[Rocksdb] [Multiget] Introduced multiget into db_bench

Summary:
Preliminary! Introduced the --use_multiget=1 and --keys_per_multiget=n
flags for db_bench. Also updated and tested the ReadRandom() method
to include an option to use multiget. By default,
keys_per_multiget=100.

Preliminary tests imply that multiget is at least 1.25x faster per
key than regular get.

Will continue adding Multiget for ReadMissing, ReadHot,
RandomWithVerify, ReadRandomWriteRandom; soon. Will also think
about ways to better verify benchmarks.

Test Plan:
1. make db_bench
2. ./db_bench --benchmarks=fillrandom
3. ./db_bench --benchmarks=readrandom --use_existing_db=1
	      --use_multiget=1 --threads=4 --keys_per_multiget=100
4. ./db_bench --benchmarks=readrandom --use_existing_db=1
	      --threads=4
5. Verify ops/sec (and 1000000 of 1000000 keys found)

Reviewers: haobo, MarkCallaghan, dhruba

Reviewed By: MarkCallaghan

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11127

db/db_bench.cc

@@ -301,6 +301,17 @@ static bool FLAGS_advise_random_on_open =
 static auto FLAGS_compaction_fadvice =
   leveldb::Options().access_hint_on_compaction_start;
 
+// Use multiget to access a series of keys instead of get
+static bool FLAGS_use_multiget = false;
+
+// If FLAGS_use_multiget is true, determines number of keys to group per call
+// Arbitrary default. 90 is good because it agrees with FLAGS_readwritepercent
+static long FLAGS_keys_per_multiget = 90;
+
+// Print a message to user when a key is missing in a Get/MultiGet call
+// TODO: Apply this flag to generic Get calls too. Currently only with Multiget
+static bool FLAGS_warn_missing_keys = true;
+
 // Use adaptive mutex
 static auto FLAGS_use_adaptive_mutex =
   leveldb::Options().use_adaptive_mutex;
@@ -497,7 +508,7 @@ class Stats {
 
     fprintf(stdout, "%-12s : %11.3f micros/op %ld ops/sec;%s%s\n",
             name.ToString().c_str(),
-            seconds_ * 1e6 / done_,
+            elapsed * 1e6 / done_,
             (long)throughput,
             (extra.empty() ? "" : " "),
             extra.c_str());
@@ -550,10 +561,12 @@ class Duration {
   }
 
   bool Done(int increment) {
+    if (increment <= 0) increment = 1;    // avoid Done(0) and infinite loops
     ops_ += increment;
 
     if (max_seconds_) {
-      if (!(ops_ % 1000)) {
+      // Recheck every appx 1000 ops (exact iff increment is factor of 1000)
+      if ((ops_/1000) != ((ops_-increment)/1000)) {
         double now = FLAGS_env->NowMicros();
         return ((now - start_at_) / 1000000.0) >= max_seconds_;
       } else {
@@ -712,7 +725,7 @@ class Benchmark {
     HistogramData histogramData;
     dbstats->histogramData(histogram_type, &histogramData);
     fprintf(stdout, "%s statistics Percentiles :", name.c_str());
-    fprintf(stdout, "50 : %f ",histogramData.median);
+    fprintf(stdout, "50 : %f ", histogramData.median);
     fprintf(stdout, "95 : %f ", histogramData.percentile95);
     fprintf(stdout, "99 : %f\n", histogramData.percentile99);
   }
@@ -743,6 +756,7 @@ class Benchmark {
       PrintHistogram(WAL_FILE_SYNC_MICROS, "WAL FILE SYNC MICROS");
       PrintHistogram(MANIFEST_FILE_SYNC_MICROS, "Manifest SYNC MICROS");
       PrintHistogram(TABLE_OPEN_IO_MICROS, "Table Open IO Micros");
+      PrintHistogram(DB_MULTIGET, "DB_MULTIGET");
     }
   }
 
@@ -1244,13 +1258,74 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
     thread->stats.AddBytes(bytes);
   }
 
