Improve RocksDB "get" performance by computing merge result in memtable

Summary:
Added an option (max_successive_merges) that can be used to specify the
maximum number of successive merge operations on a key in the memtable.
This can be used to improve performance of the "get" operation. If many
successive merge operations are performed on a key, the performance of "get"
operations on the key deteriorates, as the value has to be computed for each
"get" operation by applying all the successive merge operations.

FB Task ID: #3428853

Test Plan:
make all check
db_bench --benchmarks=readrandommergerandom
counter_stress_test

Reviewers: haobo, vamsi, dhruba, sdong

Reviewed By: haobo

CC: zshao

Differential Revision: https://reviews.facebook.net/D14991

db/db_bench.cc

@@ -94,6 +94,8 @@ DEFINE_string(benchmarks,
               "\tmergerandom   -- same as updaterandom/appendrandom using merge"
               " operator. "
               "Must be used with merge_operator\n"
+              "\treadrandommergerandom -- perform N random read-or-merge "
+              "operations. Must be used with merge_operator\n"
               "\tseekrandom    -- N random seeks\n"
               "\tcrc32c        -- repeated crc32c of 4K of data\n"
               "\tacquireload   -- load N*1000 times\n"
@@ -112,6 +114,11 @@ DEFINE_int64(numdistinct, 1000,
              "read/write on fewer keys so that gets are more likely to find the"
              " key and puts are more likely to update the same key");
 
+DEFINE_int64(merge_keys, -1,
+             "Number of distinct keys to use for MergeRandom and "
+             "ReadRandomMergeRandom. "
+             "If negative, there will be FLAGS_num keys.");
+
 DEFINE_int64(reads, -1, "Number of read operations to do.  "
              "If negative, do FLAGS_num reads.");
 
@@ -297,6 +304,11 @@ DEFINE_int32(readwritepercent, 90, "Ratio of reads to reads/writes (expressed"
              "default value 90 means 90% operations out of all reads and writes"
              " operations are reads. In other words, 9 gets for every 1 put.");
 
+DEFINE_int32(mergereadpercent, 70, "Ratio of merges to merges&reads (expressed"
+             " as percentage) for the ReadRandomMergeRandom workload. The"
+             " default value 70 means 70% out of all read and merge operations"
+             " are merges. In other words, 7 merges for every 3 gets.");
+
 DEFINE_int32(deletepercent, 2, "Percentage of deletes out of reads/writes/"
              "deletes (used in RandomWithVerify only). RandomWithVerify "
              "calculates writepercent as (100 - FLAGS_readwritepercent - "
@@ -446,6 +458,9 @@ DEFINE_uint64(bytes_per_sync,  rocksdb::Options().bytes_per_sync,
 DEFINE_bool(filter_deletes, false, " On true, deletes use bloom-filter and drop"
             " the delete if key not present");
 
+DEFINE_int32(max_successive_merges, 0, "Maximum number of successive merge"
+             " operations on a key in the memtable");
+
 static bool ValidatePrefixSize(const char* flagname, int32_t value) {
   if (value < 0 || value>=2000000000) {
     fprintf(stderr, "Invalid value for --%s: %d. 0<= PrefixSize <=2000000000\n",
@@ -784,6 +799,7 @@ class Benchmark {
   long long reads_;
   long long writes_;
   long long readwrites_;
+  long long merge_keys_;
   int heap_counter_;
   char keyFormat_[100]; // will contain the format of key. e.g "%016d"
   void PrintHeader() {
@@ -958,6 +974,7 @@ class Benchmark {
     readwrites_((FLAGS_writes < 0  && FLAGS_reads < 0)? FLAGS_num :
                 ((FLAGS_writes > FLAGS_reads) ? FLAGS_writes : FLAGS_reads)
                ),
+    merge_keys_(FLAGS_merge_keys < 0 ? FLAGS_num : FLAGS_merge_keys),
     heap_counter_(0) {
     std::vector<std::string> files;
     FLAGS_env->GetChildren(FLAGS_db, &files);
@@ -985,8 +1002,8 @@ class Benchmark {
   }
 
