Added RocksDB stats GET_HIT_L0 and GET_HIT_L1

Summary:
  - In statistics.h , added tickers.
  - In version_set.cc,
  -- Added a getter method for hit_file_level_ in the class FilePicker
  -- Added a line in the Get() method in case of a found, increment the corresponding counters based on the level of the file respectively.

Corresponding task: https://our.intern.facebook.com/intern/tasks/?s=506100481&t=5952818
Personal fork: sycamlaw43@0c3f2e3

Test Plan:
In terminal,
```
make -j32 db_test
ROCKSDB_TESTS=L0L1L2AndUpHitCounter ./db_test
```

Or to use debugger,
```
make -j32 db_test
export ROCKSDB_TESTS=L0L1L2AndUpHitCounter
gdb db_test
```

Reviewers: rven, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D32205

db/db_test.cc

@@ -10330,6 +10330,44 @@ TEST(DBTest, DeleteMovedFileAfterCompaction) {
   }
 }
 
+TEST(DBTest, L0L1L2AndUpHitCounter) {
+  Options options = CurrentOptions();
+  options.write_buffer_size = 32 * 1024;
+  options.target_file_size_base = 32 * 1024;
+  options.level0_file_num_compaction_trigger = 2;
+  options.level0_slowdown_writes_trigger = 2;
+  options.level0_stop_writes_trigger = 4;
+  options.max_bytes_for_level_base = 64 * 1024;
+  options.max_write_buffer_number = 2;
+  options.max_background_compactions = 8;
+  options.max_background_flushes = 8;
+  options.statistics = rocksdb::CreateDBStatistics();
+  CreateAndReopenWithCF({"mypikachu"}, options);
+
+  int numkeys = 20000;
+  for (int i = 0; i < numkeys; i++) {
+    ASSERT_OK(Put(1, Key(i), "val"));
+  }
+  ASSERT_EQ(0, TestGetTickerCount(options, GET_HIT_L0));
+  ASSERT_EQ(0, TestGetTickerCount(options, GET_HIT_L1));
+  ASSERT_EQ(0, TestGetTickerCount(options, GET_HIT_L2_AND_UP));
+
+  ASSERT_OK(Flush(1));
+  dbfull()->TEST_WaitForCompact();
+
+  for (int i = 0; i < numkeys; i++) {
+    ASSERT_EQ(Get(1, Key(i)), "val");
+  }
+
+  ASSERT_GT(TestGetTickerCount(options, GET_HIT_L0), 100);
+  ASSERT_GT(TestGetTickerCount(options, GET_HIT_L1), 100);
+  ASSERT_GT(TestGetTickerCount(options, GET_HIT_L2_AND_UP), 100);
+
+  ASSERT_EQ(numkeys, TestGetTickerCount(options, GET_HIT_L0) +
+                         TestGetTickerCount(options, GET_HIT_L1) +
+                         TestGetTickerCount(options, GET_HIT_L2_AND_UP));
+}
+
 TEST(DBTest, EncodeDecompressedBlockSizeTest) {
   // iter 0 -- zlib
   // iter 1 -- bzip2

db/version_set.cc

@@ -91,6 +91,7 @@ class FilePicker {
       const InternalKeyComparator* internal_comparator)
       : num_levels_(num_levels),
         curr_level_(-1),
+        hit_file_level_(-1),
         search_left_bound_(0),
         search_right_bound_(FileIndexer::kLevelMaxIndex),
 #ifndef NDEBUG
@@ -120,6 +121,7 @@ class FilePicker {
       while (curr_index_in_curr_level_ < curr_file_level_->num_files) {
         // Loops over all files in current level.
         FdWithKeyRange* f = &curr_file_level_->files[curr_index_in_curr_level_];
+        hit_file_level_ = curr_level_;
         int cmp_largest = -1;
 
         // Do key range filtering of files or/and fractional cascading if:
@@ -199,9 +201,14 @@ class FilePicker {
     return nullptr;
   }
 
+  // getter for current file level
+  // for GET_HIT_L0, GET_HIT_L1 & GET_HIT_L2_AND_UP counts
+  unsigned int GetHitFileLevel() { return hit_file_level_; }
+
  private:
   unsigned int num_levels_;
   unsigned int curr_level_;
+  unsigned int hit_file_level_;
   int32_t search_left_bound_;
   int32_t search_right_bound_;
 #ifndef NDEBUG
@@ -800,6 +807,13 @@ void Version::Get(const ReadOptions& read_options,
         // Keep searching in other files
         break;
       case GetContext::kFound:
+        if (fp.GetHitFileLevel() == 0) {
+          RecordTick(db_statistics_, GET_HIT_L0);
+        } else if (fp.GetHitFileLevel() == 1) {
+          RecordTick(db_statistics_, GET_HIT_L1);
+        } else if (fp.GetHitFileLevel() >= 2) {
+          RecordTick(db_statistics_, GET_HIT_L2_AND_UP);
+        }
         return;
       case GetContext::kDeleted:
         // Use empty error message for speed

include/rocksdb/statistics.h

@@ -53,6 +53,13 @@ enum Tickers : uint32_t {
   // # of memtable misses.
   MEMTABLE_MISS,
 
+  // # of Get() queries served by L0
+  GET_HIT_L0,
+  // # of Get() queries served by L1
+  GET_HIT_L1,
+  // # of Get() queries served by L2 and up
+  GET_HIT_L2_AND_UP,
+
   /**
    * COMPACTION_KEY_DROP_* count the reasons for key drop during compaction
    * There are 3 reasons currently.
@@ -150,6 +157,9 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {BLOOM_FILTER_USEFUL, "rocksdb.bloom.filter.useful"},
     {MEMTABLE_HIT, "rocksdb.memtable.hit"},
     {MEMTABLE_MISS, "rocksdb.memtable.miss"},
+    {GET_HIT_L0, "rocksdb.l0.hit"},
+    {GET_HIT_L1, "rocksdb.l1.hit"},
+    {GET_HIT_L2_AND_UP, "rocksdb.l2andup.hit"},
     {COMPACTION_KEY_DROP_NEWER_ENTRY, "rocksdb.compaction.key.drop.new"},
     {COMPACTION_KEY_DROP_OBSOLETE, "rocksdb.compaction.key.drop.obsolete"},
     {COMPACTION_KEY_DROP_USER, "rocksdb.compaction.key.drop.user"},
@@ -194,7 +204,8 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {NUMBER_SUPERVERSION_ACQUIRES, "rocksdb.number.superversion_acquires"},
     {NUMBER_SUPERVERSION_RELEASES, "rocksdb.number.superversion_releases"},
     {NUMBER_SUPERVERSION_CLEANUPS, "rocksdb.number.superversion_cleanups"},
-    {NUMBER_BLOCK_NOT_COMPRESSED, "rocksdb.number.block.not_compressed"}, };
+    {NUMBER_BLOCK_NOT_COMPRESSED, "rocksdb.number.block.not_compressed"},
+};
 
 /**
  * Keep adding histogram's here.