ceph/src/rocksdb/include/rocksdb/statistics.h

   1 // Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 #include <atomic>
   9 #include <cstddef>
  10 #include <cstdint>
  11 #include <map>
  12 #include <memory>
  13 #include <string>
  14 #include <vector>
  15
  16 #include "rocksdb/status.h"
  17
  18 namespace rocksdb {
  19
  20 /**
  21  * Keep adding ticker's here.
  22  *  1. Any ticker should be added before TICKER_ENUM_MAX.
  23  *  2. Add a readable string in TickersNameMap below for the newly added ticker.
  24  *  3. Add a corresponding enum value to TickerType.java in the java API
  25  *  4. Add the enum conversions from Java and C++ to portal.h's toJavaTickerType
  26  * and toCppTickers
  27  */
  28 enum Tickers : uint32_t {
  29   // total block cache misses
  30   // REQUIRES: BLOCK_CACHE_MISS == BLOCK_CACHE_INDEX_MISS +
  31   //                               BLOCK_CACHE_FILTER_MISS +
  32   //                               BLOCK_CACHE_DATA_MISS;
  33   BLOCK_CACHE_MISS = 0,
  34   // total block cache hit
  35   // REQUIRES: BLOCK_CACHE_HIT == BLOCK_CACHE_INDEX_HIT +
  36   //                              BLOCK_CACHE_FILTER_HIT +
  37   //                              BLOCK_CACHE_DATA_HIT;
  38   BLOCK_CACHE_HIT,
  39   // # of blocks added to block cache.
  40   BLOCK_CACHE_ADD,
  41   // # of failures when adding blocks to block cache.
  42   BLOCK_CACHE_ADD_FAILURES,
  43   // # of times cache miss when accessing index block from block cache.
  44   BLOCK_CACHE_INDEX_MISS,
  45   // # of times cache hit when accessing index block from block cache.
  46   BLOCK_CACHE_INDEX_HIT,
  47   // # of index blocks added to block cache.
  48   BLOCK_CACHE_INDEX_ADD,
  49   // # of bytes of index blocks inserted into cache
  50   BLOCK_CACHE_INDEX_BYTES_INSERT,
  51   // # of bytes of index block erased from cache
  52   BLOCK_CACHE_INDEX_BYTES_EVICT,
  53   // # of times cache miss when accessing filter block from block cache.
  54   BLOCK_CACHE_FILTER_MISS,
  55   // # of times cache hit when accessing filter block from block cache.
  56   BLOCK_CACHE_FILTER_HIT,
  57   // # of filter blocks added to block cache.
  58   BLOCK_CACHE_FILTER_ADD,
  59   // # of bytes of bloom filter blocks inserted into cache
  60   BLOCK_CACHE_FILTER_BYTES_INSERT,
  61   // # of bytes of bloom filter block erased from cache
  62   BLOCK_CACHE_FILTER_BYTES_EVICT,
  63   // # of times cache miss when accessing data block from block cache.
  64   BLOCK_CACHE_DATA_MISS,
  65   // # of times cache hit when accessing data block from block cache.
  66   BLOCK_CACHE_DATA_HIT,
  67   // # of data blocks added to block cache.
  68   BLOCK_CACHE_DATA_ADD,
  69   // # of bytes of data blocks inserted into cache
  70   BLOCK_CACHE_DATA_BYTES_INSERT,
  71   // # of bytes read from cache.
  72   BLOCK_CACHE_BYTES_READ,
  73   // # of bytes written into cache.
  74   BLOCK_CACHE_BYTES_WRITE,
  75
  76   // # of times bloom filter has avoided file reads, i.e., negatives.
  77   BLOOM_FILTER_USEFUL,
  78   // # of times bloom FullFilter has not avoided the reads.
  79   BLOOM_FILTER_FULL_POSITIVE,
  80   // # of times bloom FullFilter has not avoided the reads and data actually
  81   // exist.
  82   BLOOM_FILTER_FULL_TRUE_POSITIVE,
  83
  84   // # persistent cache hit
  85   PERSISTENT_CACHE_HIT,
  86   // # persistent cache miss
  87   PERSISTENT_CACHE_MISS,
  88
  89   // # total simulation block cache hits
  90   SIM_BLOCK_CACHE_HIT,
  91   // # total simulation block cache misses
  92   SIM_BLOCK_CACHE_MISS,
  93
  94   // # of memtable hits.
