Introduce MergeContext to Lazily Initialize merge operand list

Summary: In get operations, merge_operands is only used in few cases. Lazily initialize it can reduce average latency in some cases

Test Plan: make all check

Reviewers: haobo, kailiu, dhruba

Reviewed By: haobo

CC: igor, nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D14415

Conflicts:
	db/db_impl.cc
	db/memtable.cc

db/db_impl.cc

@@ -27,6 +27,7 @@
 #include "db/log_writer.h"
 #include "db/memtable.h"
 #include "db/memtablelist.h"
+#include "db/merge_context.h"
 #include "db/merge_helper.h"
 #include "db/prefix_filter_iterator.h"
 #include "db/table_cache.h"
@@ -2608,20 +2609,20 @@ Status DBImpl::GetImpl(const ReadOptions& options,
 
 
   // Prepare to store a list of merge operations if merge occurs.
-  std::deque<std::string> merge_operands;
+  MergeContext merge_context;
 
   // First look in the memtable, then in the immutable memtable (if any).
   // s is both in/out. When in, s could either be OK or MergeInProgress.
   // merge_operands will contain the sequence of merges in the latter case.
   LookupKey lkey(key, snapshot);
-  if (mem->Get(lkey, value, &s, &merge_operands, options_)) {
+  if (mem->Get(lkey, value, &s, merge_context, options_)) {
     // Done
     RecordTick(options_.statistics.get(), MEMTABLE_HIT);
-  } else if (imm.Get(lkey, value, &s, &merge_operands, options_)) {
+  } else if (imm.Get(lkey, value, &s, merge_context, options_)) {
     // Done
     RecordTick(options_.statistics.get(), MEMTABLE_HIT);
   } else {
-    current->Get(options, lkey, value, &s, &merge_operands, &stats,
+    current->Get(options, lkey, value, &s, &merge_context, &stats,
                  options_, value_found);
     have_stat_update = true;
     RecordTick(options_.statistics.get(), MEMTABLE_MISS);
@@ -2676,8 +2677,8 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
   bool have_stat_update = false;
   Version::GetStats stats;
 
-  // Prepare to store a list of merge operations if merge occurs.
+  // Contain a list of merge operations if merge occurs.
-  std::deque<std::string> merge_operands;
+  MergeContext merge_context;
 
   // Note: this always resizes the values array
   int numKeys = keys.size();
@@ -2692,17 +2693,17 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
   // s is both in/out. When in, s could either be OK or MergeInProgress.
   // merge_operands will contain the sequence of merges in the latter case.
   for (int i=0; i<numKeys; ++i) {
-    merge_operands.clear();
+    merge_context.Clear();
     Status& s = statList[i];
     std::string* value = &(*values)[i];
 
     LookupKey lkey(keys[i], snapshot);
-    if (mem->Get(lkey, value, &s, &merge_operands, options_)) {
+    if (mem->Get(lkey, value, &s, merge_context, options_)) {
       // Done
-    } else if (imm.Get(lkey, value, &s, &merge_operands, options_)) {
+    } else if (imm.Get(lkey, value, &s, merge_context, options_)) {
       // Done
     } else {
-      current->Get(options, lkey, value, &s, &merge_operands, &stats, options_);
+      current->Get(options, lkey, value, &s, &merge_context, &stats, options_);
       have_stat_update = true;
     }
 

db/db_impl_readonly.cc

@@ -23,6 +23,7 @@
 #include "db/log_reader.h"
 #include "db/log_writer.h"
 #include "db/memtable.h"
+#include "db/merge_context.h"
 #include "db/table_cache.h"
 #include "db/version_set.h"
 #include "db/write_batch_internal.h"
@@ -30,6 +31,7 @@
 #include "rocksdb/env.h"
 #include "rocksdb/status.h"
 #include "rocksdb/table.h"
+#include "rocksdb/merge_operator.h"
 #include "port/port.h"
 #include "table/block.h"
 #include "table/merger.h"
@@ -57,12 +59,12 @@ Status DBImplReadOnly::Get(const ReadOptions& options,
   MemTable* mem = GetMemTable();
   Version* current = versions_->current();
   SequenceNumber snapshot = versions_->LastSequence();
-  std::deque<std::string> merge_operands;
+  MergeContext merge_context;
   LookupKey lkey(key, snapshot);
-  if (mem->Get(lkey, value, &s, &merge_operands, options_)) {
+  if (mem->Get(lkey, value, &s, merge_context, options_)) {
   } else {
     Version::GetStats stats;
-    current->Get(options, lkey, value, &s, &merge_operands, &stats, options_);
+    current->Get(options, lkey, value, &s, &merge_context, &stats, options_);
   }
   return s;
 }

