Add DB property "rocksdb.estimate-table-readers-mem"

Summary:
Add a DB Property "rocksdb.estimate-table-readers-mem" to return estimated memory usage by all loaded table readers, other than allocated from block cache.

Refactor the property codes to allow getting property from a version, with DB mutex not acquired.

Test Plan: Add several checks of this new property in existing codes for various cases.

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: xjin, igor, leveldb

Differential Revision: https://reviews.facebook.net/D20733

db/db_impl.cc

@@ -514,17 +514,21 @@ void DBImpl::MaybeDumpStats() {
     // period in rare cases.
     last_stats_dump_time_microsec_ = now_micros;
 
-    DBPropertyType cf_property_type = GetPropertyType("rocksdb.cfstats");
-    DBPropertyType db_property_type = GetPropertyType("rocksdb.dbstats");
+    bool tmp1 = false;
+    bool tmp2 = false;
+    DBPropertyType cf_property_type =
+        GetPropertyType("rocksdb.cfstats", &tmp1, &tmp2);
+    DBPropertyType db_property_type =
+        GetPropertyType("rocksdb.dbstats", &tmp1, &tmp2);
     std::string stats;
     {
       MutexLock l(&mutex_);
       for (auto cfd : *versions_->GetColumnFamilySet()) {
-        cfd->internal_stats()->GetProperty(
-            cf_property_type, "rocksdb.cfstats", &stats);
+        cfd->internal_stats()->GetStringProperty(cf_property_type,
+                                                 "rocksdb.cfstats", &stats);
       }
-      default_cf_internal_stats_->GetProperty(
-          db_property_type, "rocksdb.dbstats", &stats);
+      default_cf_internal_stats_->GetStringProperty(db_property_type,
+                                                    "rocksdb.dbstats", &stats);
     }
     Log(options_.info_log, "------- DUMPING STATS -------");
     Log(options_.info_log, "%s", stats.c_str());
@@ -3321,15 +3325,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
   }
 
   // Acquire SuperVersion
-  SuperVersion* sv = nullptr;
-  // TODO(ljin): consider using GetReferencedSuperVersion() directly
-  if (LIKELY(options_.allow_thread_local)) {
-    sv = cfd->GetThreadLocalSuperVersion(&mutex_);
-  } else {
-    mutex_.Lock();
-    sv = cfd->GetSuperVersion()->Ref();
-    mutex_.Unlock();
-  }
+  SuperVersion* sv = GetAndRefSuperVersion(cfd);
 
   // Prepare to store a list of merge operations if merge occurs.
   MergeContext merge_context;
@@ -3356,22 +3352,7 @@ Status DBImpl::GetImpl(const ReadOptions& options,
 
   PERF_TIMER_START(get_post_process_time);
 
-  bool unref_sv = true;
-  if (LIKELY(options_.allow_thread_local)) {
-    unref_sv = !cfd->ReturnThreadLocalSuperVersion(sv);
-  }
-
-  if (unref_sv) {
-    // Release SuperVersion
-    if (sv->Unref()) {
-      mutex_.Lock();
-      sv->Cleanup();
-      mutex_.Unlock();
-      delete sv;
-      RecordTick(stats_, NUMBER_SUPERVERSION_CLEANUPS);
-    }
-    RecordTick(stats_, NUMBER_SUPERVERSION_RELEASES);
-  }
+  ReturnAndCleanupSuperVersion(cfd, sv);
 