+  // Calls MultiGet over a list of keys from a random distribution.
+  // Returns the total number of keys found.
+  long MultiGetRandom(ReadOptions& options, int num_keys,
+                     Random& rand, int range, const char* suffix) {
+    assert(num_keys > 0);
+    std::vector<Slice> keys(num_keys);
+    std::vector<std::string> values(num_keys);
+    std::vector<unique_ptr<char []> > gen_keys(num_keys);
+
+    int i;
+    long k;
+
+    // Fill the keys vector
+    for(i=0; i<num_keys; ++i) {
+      k = rand.Next() % range;
+      gen_keys[i] = GenerateKeyFromInt(k,suffix);
+      keys[i] = gen_keys[i].get();
+    }
+
+    if (FLAGS_use_snapshot) {
+      options.snapshot = db_->GetSnapshot();
+    }
+
+    // Apply the operation
+    std::vector<Status> statuses = db_->MultiGet(options, keys, &values);
+    assert((long)statuses.size() == num_keys);
+    assert((long)keys.size() == num_keys);  // Should always be the case.
+    assert((long)values.size() == num_keys);
+
+    if (FLAGS_use_snapshot) {
+      db_->ReleaseSnapshot(options.snapshot);
+      options.snapshot = nullptr;
+    }
+
+    // Count number found
+    long found = 0;
+    for(i=0; i<num_keys; ++i) {
+      if (statuses[i].ok()){
+        ++found;
+      } else if (FLAGS_warn_missing_keys == true) {
+        // Key not found, or error.
+        fprintf(stderr, "get error: %s\n", statuses[i].ToString().c_str());
+      }
+    }
+
+    return found;
+  }
+
   void ReadRandom(ThreadState* thread) {
     ReadOptions options(FLAGS_verify_checksum, true);
-    Iterator* iter = db_->NewIterator(options);
     Duration duration(FLAGS_duration, reads_);
 
-    std::string value;
     long found = 0;
+
+    if (FLAGS_use_multiget) {   // MultiGet
+      const long& kpg = FLAGS_keys_per_multiget;  // keys per multiget group
+      long keys_left = reads_;
+
+      // Recalculate number of keys per group, and call MultiGet until done
+      long num_keys;
+      while(num_keys = std::min(keys_left, kpg), !duration.Done(num_keys)) {
+        found += MultiGetRandom(options, num_keys, thread->rand, FLAGS_num,"");
+        thread->stats.FinishedSingleOp(db_);
+        keys_left -= num_keys;
+      }
+    } else {    // Regular case. Do one "get" at a time Get
+      Iterator* iter = db_->NewIterator(options);
+      std::string value;
       while (!duration.Done(1)) {
         const int k = thread->rand.Next() % FLAGS_num;
         unique_ptr<char []> key = GenerateKeyFromInt(k);
@@ -1289,37 +1364,81 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
 
         thread->stats.FinishedSingleOp(db_);
       }
+
       delete iter;
+    }
+
     char msg[100];
-    snprintf(msg, sizeof(msg), "(%ld of %ld found)", found, num_);
+    snprintf(msg, sizeof(msg), "(%ld of %ld found)", found, reads_);
     thread->stats.AddMessage(msg);
   }
 
-  void ReadMissing(ThreadState* thread) {
+void ReadMissing(ThreadState* thread) {
+    FLAGS_warn_missing_keys = false;    // Never warn about missing keys
+
     Duration duration(FLAGS_duration, reads_);
     ReadOptions options(FLAGS_verify_checksum, true);
+
+    if (FLAGS_use_multiget) {
+      const long& kpg = FLAGS_keys_per_multiget;  // keys per multiget group
+      long keys_left = reads_;
+
+      // Recalculate number of keys per group, and call MultiGet until done
+      long num_keys;
+      long found;
+      while(num_keys = std::min(keys_left, kpg), !duration.Done(num_keys)) {
+        found = MultiGetRandom(options, num_keys, thread->rand, FLAGS_num,".");
+        assert(!found);
+        thread->stats.FinishedSingleOp(db_);
+        keys_left -= num_keys;
+      }
+    } else {  // Regular case (not MultiGet)
       std::string value;
+      Status s;
       while (!duration.Done(1)) {
         const int k = thread->rand.Next() % FLAGS_num;
         unique_ptr<char []> key = GenerateKeyFromInt(k, ".");
-      db_->Get(options, key.get(), &value);
+        s = db_->Get(options, key.get(), &value);
+        assert(!s.ok() && s.IsNotFound());
         thread->stats.FinishedSingleOp(db_);
       }
     }
+  }
 