  95   MEMTABLE_HIT,
  96   // # of memtable misses.
  97   MEMTABLE_MISS,
  98
  99   // # of Get() queries served by L0
 100   GET_HIT_L0,
 101   // # of Get() queries served by L1
 102   GET_HIT_L1,
 103   // # of Get() queries served by L2 and up
 104   GET_HIT_L2_AND_UP,
 105
 106   /**
 107    * COMPACTION_KEY_DROP_* count the reasons for key drop during compaction
 108    * There are 4 reasons currently.
 109    */
 110   COMPACTION_KEY_DROP_NEWER_ENTRY,  // key was written with a newer value.
 111                                     // Also includes keys dropped for range del.
 112   COMPACTION_KEY_DROP_OBSOLETE,     // The key is obsolete.
 113   COMPACTION_KEY_DROP_RANGE_DEL,    // key was covered by a range tombstone.
 114   COMPACTION_KEY_DROP_USER,  // user compaction function has dropped the key.
 115   COMPACTION_RANGE_DEL_DROP_OBSOLETE,  // all keys in range were deleted.
 116   // Deletions obsoleted before bottom level due to file gap optimization.
 117   COMPACTION_OPTIMIZED_DEL_DROP_OBSOLETE,
 118   // If a compaction was cancelled in sfm to prevent ENOSPC
 119   COMPACTION_CANCELLED,
 120
 121   // Number of keys written to the database via the Put and Write call's
 122   NUMBER_KEYS_WRITTEN,
 123   // Number of Keys read,
 124   NUMBER_KEYS_READ,
 125   // Number keys updated, if inplace update is enabled
 126   NUMBER_KEYS_UPDATED,
 127   // The number of uncompressed bytes issued by DB::Put(), DB::Delete(),
 128   // DB::Merge(), and DB::Write().
 129   BYTES_WRITTEN,
 130   // The number of uncompressed bytes read from DB::Get().  It could be
 131   // either from memtables, cache, or table files.
 132   // For the number of logical bytes read from DB::MultiGet(),
 133   // please use NUMBER_MULTIGET_BYTES_READ.
 134   BYTES_READ,
 135   // The number of calls to seek/next/prev
 136   NUMBER_DB_SEEK,
 137   NUMBER_DB_NEXT,
 138   NUMBER_DB_PREV,
 139   // The number of calls to seek/next/prev that returned data
 140   NUMBER_DB_SEEK_FOUND,
 141   NUMBER_DB_NEXT_FOUND,
 142   NUMBER_DB_PREV_FOUND,
 143   // The number of uncompressed bytes read from an iterator.
 144   // Includes size of key and value.
 145   ITER_BYTES_READ,
 146   NO_FILE_CLOSES,
 147   NO_FILE_OPENS,
 148   NO_FILE_ERRORS,
 149   // DEPRECATED Time system had to wait to do LO-L1 compactions
 150   STALL_L0_SLOWDOWN_MICROS,
 151   // DEPRECATED Time system had to wait to move memtable to L1.
 152   STALL_MEMTABLE_COMPACTION_MICROS,
 153   // DEPRECATED write throttle because of too many files in L0
 154   STALL_L0_NUM_FILES_MICROS,
 155   // Writer has to wait for compaction or flush to finish.
 156   STALL_MICROS,
 157   // The wait time for db mutex.
 158   // Disabled by default. To enable it set stats level to kAll
 159   DB_MUTEX_WAIT_MICROS,
 160   RATE_LIMIT_DELAY_MILLIS,
 161   // DEPRECATED number of iterators currently open
 162   NO_ITERATORS,
 163
 164   // Number of MultiGet calls, keys read, and bytes read
 165   NUMBER_MULTIGET_CALLS,
 166   NUMBER_MULTIGET_KEYS_READ,
 167   NUMBER_MULTIGET_BYTES_READ,
 168
 169   // Number of deletes records that were not required to be
 170   // written to storage because key does not exist
 171   NUMBER_FILTERED_DELETES,
 172   NUMBER_MERGE_FAILURES,
 173
 174   // number of times bloom was checked before creating iterator on a
 175   // file, and the number of times the check was useful in avoiding
 176   // iterator creation (and thus likely IOPs).
 177   BLOOM_FILTER_PREFIX_CHECKED,
 178   BLOOM_FILTER_PREFIX_USEFUL,
 179
 180   // Number of times we had to reseek inside an iteration to skip
 181   // over large number of keys with same userkey.
