Remove wasteful instrumentation in FullMerge (stacked on D59577)

Summary:
[ This diff is stacked on top of D59577 ]

We keep calling timer.ElapsedNanos() on every call to MergeOperator::FullMerge even when statistics are disabled, this is wasteful.

I run the readseq benchmark on a DB containing 100K merge operands for 100K keys (1 operand per key) with 1GB block cache
I see slight performance improvment

Original results

```
$ ./db_bench --benchmarks="readseq,readseq,readseq,readseq,readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --merge_keys=100000 --num=100000 --db="/dev/shm/100K_merge_compacted/" --cache_size=1073741824 --use_existing_db --disable_auto_compactions
------------------------------------------------
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.498 micros/op 2006597 ops/sec;  222.0 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.295 micros/op 3393627 ops/sec;  375.4 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.285 micros/op 3511155 ops/sec;  388.4 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.286 micros/op 3500470 ops/sec;  387.2 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.283 micros/op 3530751 ops/sec;  390.6 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.289 micros/op 3464811 ops/sec;  383.3 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.277 micros/op 3612814 ops/sec;  399.7 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.283 micros/op 3539640 ops/sec;  391.6 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.285 micros/op 3503766 ops/sec;  387.6 MB/s
```

After patch

```
$ ./db_bench --benchmarks="readseq,readseq,readseq,readseq,readseq,readseq,readseq,readseq,readseq" --merge_operator="max" --merge_keys=100000 --num=100000 --db="/dev/shm/100K_merge_compacted/" --cache_size=1073741824 --use_existing_db --disable_auto_compactions
------------------------------------------------
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.476 micros/op 2100119 ops/sec;  232.3 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.278 micros/op 3600887 ops/sec;  398.4 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.275 micros/op 3636698 ops/sec;  402.3 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.271 micros/op 3691661 ops/sec;  408.4 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.273 micros/op 3661534 ops/sec;  405.1 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.276 micros/op 3627106 ops/sec;  401.3 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.272 micros/op 3682635 ops/sec;  407.4 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.266 micros/op 3758331 ops/sec;  415.8 MB/s
DB path: [/dev/shm/100K_merge_compacted/]
readseq      :       0.266 micros/op 3761907 ops/sec;  416.2 MB/s
```

Test Plan: make check -j64

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D59583

1 files changed, 3 insertions, 2 deletions

diff --git a/db/merge_helper.cc b/db/merge_helper.cc
index f87b5ee37..2ba22d977 100644
--- a/db/merge_helper.cc
+++ b/db/merge_helper.cc
@@ -39,7 +39,8 @@ Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
     // Do the merge
     success = merge_operator->FullMerge(key, value, operands, result, logger);
 
-    RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME, timer.ElapsedNanos());
+    RecordTick(statistics, MERGE_OPERATION_TOTAL_TIME,
+               statistics ? timer.ElapsedNanos() : 0);
   }
 
   if (!success) {
@@ -254,7 +255,7 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
             std::deque<Slice>(operands_.begin(), operands_.end()),
             &merge_result, logger_);
         RecordTick(stats_, MERGE_OPERATION_TOTAL_TIME,
-                   timer.ElapsedNanosSafe());
+                   stats_ ? timer.ElapsedNanosSafe() : 0);
       }
       if (merge_success) {
         // Merging of operands (associative merge) was successful.