Histogram of number of merge operands

Summary:
Add a histogram in statistics to help users understand how many merge operands they merge.
Closes https://github.com/facebook/rocksdb/pull/2373

Differential Revision: D5139983

Pulled By: siying

fbshipit-source-id: 61b9ba8ca83f358530a4833d68f0103b56a0e182

db/db_iter.cc

@@ -562,7 +562,7 @@ void DBIter::MergeValuesNewToOld() {
       const Slice val = iter_->value();
       s = MergeHelper::TimedFullMerge(
           merge_operator_, ikey.user_key, &val, merge_context_.GetOperands(),
-          &saved_value_, logger_, statistics_, env_, &pinned_value_);
+          &saved_value_, logger_, statistics_, env_, &pinned_value_, true);
       if (!s.ok()) {
         status_ = s;
       }
@@ -587,7 +587,7 @@ void DBIter::MergeValuesNewToOld() {
   s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                   nullptr, merge_context_.GetOperands(),
                                   &saved_value_, logger_, statistics_, env_,
-                                  &pinned_value_);
+                                  &pinned_value_, true);
   if (!s.ok()) {
     status_ = s;
   }
@@ -806,13 +806,13 @@ bool DBIter::FindValueForCurrentKey() {
         s = MergeHelper::TimedFullMerge(
             merge_operator_, saved_key_.GetUserKey(), nullptr,
             merge_context_.GetOperands(), &saved_value_, logger_, statistics_,
-            env_, &pinned_value_);
+            env_, &pinned_value_, true);
       } else {
         assert(last_not_merge_type == kTypeValue);
         s = MergeHelper::TimedFullMerge(
             merge_operator_, saved_key_.GetUserKey(), &pinned_value_,
             merge_context_.GetOperands(), &saved_value_, logger_, statistics_,
-            env_, &pinned_value_);
+            env_, &pinned_value_, true);
       }
       break;
     case kTypeValue:
@@ -884,7 +884,7 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
     s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                     nullptr, merge_context_.GetOperands(),
                                     &saved_value_, logger_, statistics_, env_,
-                                    &pinned_value_);
+                                    &pinned_value_, true);
     // Make iter_ valid and point to saved_key_
     if (!iter_->Valid() ||
         !user_comparator_->Equal(ikey.user_key, saved_key_.GetUserKey())) {
@@ -902,7 +902,7 @@ bool DBIter::FindValueForCurrentKeyUsingSeek() {
   s = MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetUserKey(),
                                   &val, merge_context_.GetOperands(),
                                   &saved_value_, logger_, statistics_, env_,
-                                  &pinned_value_);
+                                  &pinned_value_, true);
   valid_ = true;
   if (!s.ok()) {
     status_ = s;

db/memtable.cc

@@ -578,7 +578,7 @@ static bool SaveValue(void* arg, const char* entry) {
           *(s->status) = MergeHelper::TimedFullMerge(
               merge_operator, s->key->user_key(), &v,
               merge_context->GetOperands(), s->value, s->logger, s->statistics,
-              s->env_);
+              s->env_, nullptr /* result_operand */, true);
         } else if (s->value != nullptr) {
           s->value->assign(v.data(), v.size());
         }
@@ -595,7 +595,7 @@ static bool SaveValue(void* arg, const char* entry) {
           *(s->status) = MergeHelper::TimedFullMerge(
               merge_operator, s->key->user_key(), nullptr,
               merge_context->GetOperands(), s->value, s->logger, s->statistics,
-              s->env_);
+              s->env_, nullptr /* result_operand */, true);
         } else {
           *(s->status) = Status::NotFound();
         }

db/merge_helper.cc

@@ -25,7 +25,8 @@ Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
                                    const std::vector<Slice>& operands,
                                    std::string* result, Logger* logger,
                                    Statistics* statistics, Env* env,
-                                   Slice* result_operand) {
+                                   Slice* result_operand,
+                                   bool update_num_ops_stats) {
   assert(merge_operator != nullptr);
 
   if (operands.size() == 0) {
@@ -34,6 +35,11 @@ Status MergeHelper::TimedFullMerge(const MergeOperator* merge_operator,
     return Status::OK();
   }
 
+  if (update_num_ops_stats) {
+    MeasureTime(statistics, READ_NUM_MERGE_OPERANDS,
+                static_cast<uint64_t>(operands.size()));
+  }
+
   bool success;
   Slice tmp_result_operand(nullptr, 0);
   const MergeOperator::MergeOperationInput merge_in(key, value, operands,

db/merge_helper.h

@@ -56,6 +56,8 @@ class MergeHelper {
   // Wrapper around MergeOperator::FullMergeV2() that records perf statistics.
   // Result of merge will be written to result if status returned is OK.
   // If operands is empty, the value will simply be copied to result.
+  // Set `update_num_ops_stats` to true if it is from a user read, so that
+  // the latency is sensitive.
   // Returns one of the following statuses:
   // - OK: Entries were successfully merged.
   // - Corruption: Merge operator reported unsuccessful merge.
@@ -64,7 +66,8 @@ class MergeHelper {
                                const std::vector<Slice>& operands,
                                std::string* result, Logger* logger,
                                Statistics* statistics, Env* env,
-                               Slice* result_operand = nullptr);
+                               Slice* result_operand = nullptr,
+                               bool update_num_ops_stats = false);
 
   // Merge entries until we hit
   //     - a corrupted key

db/version_set.cc

@@ -1016,7 +1016,8 @@ void Version::Get(const ReadOptions& read_options, const LookupKey& k,
     std::string* str_value = value != nullptr ? value->GetSelf() : nullptr;
     *status = MergeHelper::TimedFullMerge(
         merge_operator_, user_key, nullptr, merge_context->GetOperands(),
-        str_value, info_log_, db_statistics_, env_);
+        str_value, info_log_, db_statistics_, env_,
+        nullptr /* result_operand */, true);
     if (LIKELY(value != nullptr)) {
       value->PinSelf();
     }

include/rocksdb/statistics.h

@@ -370,6 +370,9 @@ enum Histograms : uint32_t {
   BYTES_DECOMPRESSED,
   COMPRESSION_TIMES_NANOS,
   DECOMPRESSION_TIMES_NANOS,
+  // Number of merge operands passed to the merge operator in user read
+  // requests.
+  READ_NUM_MERGE_OPERANDS,
 
   HISTOGRAM_ENUM_MAX,  // TODO(ldemailly): enforce HistogramsNameMap match
 };
@@ -405,6 +408,7 @@ const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {
     {BYTES_DECOMPRESSED, "rocksdb.bytes.decompressed"},
     {COMPRESSION_TIMES_NANOS, "rocksdb.compression.times.nanos"},
     {DECOMPRESSION_TIMES_NANOS, "rocksdb.decompression.times.nanos"},
+    {READ_NUM_MERGE_OPERANDS, "rocksdb.read.num.merge_operands"},
 };
 
 struct HistogramData {