   RecordTick(stats_, NUMBER_KEYS_READ);
   RecordTick(stats_, BYTES_READ, value->size());
@@ -4372,21 +4353,92 @@ const Options& DBImpl::GetOptions(ColumnFamilyHandle* column_family) const {
 
 bool DBImpl::GetProperty(ColumnFamilyHandle* column_family,
                          const Slice& property, std::string* value) {
+  bool is_int_property;
+  bool need_out_of_mutex;
+  DBPropertyType property_type =
+      GetPropertyType(property, &is_int_property, &need_out_of_mutex);
+
   value->clear();
+  if (is_int_property) {
+    uint64_t int_value;
+    bool ret_value = GetIntPropertyInternal(column_family, property_type,
+                                            need_out_of_mutex, &int_value);
+    if (ret_value) {
+      *value = std::to_string(int_value);
+    }
+    return ret_value;
+  } else {
     auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family);
     auto cfd = cfh->cfd();
-  DBPropertyType property_type = GetPropertyType(property);
     MutexLock l(&mutex_);
-  return cfd->internal_stats()->GetProperty(property_type, property, value);
+    return cfd->internal_stats()->GetStringProperty(property_type, property,
+                                                    value);
+  }
 }
 
 bool DBImpl::GetIntProperty(ColumnFamilyHandle* column_family,
                             const Slice& property, uint64_t* value) {
+  bool is_int_property;
+  bool need_out_of_mutex;
+  DBPropertyType property_type =
+      GetPropertyType(property, &is_int_property, &need_out_of_mutex);
+  if (!is_int_property) {
+    return false;
+  }
+  return GetIntPropertyInternal(column_family, property_type, need_out_of_mutex,
+                                value);
+}
+
+bool DBImpl::GetIntPropertyInternal(ColumnFamilyHandle* column_family,
+                                    DBPropertyType property_type,
+                                    bool need_out_of_mutex, uint64_t* value) {
   auto cfh = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family);
   auto cfd = cfh->cfd();
-  DBPropertyType property_type = GetPropertyType(property);
+
+  if (!need_out_of_mutex) {
     MutexLock l(&mutex_);
-  return cfd->internal_stats()->GetIntProperty(property_type, property, value);
+    return cfd->internal_stats()->GetIntProperty(property_type, value);
+  } else {
+    SuperVersion* sv = GetAndRefSuperVersion(cfd);
+
+    bool ret = cfd->internal_stats()->GetIntPropertyOutOfMutex(
+        property_type, sv->current, value);
+
+    ReturnAndCleanupSuperVersion(cfd, sv);
+
+    return ret;
+  }
+}
+
+SuperVersion* DBImpl::GetAndRefSuperVersion(ColumnFamilyData* cfd) {
+  // TODO(ljin): consider using GetReferencedSuperVersion() directly
+  if (LIKELY(options_.allow_thread_local)) {
+    return cfd->GetThreadLocalSuperVersion(&mutex_);
+  } else {
+    MutexLock l(&mutex_);
+    return cfd->GetSuperVersion()->Ref();
+  }
+}
+
+void DBImpl::ReturnAndCleanupSuperVersion(ColumnFamilyData* cfd,
+                                          SuperVersion* sv) {
+  bool unref_sv = true;
+  if (LIKELY(options_.allow_thread_local)) {
+    unref_sv = !cfd->ReturnThreadLocalSuperVersion(sv);
+  }
+
+  if (unref_sv) {
+    // Release SuperVersion
+    if (sv->Unref()) {
+      {
+        MutexLock l(&mutex_);
+        sv->Cleanup();
+      }
+      delete sv;
+      RecordTick(stats_, NUMBER_SUPERVERSION_CLEANUPS);
+    }
+    RecordTick(stats_, NUMBER_SUPERVERSION_RELEASES);
+  }
 }
 
 void DBImpl::GetApproximateSizes(ColumnFamilyHandle* column_family,

db/db_impl.h

@@ -611,6 +611,16 @@ class DBImpl : public DB {
   void InstallSuperVersion(ColumnFamilyData* cfd,
                            DeletionState& deletion_state);
 