   void ReadHot(ThreadState* thread) {
     Duration duration(FLAGS_duration, reads_);
     ReadOptions options(FLAGS_verify_checksum, true);
-    std::string value;
     const long range = (FLAGS_num + 99) / 100;
+    long found = 0;
+
+    if (FLAGS_use_multiget) {
+      const long& kpg = FLAGS_keys_per_multiget;  // keys per multiget group
+      long keys_left = reads_;
+
+      // Recalculate number of keys per group, and call MultiGet until done
+      long num_keys;
+      while(num_keys = std::min(keys_left, kpg), !duration.Done(num_keys)) {
+        found += MultiGetRandom(options, num_keys, thread->rand, range, "");
+        thread->stats.FinishedSingleOp(db_);
+        keys_left -= num_keys;
+      }
+    } else {
+      std::string value;
       while (!duration.Done(1)) {
         const int k = thread->rand.Next() % range;
         unique_ptr<char []> key = GenerateKeyFromInt(k);
-      db_->Get(options, key.get(), &value);
+        if (db_->Get(options, key.get(), &value).ok()){
+          ++found;
+        }
         thread->stats.FinishedSingleOp(db_);
       }
     }
 
+    char msg[100];
+    snprintf(msg, sizeof(msg), "(%ld of %ld found)", found, reads_);
+    thread->stats.AddMessage(msg);
+  }
+
   void SeekRandom(ThreadState* thread) {
     Duration duration(FLAGS_duration, reads_);
     ReadOptions options(FLAGS_verify_checksum, true);
@@ -1424,8 +1543,8 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
   }
 
   // Given a key K and value V, this puts (K+"0", V), (K+"1", V), (K+"2", V)
-  // in DB atomically i.e in a single batch. Also refer MultiGet.
+  // in DB atomically i.e in a single batch. Also refer GetMany.
-  Status MultiPut(const WriteOptions& writeoptions,
+  Status PutMany(const WriteOptions& writeoptions,
                   const Slice& key, const Slice& value) {
     std::string suffixes[3] = {"2", "1", "0"};
     std::string keys[3];
@@ -1443,8 +1562,8 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
 
 
   // Given a key K, this deletes (K+"0", V), (K+"1", V), (K+"2", V)
-  // in DB atomically i.e in a single batch. Also refer MultiGet.
+  // in DB atomically i.e in a single batch. Also refer GetMany.
-  Status MultiDelete(const WriteOptions& writeoptions,
+  Status DeleteMany(const WriteOptions& writeoptions,
                   const Slice& key) {
     std::string suffixes[3] = {"1", "2", "0"};
     std::string keys[3];
@@ -1462,8 +1581,8 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
 
   // Given a key K and value V, this gets values for K+"0", K+"1" and K+"2"
   // in the same snapshot, and verifies that all the values are identical.
-  // ASSUMES that MultiPut was used to put (K, V) into the DB.
+  // ASSUMES that PutMany was used to put (K, V) into the DB.
-  Status MultiGet(const ReadOptions& readoptions,
+  Status GetMany(const ReadOptions& readoptions,
                   const Slice& key, std::string* value) {
     std::string suffixes[3] = {"0", "1", "2"};
     std::string keys[3];
@@ -1501,11 +1620,12 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
   }
 