   unique_ptr<char []> GenerateKeyFromInt(long long v, const char* suffix = "") {
-    unique_ptr<char []> keyInStr(new char[kMaxKeySize]);
+    unique_ptr<char []> keyInStr(new char[kMaxKeySize + 1]);
-    snprintf(keyInStr.get(), kMaxKeySize, keyFormat_, v, suffix);
+    snprintf(keyInStr.get(), kMaxKeySize + 1, keyFormat_, v, suffix);
     return keyInStr;
   }
 
@@ -1087,6 +1104,14 @@ class Benchmark {
         method = &Benchmark::ReadWhileWriting;
       } else if (name == Slice("readrandomwriterandom")) {
         method = &Benchmark::ReadRandomWriteRandom;
+      } else if (name == Slice("readrandommergerandom")) {
+        if (FLAGS_merge_operator.empty()) {
+          fprintf(stdout, "%-12s : skipped (--merge_operator is unknown)\n",
+                  name.ToString().c_str());
+          method = nullptr;
+        } else {
+          method = &Benchmark::ReadRandomMergeRandom;
+        }
       } else if (name == Slice("updaterandom")) {
         method = &Benchmark::UpdateRandom;
       } else if (name == Slice("appendrandom")) {
@@ -1421,6 +1446,7 @@ class Benchmark {
               FLAGS_merge_operator.c_str());
       exit(1);
     }
+    options.max_successive_merges = FLAGS_max_successive_merges;
 
     // set universal style compaction configurations, if applicable
     if (FLAGS_universal_size_ratio != 0) {
@@ -2375,13 +2401,16 @@ class Benchmark {
   //
   // For example, use FLAGS_merge_operator="uint64add" and FLAGS_value_size=8
   // to simulate random additions over 64-bit integers using merge.
+  //
+  // The number of merges on the same key can be controlled by adjusting
+  // FLAGS_merge_keys.
   void MergeRandom(ThreadState* thread) {
     RandomGenerator gen;
 
     // The number of iterations is the larger of read_ or write_
     Duration duration(FLAGS_duration, readwrites_);
     while (!duration.Done(1)) {
-      const long long k = thread->rand.Next() % FLAGS_num;
+      const long long k = thread->rand.Next() % merge_keys_;
       unique_ptr<char []> key = GenerateKeyFromInt(k);
 
       Status s = db_->Merge(write_options_, key.get(),
@@ -2400,6 +2429,68 @@ class Benchmark {
     thread->stats.AddMessage(msg);
   }
 
+  // Read and merge random keys. The amount of reads and merges are controlled
+  // by adjusting FLAGS_num and FLAGS_mergereadpercent. The number of distinct
+  // keys (and thus also the number of reads and merges on the same key) can be
+  // adjusted with FLAGS_merge_keys.
+  //
+  // As with MergeRandom, the merge operator to use should be defined by
+  // FLAGS_merge_operator.
+  void ReadRandomMergeRandom(ThreadState* thread) {
+    ReadOptions options(FLAGS_verify_checksum, true);
+    RandomGenerator gen;
+    std::string value;
+    long long num_hits = 0;
+    long long num_gets = 0;
+    long long num_merges = 0;
+    size_t max_length = 0;
+
+    // the number of iterations is the larger of read_ or write_
+    Duration duration(FLAGS_duration, readwrites_);
+
+    while (!duration.Done(1)) {
+      const long long k = thread->rand.Next() % merge_keys_;
+      unique_ptr<char []> key = GenerateKeyFromInt(k);
+
+      bool do_merge = int(thread->rand.Next() % 100) < FLAGS_mergereadpercent;
+
+      if (do_merge) {
+        Status s = db_->Merge(write_options_, key.get(),
+                              gen.Generate(value_size_));
+        if (!s.ok()) {
+          fprintf(stderr, "merge error: %s\n", s.ToString().c_str());
+          exit(1);
+        }
+
+        num_merges++;
+
+      } else {
+        Status s = db_->Get(options, key.get(), &value);
+        if (value.length() > max_length)
+          max_length = value.length();
+
+        if (!s.ok() && !s.IsNotFound()) {
+          fprintf(stderr, "get error: %s\n", s.ToString().c_str());
+          // we continue after error rather than exiting so that we can
+          // find more errors if any
+        } else if (!s.IsNotFound()) {
+          num_hits++;
+        }
+
+        num_gets++;
+
+      }
+
+      thread->stats.FinishedSingleOp(db_);
+    }
+    char msg[100];
+    snprintf(msg, sizeof(msg),
+             "(reads:%lld merges:%lld total:%lld hits:%lld maxlength:%zu)",
+             num_gets, num_merges, readwrites_, num_hits, max_length);
+    thread->stats.AddMessage(msg);
+  }
+
+
   void Compact(ThreadState* thread) {
     db_->CompactRange(nullptr, nullptr);
   }