db/memtable.cc

@@ -12,6 +12,7 @@
 #include <memory>
 
 #include "db/dbformat.h"
+#include "db/merge_context.h"
 #include "rocksdb/comparator.h"
 #include "rocksdb/env.h"
 #include "rocksdb/iterator.h"
@@ -162,15 +163,12 @@ void MemTable::Add(SequenceNumber s, ValueType type,
 }
 
 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
-                   std::deque<std::string>* operands, const Options& options) {
+                   MergeContext& merge_context, const Options& options) {
   Slice memkey = key.memtable_key();
   std::shared_ptr<MemTableRep::Iterator> iter(
     table_->GetIterator(key.user_key()));
   iter->Seek(memkey.data());
 
-  // It is the caller's responsibility to allocate/delete operands list
-  assert(operands != nullptr);
-
   bool merge_in_progress = s->IsMergeInProgress();
   auto merge_operator = options.merge_operator.get();
   auto logger = options.info_log;
@@ -202,8 +200,9 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
           *s = Status::OK();
           if (merge_in_progress) {
             assert(merge_operator);
-            if (!merge_operator->FullMerge(key.user_key(), &v, *operands,
+          if (!merge_operator->FullMerge(key.user_key(), &v,
-                                           value, logger.get())) {
+                                         merge_context.GetOperands(), value,
+                                         logger.get())) {
               RecordTick(options.statistics.get(), NUMBER_MERGE_FAILURES);
               *s = Status::Corruption("Error: Could not perform merge.");
             }
@@ -219,8 +218,9 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
           if (merge_in_progress) {
             assert(merge_operator);
             *s = Status::OK();
-            if (!merge_operator->FullMerge(key.user_key(), nullptr, *operands,
+          if (!merge_operator->FullMerge(key.user_key(), nullptr,
-                                           value, logger.get())) {
+                                         merge_context.GetOperands(), value,
+                                         logger.get())) {
               RecordTick(options.statistics.get(), NUMBER_MERGE_FAILURES);
               *s = Status::Corruption("Error: Could not perform merge.");
             }
@@ -232,16 +232,14 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
         case kTypeMerge: {
           Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
           merge_in_progress = true;
-          operands->push_front(v.ToString());
+          merge_context.PushOperand(v);
-          while(operands->size() >= 2) {
+          while(merge_context.GetNumOperands() >= 2) {
             // Attempt to associative merge. (Returns true if successful)
           if (merge_operator->PartialMerge(key.user_key(),
-                                             Slice((*operands)[0]),
+                                           merge_context.GetOperand(0),
-                                             Slice((*operands)[1]),
+                                           merge_context.GetOperand(1),
-                                             &merge_result,
+                                           &merge_result, logger.get())) {
-                                             logger.get())) {
+              merge_context.PushPartialMergeResult(merge_result);
-              operands->pop_front();
-              swap(operands->front(), merge_result);
             } else {
               // Stack them because user can't associative merge
               break;

db/memtable.h

@@ -22,6 +22,7 @@ namespace rocksdb {
 
 class Mutex;
 class MemTableIterator;
+class MergeContext;
 
 class MemTable {
  public:
@@ -94,7 +95,7 @@ class MemTable {
   //   store MergeInProgress in s, and return false.
   // Else, return false.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           std::deque<std::string>* operands, const Options& options);
+           MergeContext& merge_context, const Options& options);
 