 182   NUMBER_OF_RESEEKS_IN_ITERATION,
 183
 184   // Record the number of calls to GetUpadtesSince. Useful to keep track of
 185   // transaction log iterator refreshes
 186   GET_UPDATES_SINCE_CALLS,
 187   BLOCK_CACHE_COMPRESSED_MISS,  // miss in the compressed block cache
 188   BLOCK_CACHE_COMPRESSED_HIT,   // hit in the compressed block cache
 189   // Number of blocks added to compressed block cache
 190   BLOCK_CACHE_COMPRESSED_ADD,
 191   // Number of failures when adding blocks to compressed block cache
 192   BLOCK_CACHE_COMPRESSED_ADD_FAILURES,
 193   WAL_FILE_SYNCED,  // Number of times WAL sync is done
 194   WAL_FILE_BYTES,   // Number of bytes written to WAL
 195
 196   // Writes can be processed by requesting thread or by the thread at the
 197   // head of the writers queue.
 198   WRITE_DONE_BY_SELF,
 199   WRITE_DONE_BY_OTHER,  // Equivalent to writes done for others
 200   WRITE_TIMEDOUT,       // Number of writes ending up with timed-out.
 201   WRITE_WITH_WAL,       // Number of Write calls that request WAL
 202   COMPACT_READ_BYTES,   // Bytes read during compaction
 203   COMPACT_WRITE_BYTES,  // Bytes written during compaction
 204   FLUSH_WRITE_BYTES,    // Bytes written during flush
 205
 206   // Number of table's properties loaded directly from file, without creating
 207   // table reader object.
 208   NUMBER_DIRECT_LOAD_TABLE_PROPERTIES,
 209   NUMBER_SUPERVERSION_ACQUIRES,
 210   NUMBER_SUPERVERSION_RELEASES,
 211   NUMBER_SUPERVERSION_CLEANUPS,
 212
 213   // # of compressions/decompressions executed
 214   NUMBER_BLOCK_COMPRESSED,
 215   NUMBER_BLOCK_DECOMPRESSED,
 216
 217   NUMBER_BLOCK_NOT_COMPRESSED,
 218   MERGE_OPERATION_TOTAL_TIME,
 219   FILTER_OPERATION_TOTAL_TIME,
 220
 221   // Row cache.
 222   ROW_CACHE_HIT,
 223   ROW_CACHE_MISS,
 224
 225   // Read amplification statistics.
 226   // Read amplification can be calculated using this formula
 227   // (READ_AMP_TOTAL_READ_BYTES / READ_AMP_ESTIMATE_USEFUL_BYTES)
 228   //
 229   // REQUIRES: ReadOptions::read_amp_bytes_per_bit to be enabled
 230   READ_AMP_ESTIMATE_USEFUL_BYTES,  // Estimate of total bytes actually used.
 231   READ_AMP_TOTAL_READ_BYTES,       // Total size of loaded data blocks.
 232
 233   // Number of refill intervals where rate limiter's bytes are fully consumed.
 234   NUMBER_RATE_LIMITER_DRAINS,
 235
 236   // Number of internal keys skipped by Iterator
 237   NUMBER_ITER_SKIP,
 238
 239   // BlobDB specific stats
 240   // # of Put/PutTTL/PutUntil to BlobDB.
 241   BLOB_DB_NUM_PUT,
 242   // # of Write to BlobDB.
 243   BLOB_DB_NUM_WRITE,
 244   // # of Get to BlobDB.
 245   BLOB_DB_NUM_GET,
 246   // # of MultiGet to BlobDB.
 247   BLOB_DB_NUM_MULTIGET,
 248   // # of Seek/SeekToFirst/SeekToLast/SeekForPrev to BlobDB iterator.
 249   BLOB_DB_NUM_SEEK,
 250   // # of Next to BlobDB iterator.
 251   BLOB_DB_NUM_NEXT,
 252   // # of Prev to BlobDB iterator.
 253   BLOB_DB_NUM_PREV,
 254   // # of keys written to BlobDB.
 255   BLOB_DB_NUM_KEYS_WRITTEN,
 256   // # of keys read from BlobDB.
 257   BLOB_DB_NUM_KEYS_READ,
 258   // # of bytes (key + value) written to BlobDB.
 259   BLOB_DB_BYTES_WRITTEN,
 260   // # of bytes (keys + value) read from BlobDB.