db/memtable.cc

@@ -326,4 +326,37 @@ bool MemTable::Update(SequenceNumber seq, ValueType type,
   // Key doesn't exist
   return false;
 }
+
+size_t MemTable::CountSuccessiveMergeEntries(const LookupKey& key) {
+  Slice memkey = key.memtable_key();
+
+  // A total ordered iterator is costly for some memtablerep (prefix aware
+  // reps). By passing in the user key, we allow efficient iterator creation.
+  // The iterator only needs to be ordered within the same user key.
+  std::shared_ptr<MemTableRep::Iterator> iter(
+    table_->GetIterator(key.user_key()));
+  iter->Seek(memkey.data());
+
+  size_t num_successive_merges = 0;
+
+  for (; iter->Valid(); iter->Next()) {
+    const char* entry = iter->key();
+    uint32_t key_length;
+    const char* iter_key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
+    if (!comparator_.comparator.user_comparator()->Compare(
+        Slice(iter_key_ptr, key_length - 8), key.user_key()) == 0) {
+      break;
+    }
+
+    const uint64_t tag = DecodeFixed64(iter_key_ptr + key_length - 8);
+    if (static_cast<ValueType>(tag & 0xff) != kTypeMerge) {
+      break;
+    }
+
+    ++num_successive_merges;
+  }
+
+  return num_successive_merges;
+}
+
 }  // namespace rocksdb

db/memtable.h

@@ -107,6 +107,11 @@ class MemTable {
               const Slice& key,
               const Slice& value);
 
+  // Returns the number of successive merge entries starting from the newest
+  // entry for the key up to the last non-merge entry or last entry for the
+  // key in the memtable.
+  size_t CountSuccessiveMergeEntries(const LookupKey& key);
+
   // Returns the edits area that is needed for flushing the memtable
   VersionEdit* GetEdits() { return &edit_; }
 

db/merge_test.cc

@@ -14,6 +14,7 @@
 #include "rocksdb/merge_operator.h"
 #include "db/dbformat.h"
 #include "db/db_impl.h"
+#include "db/write_batch_internal.h"
 #include "utilities/merge_operators.h"
 #include "util/testharness.h"
 #include "utilities/utility_db.h"
@@ -21,13 +22,52 @@
 using namespace std;
 using namespace rocksdb;
 
+namespace {
+  int numMergeOperatorCalls;
 