   // Update the value and return status ok,
   //   if key exists in current memtable

db/memtablelist.cc

@@ -204,10 +204,9 @@ size_t MemTableList::ApproximateMemoryUsage() {
 // Return the most recent value found, if any.
 // Operands stores the list of merge operations to apply, so far.
 bool MemTableList::Get(const LookupKey& key, std::string* value, Status* s,
-                       std::deque<std::string>* operands,
+                       MergeContext& merge_context, const Options& options) {
-                       const Options& options) {
   for (auto &memtable : memlist_) {
-    if (memtable->Get(key, value, s, operands, options)) {
+    if (memtable->Get(key, value, s, merge_context, options)) {
       return true;
     }
   }

db/memtablelist.h

@@ -78,7 +78,7 @@ class MemTableList {
   // Search all the memtables starting from the most recent one.
   // Return the most recent value found, if any.
   bool Get(const LookupKey& key, std::string* value, Status* s,
-           std::deque<std::string>* operands, const Options& options);
+           MergeContext& merge_context, const Options& options);
 
   // Returns the list of underlying memtables.
   void GetMemTables(std::vector<MemTable*>* list);

db/merge_context.h

@@ -0,0 +1,69 @@
+//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+#pragma once
+#include "db/dbformat.h"
+#include "rocksdb/slice.h"
+#include <string>
+#include <deque>
+
+namespace rocksdb {
+
+const std::deque<std::string> empty_operand_list;
+
+// The merge context for merging a user key.
+// When doing a Get(), DB will create such a class and pass it when
+// issuing Get() operation to memtables and version_set. The operands
+// will be fetched from the context when issuing partial of full merge.
+class MergeContext {
+public:
+  // Clear all the operands
+  void Clear() {
+    if (operand_list) {
+      operand_list->clear();
+    }
+  }
+  // Replace the first two operands of merge_result, which are expected be the
+  // merge results of them.
+  void PushPartialMergeResult(std::string& merge_result) {
+    assert (operand_list);
+    operand_list->pop_front();
+    swap(operand_list->front(), merge_result);
+  }
+  // Push a merge operand
+  void PushOperand(const Slice& operand_slice) {
+    Initialize();
+    operand_list->push_front(operand_slice.ToString());
+  }
+  // return total number of operands in the list
+  size_t GetNumOperands() const {
+    if (!operand_list) {
+      return 0;
+    }
+    return operand_list->size();
+  }
+  // Get the operand at the index.
+  Slice GetOperand(int index) const {
+    assert (operand_list);
+    return (*operand_list)[index];
+  }
+  // Return all the operands.
+  const std::deque<std::string>& GetOperands() const {
+    if (!operand_list) {
+      return empty_operand_list;
+    }
+    return *operand_list;
+  }
+private:
+  void Initialize() {
+    if (!operand_list) {
+      operand_list.reset(new std::deque<std::string>());
+    }
+  }
+  std::unique_ptr<std::deque<std::string>> operand_list;
+};
+
+} // namespace rocksdb
+

db/version_set.cc

@@ -16,6 +16,7 @@
 #include "db/log_reader.h"
 #include "db/log_writer.h"
 #include "db/memtable.h"
+#include "db/merge_context.h"
 #include "db/table_cache.h"
 #include "rocksdb/env.h"
 #include "rocksdb/merge_operator.h"
@@ -287,7 +288,8 @@ struct Saver {
   bool* value_found; // Is value set correctly? Used by KeyMayExist
   std::string* value;
   const MergeOperator* merge_operator;
-  std::deque<std::string>* merge_operands;  // the merge operations encountered
+  // the merge operations encountered;
+  MergeContext* merge_context;
   Logger* logger;
   bool didIO;    // did we do any disk io?
   Statistics* statistics;
@@ -309,10 +311,10 @@ static void MarkKeyMayExist(void* arg) {
 
 static bool SaveValue(void* arg, const Slice& ikey, const Slice& v, bool didIO){
   Saver* s = reinterpret_cast<Saver*>(arg);
-  std::deque<std::string>* const ops = s->merge_operands; // shorter alias
+  MergeContext* merge_contex = s->merge_context;
   std::string merge_result;  // temporary area for merge results later
 
-  assert(s != nullptr && ops != nullptr);
+  assert(s != nullptr && merge_contex != nullptr);
 