 261   BLOB_DB_BYTES_READ,
 262   // # of keys written by BlobDB as non-TTL inlined value.
 263   BLOB_DB_WRITE_INLINED,
 264   // # of keys written by BlobDB as TTL inlined value.
 265   BLOB_DB_WRITE_INLINED_TTL,
 266   // # of keys written by BlobDB as non-TTL blob value.
 267   BLOB_DB_WRITE_BLOB,
 268   // # of keys written by BlobDB as TTL blob value.
 269   BLOB_DB_WRITE_BLOB_TTL,
 270   // # of bytes written to blob file.
 271   BLOB_DB_BLOB_FILE_BYTES_WRITTEN,
 272   // # of bytes read from blob file.
 273   BLOB_DB_BLOB_FILE_BYTES_READ,
 274   // # of times a blob files being synced.
 275   BLOB_DB_BLOB_FILE_SYNCED,
 276   // # of blob index evicted from base DB by BlobDB compaction filter because
 277   // of expiration.
 278   BLOB_DB_BLOB_INDEX_EXPIRED_COUNT,
 279   // size of blob index evicted from base DB by BlobDB compaction filter
 280   // because of expiration.
 281   BLOB_DB_BLOB_INDEX_EXPIRED_SIZE,
 282   // # of blob index evicted from base DB by BlobDB compaction filter because
 283   // of corresponding file deleted.
 284   BLOB_DB_BLOB_INDEX_EVICTED_COUNT,
 285   // size of blob index evicted from base DB by BlobDB compaction filter
 286   // because of corresponding file deleted.
 287   BLOB_DB_BLOB_INDEX_EVICTED_SIZE,
 288   // # of blob files being garbage collected.
 289   BLOB_DB_GC_NUM_FILES,
 290   // # of blob files generated by garbage collection.
 291   BLOB_DB_GC_NUM_NEW_FILES,
 292   // # of BlobDB garbage collection failures.
 293   BLOB_DB_GC_FAILURES,
 294   // # of keys drop by BlobDB garbage collection because they had been
 295   // overwritten.
 296   BLOB_DB_GC_NUM_KEYS_OVERWRITTEN,
 297   // # of keys drop by BlobDB garbage collection because of expiration.
 298   BLOB_DB_GC_NUM_KEYS_EXPIRED,
 299   // # of keys relocated to new blob file by garbage collection.
 300   BLOB_DB_GC_NUM_KEYS_RELOCATED,
 301   // # of bytes drop by BlobDB garbage collection because they had been
 302   // overwritten.
 303   BLOB_DB_GC_BYTES_OVERWRITTEN,
 304   // # of bytes drop by BlobDB garbage collection because of expiration.
 305   BLOB_DB_GC_BYTES_EXPIRED,
 306   // # of bytes relocated to new blob file by garbage collection.
 307   BLOB_DB_GC_BYTES_RELOCATED,
 308   // # of blob files evicted because of BlobDB is full.
 309   BLOB_DB_FIFO_NUM_FILES_EVICTED,
 310   // # of keys in the blob files evicted because of BlobDB is full.
 311   BLOB_DB_FIFO_NUM_KEYS_EVICTED,
 312   // # of bytes in the blob files evicted because of BlobDB is full.
 313   BLOB_DB_FIFO_BYTES_EVICTED,
 314
 315   // These counters indicate a performance issue in WritePrepared transactions.
 316   // We should not seem them ticking them much.
 317   // # of times prepare_mutex_ is acquired in the fast path.
 318   TXN_PREPARE_MUTEX_OVERHEAD,
 319   // # of times old_commit_map_mutex_ is acquired in the fast path.
 320   TXN_OLD_COMMIT_MAP_MUTEX_OVERHEAD,
 321   // # of times we checked a batch for duplicate keys.
 322   TXN_DUPLICATE_KEY_OVERHEAD,
 323   // # of times snapshot_mutex_ is acquired in the fast path.
