MergeHelper::FilterMerge() calling ElapsedNanosSafe() upon exit even … (#7867)

Summary:
…when unused.  Causes many calls to clock_gettime, impacting performance.

Was looking for something else via Linux "perf" command when I spotted heavy usage of clock_gettime during a compaction.  Our product heavily uses the rocksdb::Options::merge_operator.  MergeHelper::FilterMerge() properly tests if timing is enabled/disabled upon entry, but not on exit.  This patch fixes the exit.

Note:  the entry test also verifies if "nullptr!=stats_".  This test is redundant to code within ShouldReportDetailedTime().  Therefore I omitted it in my change.

merge_test.cc updated with test that shows failure before merge_helper.cc change ... and fix after change.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7867

Reviewed By: jay-zhuang

Differential Revision: D25960175

Pulled By: ajkr

fbshipit-source-id: 56e66d7eb6ae5eae89c8e0d5a262bd2905a226b6

db/merge_helper.cc

@@ -183,7 +183,6 @@ Status MergeHelper::MergeUntil(InternalIterator* iter,
 
     assert(IsValueType(ikey.type));
     if (ikey.type != kTypeMerge) {
-
       // hit a put/delete/single delete
       //   => merge the put value or a nullptr with operands_
       //   => store result in operands_.back() (and update keys_.back())
@@ -414,7 +413,9 @@ CompactionFilter::Decision MergeHelper::FilterMerge(const Slice& user_key,
                                                        kValueTypeForSeek);
     }
   }
+  if (stats_ != nullptr && ShouldReportDetailedTime(env_, stats_)) {
     total_filter_time_ += filter_timer_.ElapsedNanosSafe();
+  }
   return ret;
 }
 

db/merge_test.cc

@@ -49,11 +49,7 @@ class CountMergeOperator : public AssociativeMergeOperator {
       return true;
     }
 
-    return mergeOperator_->PartialMerge(
-        key,
-        *existing_value,
-        value,
-        new_value,
+    return mergeOperator_->PartialMerge(key, *existing_value, value, new_value,
                                         logger);
   }
 
@@ -73,6 +69,29 @@ class CountMergeOperator : public AssociativeMergeOperator {
   std::shared_ptr<MergeOperator> mergeOperator_;
 };
 
+class EnvMergeTest : public EnvWrapper {
+ public:
+  EnvMergeTest() : EnvWrapper(Env::Default()) {}
+  //  ~EnvMergeTest() override {}
+
+  uint64_t NowNanos() override {
+    ++now_nanos_count_;
+    return target()->NowNanos();
+  }
+
+  static uint64_t now_nanos_count_;
+
+  static std::unique_ptr<EnvMergeTest> singleton_;
+
+  static EnvMergeTest* GetInstance() {
+    if (nullptr == singleton_) singleton_.reset(new EnvMergeTest);
+    return singleton_.get();
+  }
+};
+
+uint64_t EnvMergeTest::now_nanos_count_{0};
+std::unique_ptr<EnvMergeTest> EnvMergeTest::singleton_;
+
 std::shared_ptr<DB> OpenDb(const std::string& dbname, const bool ttl = false,
                            const size_t max_successive_merges = 0) {
   DB* db;
@@ -80,6 +99,7 @@ std::shared_ptr<DB> OpenDb(const std::string& dbname, const bool ttl = false,
   options.create_if_missing = true;
   options.merge_operator = std::make_shared<CountMergeOperator>();
   options.max_successive_merges = max_successive_merges;
+  options.env = EnvMergeTest::GetInstance();
   EXPECT_OK(DestroyDB(dbname, Options()));
   Status s;
 // DBWithTTL is not supported in ROCKSDB_LITE
@@ -106,7 +126,6 @@ std::shared_ptr<DB> OpenDb(const std::string& dbname, const bool ttl = false,
 // set, add, get and remove
 // This is a quick implementation without a Merge operation.
 class Counters {
-
  protected:
   std::shared_ptr<DB> db_;
 
@@ -190,7 +209,6 @@ class Counters {
     return get(key, &base) && set(key, base + value);
   }
 
-
   // convenience functions for testing
   void assert_set(const std::string& key, uint64_t value) {
     assert(set(key, value));
@@ -220,9 +238,7 @@ class MergeBasedCounters : public Counters {
 
  public:
   explicit MergeBasedCounters(std::shared_ptr<DB> db, uint64_t defaultCount = 0)
-      : Counters(db, defaultCount),
-        merge_option_() {
-  }
+      : Counters(db, defaultCount), merge_option_() {}
 
   // mapped to a rocksdb Merge operation
   bool add(const std::string& key, uint64_t value) override {
@@ -250,7 +266,6 @@ void dumpDb(DB* db) {
 }
 
 void testCounters(Counters& counters, DB* db, bool test_compaction) {
-
   FlushOptions o;
   o.wait = true;
 
@@ -392,7 +407,6 @@ void testCountersWithFlushAndCompaction(Counters& counters, DB* db) {
 
 void testSuccessiveMerge(Counters& counters, size_t max_num_merges,
                          size_t num_merges) {
-
   counters.assert_remove("z");
   uint64_t sum = 0;
 
@@ -449,6 +463,7 @@ void testPartialMerge(Counters* counters, DB* db, size_t max_merge,
   ASSERT_OK(db->CompactRange(CompactRangeOptions(), nullptr, nullptr));
   ASSERT_EQ(tmp_sum, counters->assert_get("c"));
   ASSERT_EQ(num_partial_merge_calls, 0U);
+  ASSERT_EQ(EnvMergeTest::now_nanos_count_, 0U);
 }
 
 void testSingleBatchSuccessiveMerge(DB* db, size_t max_num_merges,
@@ -486,7 +501,6 @@ void testSingleBatchSuccessiveMerge(DB* db, size_t max_num_merges,
 }
 
 void runTest(const std::string& dbname, const bool use_ttl = false) {
-
   {
     auto db = OpenDb(dbname, use_ttl);