Update stats to help users estimate MultiGet async IO impact (#10182)

Summary:
Add a couple of stats to help users estimate the impact of potential MultiGet perf improvements -
1. NUM_LEVEL_READ_PER_MULTIGET - A histogram stat for number of levels that required MultiGet to read from a file
2. MULTIGET_COROUTINE_COUNT - A ticker stat to count the number of times the coroutine version of MultiGetFromSST was used

The NUM_DATA_BLOCKS_READ_PER_LEVEL stat is obsoleted as it doesn't provide useful information for MultiGet optimization.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10182

Reviewed By: akankshamahajan15

Differential Revision: D37213296

Pulled By: anand1976

fbshipit-source-id: 5d2b7708017c0e278578ae4bffac3926f6530efb

HISTORY.md

@@ -25,6 +25,7 @@
   * And Add C API's for Transaction, SstFileWriter, Compaction as mentioned [here](https://github.com/facebook/rocksdb/wiki/User-defined-Timestamp-(Experimental))
 * The contract for implementations of Comparator::IsSameLengthImmediateSuccessor has been updated to work around a design bug in `auto_prefix_mode`.
 * The API documentation for `auto_prefix_mode` now notes some corner cases in which it returns different results than `total_order_seek`, due to design bugs that are not easily fixed. Users using built-in comparators and keys at least the size of a fixed prefix length are not affected.
+* Obsoleted the NUM_DATA_BLOCKS_READ_PER_LEVEL stat and introduced the NUM_LEVEL_READ_PER_MULTIGET and MULTIGET_COROUTINE_COUNT stats
 
 ### New Features
 * Add FileSystem::ReadAsync API in io_tracing

db/db_basic_test.cc

@@ -2221,6 +2221,7 @@ TEST_F(DBMultiGetAsyncIOTest, GetFromL0) {
   // No async IO in this case since we don't do parallel lookup in L0
   ASSERT_EQ(multiget_io_batch_size.count, 0);
   ASSERT_EQ(multiget_io_batch_size.max, 0);
+  ASSERT_EQ(statistics()->getTickerCount(MULTIGET_COROUTINE_COUNT), 0);
 }
 
 TEST_F(DBMultiGetAsyncIOTest, GetFromL1) {
@@ -2257,6 +2258,7 @@ TEST_F(DBMultiGetAsyncIOTest, GetFromL1) {
   // A batch of 3 async IOs is expected, one for each overlapping file in L1
   ASSERT_EQ(multiget_io_batch_size.count, 1);
   ASSERT_EQ(multiget_io_batch_size.max, 3);
+  ASSERT_EQ(statistics()->getTickerCount(MULTIGET_COROUTINE_COUNT), 3);
 }
 
 TEST_F(DBMultiGetAsyncIOTest, LastKeyInFile) {
@@ -2407,27 +2409,37 @@ TEST_F(DBBasicTest, MultiGetStats) {
                 values.data(), s.data(), false);
 
   ASSERT_EQ(values.size(), kMultiGetBatchSize);
-  HistogramData hist_data_blocks;
+  HistogramData hist_level;
   HistogramData hist_index_and_filter_blocks;
   HistogramData hist_sst;
 
-  options.statistics->histogramData(NUM_DATA_BLOCKS_READ_PER_LEVEL,
-                                    &hist_data_blocks);
+  options.statistics->histogramData(NUM_LEVEL_READ_PER_MULTIGET, &hist_level);
   options.statistics->histogramData(NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL,
                                     &hist_index_and_filter_blocks);
   options.statistics->histogramData(NUM_SST_READ_PER_LEVEL, &hist_sst);
 
   // Maximum number of blocks read from a file system in a level.
-  ASSERT_EQ(hist_data_blocks.max, 32);
+  ASSERT_EQ(hist_level.max, 1);
   ASSERT_GT(hist_index_and_filter_blocks.max, 0);
   // Maximum number of sst files read from file system in a level.
   ASSERT_EQ(hist_sst.max, 2);
 
   // Minimun number of blocks read in a level.
-  ASSERT_EQ(hist_data_blocks.min, 4);
+  ASSERT_EQ(hist_level.min, 1);
   ASSERT_GT(hist_index_and_filter_blocks.min, 0);
   // Minimun number of sst files read in a level.
   ASSERT_EQ(hist_sst.min, 1);
+
+  for (PinnableSlice& value : values) {
+    value.Reset();
+  }
+  for (Status& status : s) {
+    status = Status::OK();
+  }
+  db_->MultiGet(read_opts, handles_[1], kMultiGetBatchSize, &keys[950],
+                values.data(), s.data(), false);
+  options.statistics->histogramData(NUM_LEVEL_READ_PER_MULTIGET, &hist_level);
+  ASSERT_EQ(hist_level.max, 2);
 }
 
 // Test class for batched MultiGet with prefix extractor

db/version_set.cc

@@ -2252,8 +2252,8 @@ void Version::MultiGet(const ReadOptions& read_options, MultiGetRange* range,
   Status s;
   uint64_t num_index_read = 0;
   uint64_t num_filter_read = 0;
-  uint64_t num_data_read = 0;
   uint64_t num_sst_read = 0;
+  uint64_t num_level_read = 0;
 