-std::shared_ptr<DB> OpenDb(const string& dbname, const bool ttl = false) {
+  void resetNumMergeOperatorCalls() {
+    numMergeOperatorCalls = 0;
+  }
+}
+
+class CountMergeOperator : public AssociativeMergeOperator {
+ public:
+  CountMergeOperator() {
+    mergeOperator_ = MergeOperators::CreateUInt64AddOperator();
+  }
+
+  virtual bool Merge(const Slice& key,
+                     const Slice* existing_value,
+                     const Slice& value,
+                     std::string* new_value,
+                     Logger* logger) const override {
+    ++numMergeOperatorCalls;
+    return mergeOperator_->PartialMerge(
+        key,
+        *existing_value,
+        value,
+        new_value,
+        logger);
+  }
+
+  virtual const char* Name() const override {
+    return "UInt64AddOperator";
+  }
+
+ private:
+  std::shared_ptr<MergeOperator> mergeOperator_;
+};
+
+std::shared_ptr<DB> OpenDb(
+    const string& dbname,
+    const bool ttl = false,
+    const unsigned max_successive_merges = 0) {
   DB* db;
   StackableDB* sdb;
   Options options;
   options.create_if_missing = true;
-  options.merge_operator = MergeOperators::CreateUInt64AddOperator();
+  options.merge_operator = std::make_shared<CountMergeOperator>();
+  options.max_successive_merges = max_successive_merges;
   Status s;
   DestroyDB(dbname, Options());
   if (ttl) {
@@ -243,6 +283,67 @@ void testCounters(Counters& counters, DB* db, bool test_compaction) {
   }
 }
 
+void testSuccessiveMerge(
+    Counters& counters, int max_num_merges, int num_merges) {
+
+  counters.assert_remove("z");
+  uint64_t sum = 0;
+
+  for (int i = 1; i <= num_merges; ++i) {
+    resetNumMergeOperatorCalls();
+    counters.assert_add("z", i);
+    sum += i;
+
+    if (i % (max_num_merges + 1) == 0) {
+      assert(numMergeOperatorCalls == max_num_merges + 1);
+    } else {
+      assert(numMergeOperatorCalls == 0);
+    }
+
+    resetNumMergeOperatorCalls();
+    assert(counters.assert_get("z") == sum);
+    assert(numMergeOperatorCalls == i % (max_num_merges + 1));
+  }
+}
+
+void testSingleBatchSuccessiveMerge(
+    DB* db,
+    int max_num_merges,
+    int num_merges) {
+  assert(num_merges > max_num_merges);
+
+  Slice key("BatchSuccessiveMerge");
+  uint64_t merge_value = 1;
+  Slice merge_value_slice((char *)&merge_value, sizeof(merge_value));
+
+  // Create the batch
+  WriteBatch batch;
+  for (int i = 0; i < num_merges; ++i) {
+    batch.Merge(key, merge_value_slice);
+  }
+
+  // Apply to memtable and count the number of merges
+  resetNumMergeOperatorCalls();
+  {
+    Status s = db->Write(WriteOptions(), &batch);
+    assert(s.ok());
+  }
+  assert(numMergeOperatorCalls ==
+      num_merges - (num_merges % (max_num_merges + 1)));
+
+  // Get the value
+  resetNumMergeOperatorCalls();
+  string get_value_str;
+  {
+    Status s = db->Get(ReadOptions(), key, &get_value_str);
+    assert(s.ok());
+  }
+  assert(get_value_str.size() == sizeof(uint64_t));
+  uint64_t get_value = DecodeFixed64(&get_value_str[0]);
+  assert(get_value == num_merges * merge_value);
+  assert(numMergeOperatorCalls == (num_merges % (max_num_merges + 1)));
+}
+
 void runTest(int argc, const string& dbname, const bool use_ttl = false) {
   auto db = OpenDb(dbname, use_ttl);
 
@@ -265,6 +366,19 @@ void runTest(int argc, const string& dbname, const bool use_ttl = false) {
   }
 