 324   TXN_SNAPSHOT_MUTEX_OVERHEAD,
 325
 326   // Number of keys actually found in MultiGet calls (vs number requested by
 327   // caller)
 328   // NUMBER_MULTIGET_KEYS_READ gives the number requested by caller
 329   NUMBER_MULTIGET_KEYS_FOUND,
 330
 331   NO_ITERATOR_CREATED,  // number of iterators created
 332   NO_ITERATOR_DELETED,  // number of iterators deleted
 333
 334   BLOCK_CACHE_COMPRESSION_DICT_MISS,
 335   BLOCK_CACHE_COMPRESSION_DICT_HIT,
 336   BLOCK_CACHE_COMPRESSION_DICT_ADD,
 337   BLOCK_CACHE_COMPRESSION_DICT_BYTES_INSERT,
 338   BLOCK_CACHE_COMPRESSION_DICT_BYTES_EVICT,
 339   TICKER_ENUM_MAX
 340 };
 341
 342 // The order of items listed in  Tickers should be the same as
 343 // the order listed in TickersNameMap
 344 extern const std::vector<std::pair<Tickers, std::string>> TickersNameMap;
 345
 346 /**
 347  * Keep adding histogram's here.
 348  * Any histogram should have value less than HISTOGRAM_ENUM_MAX
 349  * Add a new Histogram by assigning it the current value of HISTOGRAM_ENUM_MAX
 350  * Add a string representation in HistogramsNameMap below
 351  * And increment HISTOGRAM_ENUM_MAX
 352  * Add a corresponding enum value to HistogramType.java in the java API
 353  */
 354 enum Histograms : uint32_t {
 355   DB_GET = 0,
 356   DB_WRITE,
 357   COMPACTION_TIME,
 358   COMPACTION_CPU_TIME,
 359   SUBCOMPACTION_SETUP_TIME,
 360   TABLE_SYNC_MICROS,
 361   COMPACTION_OUTFILE_SYNC_MICROS,
 362   WAL_FILE_SYNC_MICROS,
 363   MANIFEST_FILE_SYNC_MICROS,
 364   // TIME SPENT IN IO DURING TABLE OPEN
 365   TABLE_OPEN_IO_MICROS,
 366   DB_MULTIGET,
 367   READ_BLOCK_COMPACTION_MICROS,
 368   READ_BLOCK_GET_MICROS,
 369   WRITE_RAW_BLOCK_MICROS,
 370   STALL_L0_SLOWDOWN_COUNT,
 371   STALL_MEMTABLE_COMPACTION_COUNT,
 372   STALL_L0_NUM_FILES_COUNT,
 373   HARD_RATE_LIMIT_DELAY_COUNT,
 374   SOFT_RATE_LIMIT_DELAY_COUNT,
 375   NUM_FILES_IN_SINGLE_COMPACTION,
 376   DB_SEEK,
 377   WRITE_STALL,
 378   SST_READ_MICROS,
 379   // The number of subcompactions actually scheduled during a compaction
 380   NUM_SUBCOMPACTIONS_SCHEDULED,
 381   // Value size distribution in each operation
 382   BYTES_PER_READ,
 383   BYTES_PER_WRITE,
 384   BYTES_PER_MULTIGET,
 385
 386   // number of bytes compressed/decompressed
 387   // number of bytes is when uncompressed; i.e. before/after respectively
 388   BYTES_COMPRESSED,
 389   BYTES_DECOMPRESSED,
 390   COMPRESSION_TIMES_NANOS,
 391   DECOMPRESSION_TIMES_NANOS,
 392   // Number of merge operands passed to the merge operator in user read
 393   // requests.
 394   READ_NUM_MERGE_OPERANDS,
 395
 396   // BlobDB specific stats
 397   // Size of keys written to BlobDB.
 398   BLOB_DB_KEY_SIZE,
 399   // Size of values written to BlobDB.
 400   BLOB_DB_VALUE_SIZE,
 401   // BlobDB Put/PutWithTTL/PutUntil/Write latency.
 402   BLOB_DB_WRITE_MICROS,
 403   // BlobDB Get lagency.
 404   BLOB_DB_GET_MICROS,
 405   // BlobDB MultiGet latency.
 406   BLOB_DB_MULTIGET_MICROS,
 407   // BlobDB Seek/SeekToFirst/SeekToLast/SeekForPrev latency.
 408   BLOB_DB_SEEK_MICROS,
 409   // BlobDB Next latency.
 410   BLOB_DB_NEXT_MICROS,
 411   // BlobDB Prev latency.
 412   BLOB_DB_PREV_MICROS,
 413   // Blob file write latency.
 414   BLOB_DB_BLOB_FILE_WRITE_MICROS,
 415   // Blob file read latency.
 416   BLOB_DB_BLOB_FILE_READ_MICROS,
 417   // Blob file sync latency.
 418   BLOB_DB_BLOB_FILE_SYNC_MICROS,
 419   // BlobDB garbage collection time.