+  // Find Super version and reference it. Based on options, it might return
+  // the thread local cached one.
+  inline SuperVersion* GetAndRefSuperVersion(ColumnFamilyData* cfd);
+
+  // Un-reference the super version and return it to thread local cache if
+  // needed. If it is the last reference of the super version. Clean it up
+  // after un-referencing it.
+  inline void ReturnAndCleanupSuperVersion(ColumnFamilyData* cfd,
+                                           SuperVersion* sv);
+
 #ifndef ROCKSDB_LITE
   using DB::GetPropertiesOfAllTables;
   virtual Status GetPropertiesOfAllTables(ColumnFamilyHandle* column_family,
@@ -623,6 +633,10 @@ class DBImpl : public DB {
   Status GetImpl(const ReadOptions& options, ColumnFamilyHandle* column_family,
                  const Slice& key, std::string* value,
                  bool* value_found = nullptr);
+
+  bool GetIntPropertyInternal(ColumnFamilyHandle* column_family,
+                              DBPropertyType property_type,
+                              bool need_out_of_mutex, uint64_t* value);
 };
 
 // Sanitize db options.  The caller should delete result.info_log if

db/db_test.cc

@@ -1186,6 +1186,10 @@ TEST(DBTest, IndexAndFilterBlocksOfNewTableAddedToCache) {
   ASSERT_EQ(2, /* only index/filter were added */
             TestGetTickerCount(options, BLOCK_CACHE_ADD));
   ASSERT_EQ(0, TestGetTickerCount(options, BLOCK_CACHE_DATA_MISS));
+  uint64_t int_num;
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_EQ(int_num, 0U);
 
   // Make sure filter block is in cache.
   std::string value;
@@ -2489,6 +2493,10 @@ TEST(DBTest, GetProperty) {
   uint64_t int_num;
   SetPerfLevel(kEnableTime);
 
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_EQ(int_num, 0U);
+
   ASSERT_OK(dbfull()->Put(writeOpt, "k1", big_value));
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.num-immutable-mem-table", &num));
   ASSERT_EQ(num, "0");
@@ -2525,6 +2533,10 @@ TEST(DBTest, GetProperty) {
   ASSERT_TRUE(dbfull()->GetIntProperty("rocksdb.estimate-num-keys", &int_num));
   ASSERT_EQ(int_num, 4U);
 
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_EQ(int_num, 0U);
+
   sleeping_task_high.WakeUp();
   sleeping_task_high.WaitUntilDone();
   dbfull()->TEST_WaitForFlushMemTable();
@@ -2538,8 +2550,29 @@ TEST(DBTest, GetProperty) {
   ASSERT_EQ(num, "1");
   ASSERT_TRUE(dbfull()->GetProperty("rocksdb.estimate-num-keys", &num));
   ASSERT_EQ(num, "4");
+
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_GT(int_num, 0U);
+
   sleeping_task_low.WakeUp();
   sleeping_task_low.WaitUntilDone();
+
+  dbfull()->TEST_WaitForFlushMemTable();
+  options.max_open_files = 10;
+  Reopen(&options);
+  // After reopening, no table reader is loaded, so no memory for table readers
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_EQ(int_num, 0U);
+  ASSERT_TRUE(dbfull()->GetIntProperty("rocksdb.estimate-num-keys", &int_num));
+  ASSERT_GT(int_num, 0U);
+
+  // After reading a key, at least one table reader is loaded.
+  Get("k5");
+  ASSERT_TRUE(
+      dbfull()->GetIntProperty("rocksdb.estimate-table-readers-mem", &int_num));
+  ASSERT_GT(int_num, 0U);
 }
 
 TEST(DBTest, FLUSH) {

db/internal_stats.cc

@@ -86,12 +86,17 @@ void PrintLevelStats(char* buf, size_t len, const std::string& name,
 
 }
 
-DBPropertyType GetPropertyType(const Slice& property) {
+DBPropertyType GetPropertyType(const Slice& property, bool* is_int_property,
+                               bool* need_out_of_mutex) {
+  assert(is_int_property != nullptr);
+  assert(need_out_of_mutex != nullptr);
   Slice in = property;
   Slice prefix("rocksdb.");
+  *need_out_of_mutex = false;
   if (!in.starts_with(prefix)) return kUnknown;
   in.remove_prefix(prefix.size());
 