   DestroyDB(dbname, Options());
+  db.reset();
+
+  {
+    cout << "Test merge in memtable... \n";
+    unsigned maxMerge = 5;
+    auto db = OpenDb(dbname, use_ttl, maxMerge);
+    MergeBasedCounters counters(db, 0);
+    testCounters(counters, db.get(), compact);
+    testSuccessiveMerge(counters, maxMerge, maxMerge * 2);
+    testSingleBatchSuccessiveMerge(db.get(), 5, 7);
+    DestroyDB(dbname, Options());
+  }
+
 }
 
 int main(int argc, char *argv[]) {

db/write_batch.cc

@@ -21,6 +21,7 @@
 
 #include "rocksdb/write_batch.h"
 #include "rocksdb/options.h"
+#include "rocksdb/merge_operator.h"
 #include "db/dbformat.h"
 #include "db/db_impl.h"
 #include "db/memtable.h"
@@ -203,7 +204,62 @@ class MemTableInserter : public WriteBatch::Handler {
     sequence_++;
   }
   virtual void Merge(const Slice& key, const Slice& value) {
+    bool perform_merge = false;
+
+    if (options_->max_successive_merges > 0 && db_ != nullptr) {
+      LookupKey lkey(key, sequence_);
+
+      // Count the number of successive merges at the head
+      // of the key in the memtable
+      size_t num_merges = mem_->CountSuccessiveMergeEntries(lkey);
+
+      if (num_merges >= options_->max_successive_merges) {
+        perform_merge = true;
+      }
+    }
+
+    if (perform_merge) {
+      // 1) Get the existing value
+      std::string get_value;
+
+      // Pass in the sequence number so that we also include previous merge
+      // operations in the same batch.
+      SnapshotImpl read_from_snapshot;
+      read_from_snapshot.number_ = sequence_;
+      ReadOptions read_options;
+      read_options.snapshot = &read_from_snapshot;
+
+      db_->Get(read_options, key, &get_value);
+      Slice get_value_slice = Slice(get_value);
+
+      // 2) Apply this merge
+      auto merge_operator = options_->merge_operator.get();
+      assert(merge_operator);
+
+      std::deque<std::string> operands;
+      operands.push_front(value.ToString());
+      std::string new_value;
+      if (!merge_operator->FullMerge(key,
+                                     &get_value_slice,
+                                     operands,
+                                     &new_value,
+                                     options_->info_log.get())) {
+          // Failed to merge!
+          RecordTick(options_->statistics.get(), NUMBER_MERGE_FAILURES);
+
+          // Store the delta in memtable
+          perform_merge = false;
+      } else {
+        // 3) Add value to memtable
+        mem_->Add(sequence_, kTypeValue, key, new_value);
+      }
+    }
+
+    if (!perform_merge) {
+      // Add merge operator to memtable
       mem_->Add(sequence_, kTypeMerge, key, value);
+    }
+
     sequence_++;
   }
   virtual void Delete(const Slice& key) {

include/rocksdb/options.h

@@ -643,6 +643,17 @@ struct Options {
   // Number of locks used for inplace update
   // Default: 10000, if inplace_update_support = true, else 0.
   size_t inplace_update_num_locks;
+
+  // Maximum number of successive merge operations on a key in the memtable.
+  //
+  // When a merge operation is added to the memtable and the maximum number of
+  // successive merges is reached, the value of the key will be calculated and
+  // inserted into the memtable instead of the merge operation. This will
+  // ensure that there are never more than max_successive_merges merge
+  // operations in the memtable.
+  //
+  // Default: 0 (disabled)
+  size_t max_successive_merges;
 };
 
 //

util/options.cc

@@ -101,7 +101,8 @@ Options::Options()
       table_factory(
         std::shared_ptr<TableFactory>(new BlockBasedTableFactory())),
       inplace_update_support(false),
-      inplace_update_num_locks(10000) {
+      inplace_update_num_locks(10000),
+      max_successive_merges(0) {
   assert(memtable_factory.get() != nullptr);
 }
 
@@ -292,6 +293,8 @@ Options::Dump(Logger* log) const
         inplace_update_support);
     Log(log, "                Options.inplace_update_num_locks: %zd",
         inplace_update_num_locks);
+    Log(log, "                   Options.max_successive_merges: %zd",
+        max_successive_merges);
 }   // Options::Dump
 
 //