Add histogram statistic for number of compaction input iterators

db/db_compaction_test.cc

@@ -10733,6 +10733,41 @@ TEST_F(DBCompactionTest, RecordNewestKeyTimeForTtlCompaction) {
   ASSERT_OK(dbfull()->TEST_WaitForCompact());
   ASSERT_EQ(NumTableFilesAtLevel(0), 0);
 }
+
+TEST_F(DBCompactionTest, RecordNumberCompactionInputIterators) {
+  Options options;
+  SetTimeElapseOnlySleepOnReopen(&options);
+  options.env = CurrentOptions().env;
+  options.compaction_style = kCompactionStyleFIFO;
+  options.statistics = ROCKSDB_NAMESPACE::CreateDBStatistics();
+  options.write_buffer_size = 10 << 10;  // 10KB
+  options.arena_block_size = 4096;
+  options.compression = kNoCompression;
+  options.create_if_missing = true;
+  options.compaction_options_fifo.allow_compaction = true;
+  options.num_levels = 1;
+  env_->SetMockSleep();
+  options.env = env_;
+  options.ttl = 1 * 60 * 60;  // 1 hour
+  ASSERT_OK(TryReopen(options));
+
+  // Generate and flush 4 files, each about 10KB
+  Random rnd(301);
+  for (int i = 0; i < 4; i++) {
+    for (int j = 0; j < 10; j++) {
+      ASSERT_OK(Put(std::to_string(i * 20 + j), rnd.RandomString(980)));
+    }
+    ASSERT_OK(Flush());
+    env_->MockSleepForSeconds(5);
+  }
+  ASSERT_OK(dbfull()->TEST_WaitForCompact());
+  ASSERT_EQ(NumTableFilesAtLevel(0), 1);
+  // We expect 4 input iterators (1 per L0 file)
+  HistogramData num_compaction_input_iterators;
+  options.statistics->histogramData(NUM_COMPACTION_INPUT_ITERATORS,
+                                    &num_compaction_input_iterators);
+  ASSERT_EQ(num_compaction_input_iterators.sum, 4);
+}
 }  // namespace ROCKSDB_NAMESPACE
 
 int main(int argc, char** argv) {

db/db_impl/db_impl.h

@@ -2430,6 +2430,8 @@ class DBImpl : public DB {
                                const std::vector<CompactionInputFiles>& inputs,
                                bool* sfm_bookkeeping, LogBuffer* log_buffer);
 
+  size_t GetNumberCompactionInputIterators(Compaction* c);
+
   // Request compaction tasks token from compaction thread limiter.
   // It always succeeds if force = true or limiter is disable.
   bool RequestCompactionToken(ColumnFamilyData* cfd, bool force,

db/db_impl/db_impl_compaction_flush.cc

@@ -61,6 +61,17 @@ bool DBImpl::EnoughRoomForCompaction(
   return enough_room;
 }
 
+size_t DBImpl::GetNumberCompactionInputIterators(Compaction* c) {
+  assert(c);
+  if (c->start_level() == 0) {
+    assert(0 < c->num_input_levels());
+    size_t num_l0_files = c->num_input_files(0);
+    size_t num_non_l0_levels = c->num_input_levels() - 1;
+    return num_l0_files + num_non_l0_levels;
+  }
+  return c->num_input_levels();
+}
+
 bool DBImpl::RequestCompactionToken(ColumnFamilyData* cfd, bool force,
                                     std::unique_ptr<TaskLimiterToken>* token,
                                     LogBuffer* log_buffer) {
@@ -3720,6 +3731,10 @@ Status DBImpl::BackgroundCompaction(bool* made_progress,
             num_files += each_level.files.size();
           }
           RecordInHistogram(stats_, NUM_FILES_IN_SINGLE_COMPACTION, num_files);
+          size_t num_compaction_input_iterators =
+              GetNumberCompactionInputIterators(c.get());
+          RecordInHistogram(stats_, NUM_COMPACTION_INPUT_ITERATORS,
+                            num_compaction_input_iterators);
 
           // There are three things that can change compaction score:
           // 1) When flush or compaction finish. This case is covered by

include/rocksdb/statistics.h

@@ -667,6 +667,10 @@ enum Histograms : uint32_t {
   // system's prefetch) from the end of SST table during block based table open
   TABLE_OPEN_PREFETCH_TAIL_READ_BYTES,
 
+  // Number of iterators needed to process compaction inputs
+  // Equal to number of L0 input files + number of non-L0 input levels
+  NUM_COMPACTION_INPUT_ITERATORS,
+
   HISTOGRAM_ENUM_MAX
 };
 

java/rocksjni/portal.h

@@ -5889,9 +5889,10 @@ class HistogramTypeJni {
         return 0x3C;
       case ROCKSDB_NAMESPACE::Histograms::TABLE_OPEN_PREFETCH_TAIL_READ_BYTES:
         return 0x3D;
-      case ROCKSDB_NAMESPACE::Histograms::HISTOGRAM_ENUM_MAX:
-        // 0x3D for backwards compatibility on current minor version.
+      case ROCKSDB_NAMESPACE::Histograms::NUM_COMPACTION_INPUT_ITERATORS:
         return 0x3E;
+      case ROCKSDB_NAMESPACE::Histograms::HISTOGRAM_ENUM_MAX:
+        return 0x3F;
       default:
         // undefined/default
         return 0x0;
@@ -6034,9 +6035,9 @@ class HistogramTypeJni {
         return ROCKSDB_NAMESPACE::Histograms::
             TABLE_OPEN_PREFETCH_TAIL_READ_BYTES;
       case 0x3E:
-        // 0x1F for backwards compatibility on current minor version.
+        return ROCKSDB_NAMESPACE::Histograms::NUM_COMPACTION_INPUT_ITERATORS;
+      case 0x3F:
         return ROCKSDB_NAMESPACE::Histograms::HISTOGRAM_ENUM_MAX;
-
       default:
         // undefined/default
         return ROCKSDB_NAMESPACE::Histograms::DB_GET;

java/src/main/java/org/rocksdb/HistogramType.java

@@ -210,8 +210,13 @@ public enum HistogramType {
    */
   TABLE_OPEN_PREFETCH_TAIL_READ_BYTES((byte) 0x3D),
 
-  // 0x3E for backwards compatibility on current minor version.
-  HISTOGRAM_ENUM_MAX((byte) 0x3E);
+  /**
+   * Number of iterators needed to process compaction inputs
+   * Equal to number of L0 input files + number of non-L0 input levels
+   */
+  NUM_COMPACTION_INPUT_ITERATORS((byte) 0x3E),
+
+  HISTOGRAM_ENUM_MAX((byte) 0x3F);
 
   private final byte value;
 

monitoring/statistics.cc

@@ -340,6 +340,7 @@ const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {
     {ASYNC_PREFETCH_ABORT_MICROS, "rocksdb.async.prefetch.abort.micros"},
     {TABLE_OPEN_PREFETCH_TAIL_READ_BYTES,
      "rocksdb.table.open.prefetch.tail.read.bytes"},
+    {NUM_COMPACTION_INPUT_ITERATORS, "rocksdb.num.compaction.iterators"},
 };
 
 std::shared_ptr<Statistics> CreateDBStatistics() {