void PerfContext::Reset() {
   get_from_memtable_count = 0;
   get_post_process_time = 0;
   get_from_output_files_time = 0;
+  seek_on_memtable_time = 0;
+  seek_on_memtable_count = 0;
   seek_child_seek_time = 0;
   seek_child_seek_count = 0;
   seek_min_heap_time = 0;
@@ -78,6 +80,8 @@ std::string PerfContext::ToString() const {
      << OUTPUT(get_from_memtable_count)
      << OUTPUT(get_post_process_time)
      << OUTPUT(get_from_output_files_time)
+     << OUTPUT(seek_on_memtable_time)
+     << OUTPUT(seek_on_memtable_count)
      << OUTPUT(seek_child_seek_time)
      << OUTPUT(seek_child_seek_count)
      << OUTPUT(seek_min_heap_time)