   ParsedInternalKey parsed_key;
   // TODO: didIO and Merge?
@@ -331,7 +333,8 @@ static bool SaveValue(void* arg, const Slice& ikey, const Slice& v, bool didIO){
           } else if (kMerge == s->state) {
             assert(s->merge_operator != nullptr);
             s->state = kFound;
-            if (!s->merge_operator->FullMerge(s->user_key, &v, *ops,
+            if (!s->merge_operator->FullMerge(s->user_key, &v,
+                                              merge_contex->GetOperands(),
                                               s->value, s->logger)) {
               RecordTick(s->statistics, NUMBER_MERGE_FAILURES);
               s->state = kCorrupt;
@@ -346,7 +349,8 @@ static bool SaveValue(void* arg, const Slice& ikey, const Slice& v, bool didIO){
             s->state = kDeleted;
           } else if (kMerge == s->state) {
             s->state = kFound;
-            if (!s->merge_operator->FullMerge(s->user_key, nullptr, *ops,
+          if (!s->merge_operator->FullMerge(s->user_key, nullptr,
+                                            merge_contex->GetOperands(),
                                             s->value, s->logger)) {
               RecordTick(s->statistics, NUMBER_MERGE_FAILURES);
               s->state = kCorrupt;
@@ -359,16 +363,15 @@ static bool SaveValue(void* arg, const Slice& ikey, const Slice& v, bool didIO){
         case kTypeMerge:
           assert(s->state == kNotFound || s->state == kMerge);
           s->state = kMerge;
-          ops->push_front(v.ToString());
+          merge_contex->PushOperand(v);
-          while (ops->size() >= 2) {
+          while (merge_contex->GetNumOperands() >= 2) {
             // Attempt to merge operands together via user associateive merge
             if (s->merge_operator->PartialMerge(s->user_key,
-                                                Slice((*ops)[0]),
+                                                merge_contex->GetOperand(0),
-                                                Slice((*ops)[1]),
+                                                merge_contex->GetOperand(1),
                                                 &merge_result,
                                                 s->logger)) {
-              ops->pop_front();
+              merge_contex->PushPartialMergeResult(merge_result);
-              swap(ops->front(), merge_result);
             } else {
               // Associative merge returns false ==> stack the operands
               break;
@@ -417,7 +420,7 @@ void Version::Get(const ReadOptions& options,
                   const LookupKey& k,
                   std::string* value,
                   Status* status,
-                  std::deque<std::string>* operands,
+                  MergeContext* merge_context,
                   GetStats* stats,
                   const Options& db_options,
                   bool* value_found) {
@@ -436,7 +439,7 @@ void Version::Get(const ReadOptions& options,
   saver.value_found = value_found;
   saver.value = value;
   saver.merge_operator = merge_operator;
-  saver.merge_operands = operands;
+  saver.merge_context = merge_context;
   saver.logger = logger.get();
   saver.didIO = false;
   saver.statistics = db_options.statistics.get();
@@ -557,7 +560,8 @@ void Version::Get(const ReadOptions& options,
   if (kMerge == saver.state) {
     // merge_operands are in saver and we hit the beginning of the key history
     // do a final merge of nullptr and operands;
-    if (merge_operator->FullMerge(user_key, nullptr, *saver.merge_operands,
+    if (merge_operator->FullMerge(user_key, nullptr,
+                                  saver.merge_context->GetOperands(),
                                   value, logger.get())) {
       *status = Status::OK();
     } else {

db/version_set.h

@@ -38,6 +38,7 @@ class MemTable;
 class TableCache;
 class Version;
 class VersionSet;
+class MergeContext;
 
 // Return the smallest index i such that files[i]->largest >= key.
 // Return files.size() if there is no such file.
@@ -76,9 +77,9 @@ class Version {
     int seek_file_level;
   };
   void Get(const ReadOptions&, const LookupKey& key, std::string* val,
-           Status* status, std::deque<std::string>* operands, GetStats* stats,
+           Status* status, MergeContext* merge_context,
-           const Options& db_option,
+           GetStats* stats, const Options& db_option, bool* value_found =
-           bool* value_found = nullptr);
+               nullptr);
 
   // Adds "stats" into the current state.  Returns true if a new
   // compaction may need to be triggered, false otherwise.

include/rocksdb/merge_operator.h

@@ -6,6 +6,7 @@
 #ifndef STORAGE_ROCKSDB_INCLUDE_MERGE_OPERATOR_H_
 #define STORAGE_ROCKSDB_INCLUDE_MERGE_OPERATOR_H_
 
+#include <memory>
 #include <string>
 #include <deque>
 #include "rocksdb/slice.h"