+  *is_int_property = false;
   if (in.starts_with("num-files-at-level")) {
     return kNumFilesAtLevel;
   } else if (in == "levelstats") {
@@ -104,7 +109,10 @@ DBPropertyType GetPropertyType(const Slice& property) {
     return kDBStats;
   } else if (in == "sstables") {
     return kSsTables;
-  } else if (in == "num-immutable-mem-table") {
+  }
+
+  *is_int_property = true;
+  if (in == "num-immutable-mem-table") {
     return kNumImmutableMemTable;
   } else if (in == "mem-table-flush-pending") {
     return kMemtableFlushPending;
@@ -120,21 +128,32 @@ DBPropertyType GetPropertyType(const Slice& property) {
     return kNumEntriesInImmutableMemtable;
   } else if (in == "estimate-num-keys") {
     return kEstimatedNumKeys;
+  } else if (in == "estimate-table-readers-mem") {
+    *need_out_of_mutex = true;
+    return kEstimatedUsageByTableReaders;
   }
   return kUnknown;
 }
 
-bool InternalStats::GetProperty(DBPropertyType property_type,
-                                const Slice& property, std::string* value) {
-  if (property_type > kStartIntTypes) {
-    uint64_t int_value;
-    bool ret_value = GetIntProperty(property_type, property, &int_value);
-    if (ret_value) {
-      *value = std::to_string(int_value);
+bool InternalStats::GetIntPropertyOutOfMutex(DBPropertyType property_type,
+                                             Version* version,
+                                             uint64_t* value) const {
+  assert(value != nullptr);
+  if (property_type != kEstimatedUsageByTableReaders) {
+    return false;
   }
-    return ret_value;
+  if (version == nullptr) {
+    *value = 0;
+  } else {
+    *value = version->GetMemoryUsageByTableReaders();
   }
+  return true;
+}
 
+bool InternalStats::GetStringProperty(DBPropertyType property_type,
+                                      const Slice& property,
+                                      std::string* value) {
+  assert(value != nullptr);
   Version* current = cfd_->current();
   Slice in = property;
 
@@ -169,10 +188,10 @@ bool InternalStats::GetProperty(DBPropertyType property_type,
       return true;
     }
     case kStats: {
-      if (!GetProperty(kCFStats, "rocksdb.cfstats", value)) {
+      if (!GetStringProperty(kCFStats, "rocksdb.cfstats", value)) {
         return false;
       }
-      if (!GetProperty(kDBStats, "rocksdb.dbstats", value)) {
+      if (!GetStringProperty(kDBStats, "rocksdb.dbstats", value)) {
         return false;
       }
       return true;
@@ -194,7 +213,6 @@ bool InternalStats::GetProperty(DBPropertyType property_type,
 }
 
 bool InternalStats::GetIntProperty(DBPropertyType property_type,
-                                   const Slice& property,
                                    uint64_t* value) const {
   Version* current = cfd_->current();
 

db/internal_stats.h

@@ -41,9 +41,12 @@ enum DBPropertyType : uint32_t {
   kNumEntriesInImmutableMemtable,  // Return sum of number of entries in all
                                    // the immutable mem tables.
   kEstimatedNumKeys,  // Estimated total number of keys in the database.
+  kEstimatedUsageByTableReaders,  // Estimated memory by table readers.
 };
 
-extern DBPropertyType GetPropertyType(const Slice& property);
+extern DBPropertyType GetPropertyType(const Slice& property,
+                                      bool* is_int_property,
+                                      bool* need_out_of_mutex);
 
 class InternalStats {
  public:
@@ -191,10 +194,12 @@ class InternalStats {
 
   uint64_t BumpAndGetBackgroundErrorCount() { return ++bg_error_count_; }
 
-  bool GetProperty(DBPropertyType property_type, const Slice& property,
+  bool GetStringProperty(DBPropertyType property_type, const Slice& property,
                          std::string* value);
 