   // Differs from readrandomwriterandom in the following ways:
-  // (a) Uses MultiGet/MultiPut to read/write key values. Refer to those funcs.
+  // (a) Uses GetMany/PutMany to read/write key values. Refer to those funcs.
   // (b) Does deletes as well (per FLAGS_deletepercent)
   // (c) In order to achieve high % of 'found' during lookups, and to do
   //     multiple writes (including puts and deletes) it uses upto
   //     FLAGS_numdistinct distinct keys instead of FLAGS_num distinct keys.
+  // (d) Does not have a MultiGet option.
   void RandomWithVerify(ThreadState* thread) {
     ReadOptions options(FLAGS_verify_checksum, true);
     RandomGenerator gen;
@@ -1517,21 +1637,22 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
     long gets_done = 0;
     long puts_done = 0;
     long deletes_done = 0;
+
     // the number of iterations is the larger of read_ or write_
     for (long i = 0; i < readwrites_; i++) {
       const int k = thread->rand.Next() % (FLAGS_numdistinct);
       unique_ptr<char []> key = GenerateKeyFromInt(k);
       if (get_weight == 0 && put_weight == 0 && delete_weight == 0) {
-        // one batch complated, reinitialize for next batch
+        // one batch completed, reinitialize for next batch
         get_weight = FLAGS_readwritepercent;
         delete_weight = FLAGS_deletepercent;
         put_weight = 100 - get_weight - delete_weight;
       }
       if (get_weight > 0) {
         // do all the gets first
-        Status s = MultiGet(options, key.get(), &value);
+        Status s = GetMany(options, key.get(), &value);
         if (!s.ok() && !s.IsNotFound()) {
-          fprintf(stderr, "get error: %s\n", s.ToString().c_str());
+          fprintf(stderr, "getmany 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()) {
@@ -1542,17 +1663,17 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
       } else if (put_weight > 0) {
         // then do all the corresponding number of puts
         // for all the gets we have done earlier
-        Status s = MultiPut(write_options_, key.get(), gen.Generate(value_size_));
+        Status s = PutMany(write_options_, key.get(), gen.Generate(value_size_));
         if (!s.ok()) {
-          fprintf(stderr, "multiput error: %s\n", s.ToString().c_str());
+          fprintf(stderr, "putmany error: %s\n", s.ToString().c_str());
           exit(1);
         }
         put_weight--;
         puts_done++;
       } else if (delete_weight > 0) {
-        Status s = MultiDelete(write_options_, key.get());
+        Status s = DeleteMany(write_options_, key.get());
         if (!s.ok()) {
-          fprintf(stderr, "multidelete error: %s\n", s.ToString().c_str());
+          fprintf(stderr, "deletemany error: %s\n", s.ToString().c_str());
           exit(1);
         }
         delete_weight--;
@@ -1572,6 +1693,12 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
   // an extra thread.
   //
   void ReadRandomWriteRandom(ThreadState* thread) {
+    if (FLAGS_use_multiget){
+      // Separate function for multiget (for ease of reading)
+      ReadRandomWriteRandomMultiGet(thread);
+      return;
+    }
+
     ReadOptions options(FLAGS_verify_checksum, true);
     RandomGenerator gen;
     std::string value;
@@ -1617,9 +1744,6 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
           found++;
         }
 
-        if (db_->Get(options, key.get(), &value).ok()) {
-          found++;
-        }
         get_weight--;
         reads_done++;
 
@@ -1646,9 +1770,85 @@ unique_ptr<char []> GenerateKeyFromInt(int v, const char* suffix = "")
     thread->stats.AddMessage(msg);
   }
 