   MultiGetRange keys_with_blobs_range(*range, range->begin(), range->end());
   // blob_file => [[blob_idx, it], ...]
@@ -2275,7 +2275,7 @@ void Version::MultiGet(const ReadOptions& read_options, MultiGetRange* range,
         s = MultiGetFromSST(read_options, fp.CurrentFileRange(),
                             fp.GetHitFileLevel(), fp.IsHitFileLastInLevel(), f,
                             blob_rqs, num_filter_read, num_index_read,
-                            num_data_read, num_sst_read);
+                            num_sst_read);
         if (fp.GetHitFileLevel() == 0) {
           dump_stats_for_l0_file = true;
         }
@@ -2290,13 +2290,14 @@ void Version::MultiGet(const ReadOptions& read_options, MultiGetRange* range,
         mget_tasks.emplace_back(MultiGetFromSSTCoroutine(
             read_options, fp.CurrentFileRange(), fp.GetHitFileLevel(),
             fp.IsHitFileLastInLevel(), f, blob_rqs, num_filter_read,
-            num_index_read, num_data_read, num_sst_read));
+            num_index_read, num_sst_read));
         if (fp.KeyMaySpanNextFile()) {
           break;
         }
         f = fp.GetNextFileInLevel();
       }
       if (mget_tasks.size() > 0) {
+        RecordTick(db_statistics_, MULTIGET_COROUTINE_COUNT, mget_tasks.size());
         // Collect all results so far
         std::vector<Status> statuses = folly::coro::blockingWait(
             folly::coro::collectAllRange(std::move(mget_tasks))
@@ -2328,18 +2329,18 @@ void Version::MultiGet(const ReadOptions& read_options, MultiGetRange* range,
           (prev_level != 0 && prev_level != (int)fp.GetHitFileLevel())) {
         // Dump the stats if the search has moved to the next level and
         // reset for next level.
-        if (num_sst_read || (num_filter_read + num_index_read)) {
+        if (num_filter_read + num_index_read) {
           RecordInHistogram(db_statistics_,
                             NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL,
                             num_index_read + num_filter_read);
-          RecordInHistogram(db_statistics_, NUM_DATA_BLOCKS_READ_PER_LEVEL,
-                            num_data_read);
+        }
+        if (num_sst_read) {
           RecordInHistogram(db_statistics_, NUM_SST_READ_PER_LEVEL,
                             num_sst_read);
+          num_level_read++;
         }
         num_filter_read = 0;
         num_index_read = 0;
-        num_data_read = 0;
         num_sst_read = 0;
       }
       prev_level = fp.GetHitFileLevel();
@@ -2347,11 +2348,19 @@ void Version::MultiGet(const ReadOptions& read_options, MultiGetRange* range,
   }
 
   // Dump stats for most recent level
-  RecordInHistogram(db_statistics_, NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL,
-                    num_index_read + num_filter_read);
-  RecordInHistogram(db_statistics_, NUM_DATA_BLOCKS_READ_PER_LEVEL,
-                    num_data_read);
-  RecordInHistogram(db_statistics_, NUM_SST_READ_PER_LEVEL, num_sst_read);
+  if (num_filter_read + num_index_read) {
+    RecordInHistogram(db_statistics_,
+                      NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL,
+                      num_index_read + num_filter_read);
+  }
+  if (num_sst_read) {
+    RecordInHistogram(db_statistics_, NUM_SST_READ_PER_LEVEL, num_sst_read);
+    num_level_read++;
+  }
+  if (num_level_read) {
+    RecordInHistogram(db_statistics_, NUM_LEVEL_READ_PER_MULTIGET,
+                      num_level_read);
+  }
 
   if (s.ok() && !blob_rqs.empty()) {
     MultiGetBlob(read_options, keys_with_blobs_range, blob_rqs);

db/version_set.h

@@ -958,7 +958,7 @@ class Version {
       int hit_file_level, bool is_hit_file_last_in_level, FdWithKeyRange* f,
       std::unordered_map<uint64_t, BlobReadRequests>& blob_rqs,
       uint64_t& num_filter_read, uint64_t& num_index_read,
-      uint64_t& num_data_read, uint64_t& num_sst_read);
+      uint64_t& num_sst_read);
 
   ColumnFamilyData* cfd_;  // ColumnFamilyData to which this Version belongs
   Logger* info_log_;

db/version_set_sync_and_async.h

@@ -15,8 +15,7 @@ DEFINE_SYNC_AND_ASYNC(Status, Version::MultiGetFromSST)
 (const ReadOptions& read_options, MultiGetRange file_range, int hit_file_level,
  bool is_hit_file_last_in_level, FdWithKeyRange* f,
  std::unordered_map<uint64_t, BlobReadRequests>& blob_rqs,
- uint64_t& num_filter_read, uint64_t& num_index_read, uint64_t& num_data_read,
- uint64_t& num_sst_read) {
+ uint64_t& num_filter_read, uint64_t& num_index_read, uint64_t& num_sst_read) {
   bool timer_enabled = GetPerfLevel() >= PerfLevel::kEnableTimeExceptForMutex &&
                        get_perf_context()->per_level_perf_context_enabled;
 