-  bool GetIntProperty(DBPropertyType property_type, const Slice& property,
+  bool GetIntProperty(DBPropertyType property_type, uint64_t* value) const;
+
+  bool GetIntPropertyOutOfMutex(DBPropertyType property_type, Version* version,
                                 uint64_t* value) const;
 
  private:

db/plain_table_db_test.cc

@@ -347,11 +347,20 @@ TEST(PlainTableDBTest, Flush) {
                 NewPlainTableFactory(plain_table_options));
           }
           DestroyAndReopen(&options);
+          uint64_t int_num;
+          ASSERT_TRUE(dbfull()->GetIntProperty(
+              "rocksdb.estimate-table-readers-mem", &int_num));
+          ASSERT_EQ(int_num, 0U);
+
           ASSERT_OK(Put("1000000000000foo", "v1"));
           ASSERT_OK(Put("0000000000000bar", "v2"));
           ASSERT_OK(Put("1000000000000foo", "v3"));
           dbfull()->TEST_FlushMemTable();
 
+          ASSERT_TRUE(dbfull()->GetIntProperty(
+              "rocksdb.estimate-table-readers-mem", &int_num));
+          ASSERT_GT(int_num, 0U);
+
           TablePropertiesCollection ptc;
           reinterpret_cast<DB*>(dbfull())->GetPropertiesOfAllTables(&ptc);
           ASSERT_EQ(1U, ptc.size());

db/simple_table_db_test.cc

@@ -92,6 +92,8 @@ public:
 
   uint64_t ApproximateOffsetOf(const Slice& key) override;
 
+  virtual size_t ApproximateMemoryUsage() const override { return 0; }
+
   void SetupForCompaction() override;
 
   std::shared_ptr<const TableProperties> GetTableProperties() const override;

db/table_cache.cc

@@ -185,6 +185,29 @@ Status TableCache::GetTableProperties(
   return s;
 }
 
+size_t TableCache::GetMemoryUsageByTableReader(
+    const EnvOptions& toptions,
+    const InternalKeyComparator& internal_comparator,
+    const FileDescriptor& fd) {
+  Status s;
+  auto table_reader = fd.table_reader;
+  // table already been pre-loaded?
+  if (table_reader) {
+    return table_reader->ApproximateMemoryUsage();
+  }
+
+  Cache::Handle* table_handle = nullptr;
+  s = FindTable(toptions, internal_comparator, fd, &table_handle, true);
+  if (!s.ok()) {
+    return 0;
+  }
+  assert(table_handle);
+  auto table = GetTableReaderFromHandle(table_handle);
+  auto ret = table->ApproximateMemoryUsage();
+  ReleaseHandle(table_handle);
+  return ret;
+}
+
 void TableCache::Evict(Cache* cache, uint64_t file_number) {
   cache->Erase(GetSliceForFileNumber(&file_number));
 }

db/table_cache.h

@@ -80,6 +80,13 @@ class TableCache {
                             std::shared_ptr<const TableProperties>* properties,
                             bool no_io = false);
 
+  // Return total memory usage of the table reader of the file.
+  // 0 of table reader of the file is not loaded.
+  size_t GetMemoryUsageByTableReader(
+      const EnvOptions& toptions,
+      const InternalKeyComparator& internal_comparator,
+      const FileDescriptor& fd);
+
   // Release the handle from a cache
   void ReleaseHandle(Cache::Handle* handle);
 

db/version_set.cc

@@ -593,6 +593,18 @@ Status Version::GetPropertiesOfAllTables(TablePropertiesCollection* props) {
   return Status::OK();
 }
 