+  // ReadRandomWriteRandom (with multiget)
+  // Does FLAGS_keys_per_multiget reads (per multiget), followed by some puts.
+  // FLAGS_readwritepercent will specify the ratio of gets to puts.
+  // e.g.: If FLAGS_keys_per_multiget == 100 and FLAGS_readwritepercent == 75
+  // Then each block will do 100 multigets and 33 puts
+  // So there are 133 operations in-total: 100 of them (75%) are gets, and 33
+  // of them (25%) are puts.
+  void ReadRandomWriteRandomMultiGet(ThreadState* thread) {
+    ReadOptions options(FLAGS_verify_checksum, true);
+    RandomGenerator gen;
+
+    // For multiget
+    const long& kpg = FLAGS_keys_per_multiget;  // keys per multiget group
+
+    long keys_left = readwrites_;  // number of keys still left to read
+    long num_keys;                  // number of keys to read in current group
+    long num_put_keys;              // number of keys to put in current group
+
+    long found = 0;
+    long reads_done = 0;
+    long writes_done = 0;
+    long multigets_done = 0;
+
+    // the number of iterations is the larger of read_ or write_
+    Duration duration(FLAGS_duration, readwrites_);
+    while(true) {
+      // Read num_keys keys, then write num_put_keys keys.
+      // The ratio of num_keys to num_put_keys is always FLAGS_readwritepercent
+      // And num_keys is set to be FLAGS_keys_per_multiget (kpg)
+      // num_put_keys is calculated accordingly (to maintain the ratio)
+      // Note: On the final iteration, num_keys and num_put_keys will be smaller
+      num_keys = std::min(keys_left*(FLAGS_readwritepercent + 99)/100, kpg);
+      num_put_keys = num_keys * (100-FLAGS_readwritepercent)
+                     / FLAGS_readwritepercent;
+
+      // This will break the loop when duration is complete
+      if (duration.Done(num_keys + num_put_keys)) {
+        break;
+      }
+
+      // A quick check to make sure our formula doesn't break on edge cases
+      assert(num_keys >= 1);
+      assert(num_keys + num_put_keys <= keys_left);
+
+      // Apply the MultiGet operations
+      found += MultiGetRandom(options, num_keys, thread->rand, FLAGS_num,"");
+      ++multigets_done;
+      reads_done+=num_keys;
+      thread->stats.FinishedSingleOp(db_);
+
+      // Now do the puts
+      int i;
+      long k;
+      for(i=0; i<num_put_keys; ++i) {
+        k = thread->rand.Next() % FLAGS_num;
+        unique_ptr<char []> key = GenerateKeyFromInt(k);
+        Status s = db_->Put(write_options_, key.get(),
+                            gen.Generate(value_size_));
+        if (!s.ok()) {
+          fprintf(stderr, "put error: %s\n", s.ToString().c_str());
+          exit(1);
+        }
+        writes_done++;
+        thread->stats.FinishedSingleOp(db_);
+      }
+
+      keys_left -= (num_keys + num_put_keys);
+    }
+    char msg[100];
+    snprintf(msg, sizeof(msg),
+             "( reads:%ld writes:%ld total:%ld multiget_ops:%ld found:%ld)",
+             reads_done, writes_done, readwrites_, multigets_done, found);
+    thread->stats.AddMessage(msg);
+  }
+
   //
   // Read-modify-write for random keys
   //
+  // TODO: Implement MergeOperator tests here (Read-modify-write)
   void UpdateRandom(ThreadState* thread) {
     ReadOptions options(FLAGS_verify_checksum, true);
     RandomGenerator gen;
@@ -1971,6 +2171,12 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--use_adaptive_mutex=%d%c", &n, &junk) == 1
                && (n == 0 || n ==1 )) {
       FLAGS_use_adaptive_mutex = n;
+    } else if (sscanf(argv[i], "--use_multiget=%d%c", &n, &junk) == 1 &&
+               (n == 0 || n == 1)) {
+      FLAGS_use_multiget = n;
+    } else if (sscanf(argv[i], "--keys_per_multiget=%d%c",
+               &n, &junk) == 1) {
+      FLAGS_keys_per_multiget = n;
     } else {
       fprintf(stderr, "Invalid flag '%s'\n", argv[i]);
       exit(1);

db/db_impl.cc

@@ -2087,7 +2087,6 @@ std::vector<Status> DBImpl::MultiGet(const ReadOptions& options,
   // 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.
   // value will contain the current merge operand in the latter case.
-  // TODO: Maybe these could be run concurrently?
   for(int i=0; i<numKeys; ++i) {
     Status& s = statList[i];
     std::string* value = &(*values)[i];