Allow users to profile a query and see bottleneck of the query

Summary:
Provide a framework to profile a query in detail to figure out latency bottleneck. Currently, in Get(), Put() and iterators, 2-3 simple timing is used. We can easily add more profile counters to the framework later.

Test Plan: Enable this profiling in seveal existing tests.

Reviewers: haobo, dhruba, kailiu, emayanke, vamsi, igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14001

Conflicts:
	table/merger.cc

db/db_bench.cc

@@ -22,6 +22,7 @@
 #include "rocksdb/memtablerep.h"
 #include "rocksdb/write_batch.h"
 #include "rocksdb/statistics.h"
+#include "rocksdb/perf_context.h"
 #include "port/port.h"
 #include "util/bit_set.h"
 #include "util/crc32c.h"
@@ -350,6 +351,8 @@ DEFINE_int64(stats_interval, 0, "Stats are reported every N operations when "
 DEFINE_int32(stats_per_interval, 0, "Reports additional stats per interval when"
              " this is greater than 0.");
 
+DEFINE_int32(perf_level, 0, "Level of perf collection");
+
 static bool ValidateRateLimit(const char* flagname, double value) {
   static constexpr double EPSILON = 1e-10;
   if ( value < -EPSILON ) {
@@ -689,6 +692,7 @@ struct SharedState {
   port::Mutex mu;
   port::CondVar cv;
   int total;
+  int perf_level;
 
   // Each thread goes through the following states:
   //    (1) initializing
@@ -700,7 +704,7 @@ struct SharedState {
   long num_done;
   bool start;
 
-  SharedState() : cv(&mu) { }
+  SharedState() : cv(&mu), perf_level(FLAGS_perf_level) { }
 };
 
 // Per-thread state for concurrent executions of the same benchmark.
@@ -810,6 +814,7 @@ class Benchmark {
         fprintf(stdout, "Memtablerep: vector\n");
         break;
     }
+    fprintf(stdout, "Perf Level: %d\n", FLAGS_perf_level);
 
     PrintWarnings();
     fprintf(stdout, "------------------------------------------------\n");
@@ -1150,6 +1155,7 @@ class Benchmark {
       }
     }
 
+    SetPerfLevel(static_cast<PerfLevel> (shared->perf_level));
     thread->stats.Start(thread->tid);
     (arg->bm->*(arg->method))(thread);
     thread->stats.Stop();