@@ -63,12 +62,10 @@ DEFINE_SYNC_AND_ASYNC(Status, Version::MultiGetFromSST)
     batch_size++;
     num_index_read += get_context.get_context_stats_.num_index_read;
     num_filter_read += get_context.get_context_stats_.num_filter_read;
-    num_data_read += get_context.get_context_stats_.num_data_read;
     num_sst_read += get_context.get_context_stats_.num_sst_read;
     // Reset these stats since they're specific to a level
     get_context.get_context_stats_.num_index_read = 0;
     get_context.get_context_stats_.num_filter_read = 0;
-    get_context.get_context_stats_.num_data_read = 0;
     get_context.get_context_stats_.num_sst_read = 0;
 
     // report the counters before returning

include/rocksdb/statistics.h

@@ -432,6 +432,7 @@ enum Tickers : uint32_t {
   NON_LAST_LEVEL_READ_COUNT,
 
   BLOCK_CHECKSUM_COMPUTE_COUNT,
+  MULTIGET_COROUTINE_COUNT,
 
   TICKER_ENUM_MAX
 };
@@ -529,6 +530,7 @@ enum Histograms : uint32_t {
   // Num of index and filter blocks read from file system per level.
   NUM_INDEX_AND_FILTER_BLOCKS_READ_PER_LEVEL,
   // Num of data blocks read from file system per level.
+  // Obsolete
   NUM_DATA_BLOCKS_READ_PER_LEVEL,
   // Num of sst files read from file system per level.
   NUM_SST_READ_PER_LEVEL,
@@ -546,6 +548,9 @@ enum Histograms : uint32_t {
   // Number of IOs issued in parallel in a MultiGet batch
   MULTIGET_IO_BATCH_SIZE,
 
+  // Number of levels requiring IO for MultiGet
+  NUM_LEVEL_READ_PER_MULTIGET,
+
   HISTOGRAM_ENUM_MAX,
 };
 

monitoring/statistics.cc

@@ -226,7 +226,8 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {LAST_LEVEL_READ_COUNT, "rocksdb.last.level.read.count"},
     {NON_LAST_LEVEL_READ_BYTES, "rocksdb.non.last.level.read.bytes"},
     {NON_LAST_LEVEL_READ_COUNT, "rocksdb.non.last.level.read.count"},
-    {BLOCK_CHECKSUM_COMPUTE_COUNT, "rocksdb.block.checksum.compute.count"}};
+    {BLOCK_CHECKSUM_COMPUTE_COUNT, "rocksdb.block.checksum.compute.count"},
+    {MULTIGET_COROUTINE_COUNT, "rocksdb.multiget.coroutine.count"}};
 
 const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {
     {DB_GET, "rocksdb.db.get.micros"},
@@ -287,6 +288,7 @@ const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {
     {POLL_WAIT_MICROS, "rocksdb.poll.wait.micros"},
     {PREFETCHED_BYTES_DISCARDED, "rocksdb.prefetched.bytes.discarded"},
     {MULTIGET_IO_BATCH_SIZE, "rocksdb.multiget.io.batch.size"},
+    {NUM_LEVEL_READ_PER_MULTIGET, "rocksdb.num.level.read.per.multiget"},
 };
 
 std::shared_ptr<Statistics> CreateDBStatistics() {

table/block_based/block_based_table_reader.cc

@@ -1611,9 +1611,6 @@ Status BlockBasedTable::MaybeReadBlockAndLoadToCache(
             case BlockType::kDeprecatedFilter:
               ++get_context->get_context_stats_.num_filter_read;
               break;
-            case BlockType::kData:
-              ++get_context->get_context_stats_.num_data_read;
-              break;
             default:
               break;
           }
@@ -1771,9 +1768,6 @@ Status BlockBasedTable::RetrieveBlock(
         case BlockType::kDeprecatedFilter:
           ++(get_context->get_context_stats_.num_filter_read);
           break;
-        case BlockType::kData:
-          ++(get_context->get_context_stats_.num_data_read);
-          break;
         default:
           break;
       }

table/block_based/block_based_table_reader_sync_and_async.h

@@ -177,9 +177,6 @@ DEFINE_SYNC_AND_ASYNC(void, BlockBasedTable::RetrieveMultipleBlocks)
     size_t& req_idx = req_idx_for_block[valid_batch_idx];
     size_t& req_offset = req_offset_for_block[valid_batch_idx];
     valid_batch_idx++;
-    if (mget_iter->get_context) {
-      ++(mget_iter->get_context->get_context_stats_.num_data_read);
-    }
     FSReadRequest& req = read_reqs[req_idx];
     Status s = req.status;
     if (s.ok()) {

table/get_context.h

@@ -53,7 +53,6 @@ struct GetContextStats {
   // MultiGet stats.
   uint64_t num_filter_read = 0;
   uint64_t num_index_read = 0;
-  uint64_t num_data_read = 0;
   uint64_t num_sst_read = 0;
 };