 420   BLOB_DB_GC_MICROS,
 421   // BlobDB compression time.
 422   BLOB_DB_COMPRESSION_MICROS,
 423   // BlobDB decompression time.
 424   BLOB_DB_DECOMPRESSION_MICROS,
 425   // Time spent flushing memtable to disk
 426   FLUSH_TIME,
 427
 428   HISTOGRAM_ENUM_MAX,
 429 };
 430
 431 extern const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap;
 432
 433 struct HistogramData {
 434   double median;
 435   double percentile95;
 436   double percentile99;
 437   double average;
 438   double standard_deviation;
 439   // zero-initialize new members since old Statistics::histogramData()
 440   // implementations won't write them.
 441   double max = 0.0;
 442   uint64_t count = 0;
 443   uint64_t sum = 0;
 444   double min = 0.0;
 445 };
 446
 447 enum StatsLevel : uint8_t {
 448   // Disable timer stats, and skip histogram stats
 449   kExceptHistogramOrTimers,
 450   // Skip timer stats
 451   kExceptTimers,
 452   // Collect all stats except time inside mutex lock AND time spent on
 453   // compression.
 454   kExceptDetailedTimers,
 455   // Collect all stats except the counters requiring to get time inside the
 456   // mutex lock.
 457   kExceptTimeForMutex,
 458   // Collect all stats, including measuring duration of mutex operations.
 459   // If getting time is expensive on the platform to run, it can
 460   // reduce scalability to more threads, especially for writes.
 461   kAll,
 462 };
 463
 464 // Analyze the performance of a db
 465 class Statistics {
 466  public:
 467   virtual ~Statistics() {}
 468
 469   virtual uint64_t getTickerCount(uint32_t tickerType) const = 0;
 470   virtual void histogramData(uint32_t type,
 471                              HistogramData* const data) const = 0;
 472   virtual std::string getHistogramString(uint32_t /*type*/) const { return ""; }
 473   virtual void recordTick(uint32_t tickerType, uint64_t count = 0) = 0;
 474   virtual void setTickerCount(uint32_t tickerType, uint64_t count) = 0;
 475   virtual uint64_t getAndResetTickerCount(uint32_t tickerType) = 0;
 476   virtual void reportTimeToHistogram(uint32_t histogramType, uint64_t time) {
 477     if (get_stats_level() <= StatsLevel::kExceptTimers) {
 478       return;
 479     }
 480     recordInHistogram(histogramType, time);
 481   }
 482   // The function is here only for backward compatibility reason.
 483   // Users implementing their own Statistics class should override
 484   // recordInHistogram() instead and leave measureTime() as it is.
 485   virtual void measureTime(uint32_t /*histogramType*/, uint64_t /*time*/) {
 486     // This is not supposed to be called.
 487     assert(false);
 488   }
 489   virtual void recordInHistogram(uint32_t histogramType, uint64_t time) {
 490     // measureTime() is the old and inaccurate function name.
 491     // To keep backward compatible. If users implement their own
 492     // statistics, which overrides meareTime() but doesn't override
 493     // this function. We forward to measureTime().
 494     measureTime(histogramType, time);
 495   }
 496
 497   // Resets all ticker and histogram stats
 498   virtual Status Reset() { return Status::NotSupported("Not implemented"); }
 499
 500   // String representation of the statistic object.
 501   virtual std::string ToString() const {
 502     // Do nothing by default
 503     return std::string("ToString(): not implemented");
 504   }
 505
 506   virtual bool getTickerMap(std::map<std::string, uint64_t>*) const {
 507     // Do nothing by default
 508     return false;
 509   };
 510
 511   // Override this function to disable particular histogram collection
 512   virtual bool HistEnabledForType(uint32_t type) const {
 513     return type < HISTOGRAM_ENUM_MAX;
 514   }
 515   void set_stats_level(StatsLevel sl) {
 516     stats_level_.store(sl, std::memory_order_relaxed);
 517   }
 518   StatsLevel get_stats_level() const {
 519     return stats_level_.load(std::memory_order_relaxed);
 520   }
 521
 522  private:
 523   std::atomic<StatsLevel> stats_level_{kExceptDetailedTimers};
 524 };
 525
 526 // Create a concrete DBStatistics object
 527 std::shared_ptr<Statistics> CreateDBStatistics();
 528
 529 }  // namespace rocksdb