+size_t Version::GetMemoryUsageByTableReaders() {
+  size_t total_usage = 0;
+  for (auto& file_level : file_levels_) {
+    for (size_t i = 0; i < file_level.num_files; i++) {
+      total_usage += cfd_->table_cache()->GetMemoryUsageByTableReader(
+          vset_->storage_options_, cfd_->internal_comparator(),
+          file_level.files[i].fd);
+    }
+  }
+  return total_usage;
+}
+
 uint64_t Version::GetEstimatedActiveKeys() {
   // Estimation will be not accurate when:
   // (1) there is merge keys

db/version_set.h

@@ -234,6 +234,8 @@ class Version {
 
   uint64_t GetEstimatedActiveKeys();
 
+  size_t GetMemoryUsageByTableReaders();
+
   // used to sort files by size
   struct Fsize {
     int index;

table/block.cc

@@ -359,4 +359,15 @@ void Block::SetBlockPrefixIndex(BlockPrefixIndex* prefix_index) {
   prefix_index_.reset(prefix_index);
 }
 
+size_t Block::ApproximateMemoryUsage() const {
+  size_t usage = size();
+  if (hash_index_) {
+    usage += hash_index_->ApproximateMemoryUsage();
+  }
+  if (prefix_index_) {
+    usage += prefix_index_->ApproximateMemoryUsage();
+  }
+  return usage;
+}
+
 }  // namespace rocksdb

table/block.h

@@ -50,6 +50,9 @@ class Block {
   void SetBlockHashIndex(BlockHashIndex* hash_index);
   void SetBlockPrefixIndex(BlockPrefixIndex* prefix_index);
 
+  // Report an approximation of how much memory has been used.
+  size_t ApproximateMemoryUsage() const;
+
  private:
   const char* data_;
   size_t size_;

table/block_based_table_reader.cc

@@ -142,6 +142,10 @@ class BlockBasedTable::IndexReader {
   // The size of the index.
   virtual size_t size() const = 0;
 
+  // Report an approximation of how much memory has been used other than memory
+  // that was allocated in block cache.
+  virtual size_t ApproximateMemoryUsage() const = 0;
+
  protected:
   const Comparator* comparator_;
 };
@@ -176,6 +180,11 @@ class BinarySearchIndexReader : public IndexReader {
 
   virtual size_t size() const override { return index_block_->size(); }
 
+  virtual size_t ApproximateMemoryUsage() const override {
+    assert(index_block_);
+    return index_block_->ApproximateMemoryUsage();
+  }
+
  private:
   BinarySearchIndexReader(const Comparator* comparator, Block* index_block)
       : IndexReader(comparator), index_block_(index_block) {
@@ -292,6 +301,12 @@ class HashIndexReader : public IndexReader {
 
   virtual size_t size() const override { return index_block_->size(); }
 
+  virtual size_t ApproximateMemoryUsage() const override {
+    assert(index_block_);
+    return index_block_->ApproximateMemoryUsage() +
+           prefixes_contents_.data.size();
+  }
+
  private:
   HashIndexReader(const Comparator* comparator, Block* index_block)
       : IndexReader(comparator),
@@ -544,6 +559,17 @@ std::shared_ptr<const TableProperties> BlockBasedTable::GetTableProperties()
   return rep_->table_properties;
 }
 
+size_t BlockBasedTable::ApproximateMemoryUsage() const {
+  size_t usage = 0;
+  if (rep_->filter) {
+    usage += rep_->filter->ApproximateMemoryUsage();
+  }
+  if (rep_->index_reader) {
+    usage += rep_->index_reader->ApproximateMemoryUsage();
+  }
+  return usage;
+}
+
 // Load the meta-block from the file. On success, return the loaded meta block
 // and its iterator.
 Status BlockBasedTable::ReadMetaBlock(

table/block_based_table_reader.h

@@ -96,6 +96,8 @@ class BlockBasedTable : public TableReader {
 
   std::shared_ptr<const TableProperties> GetTableProperties() const override;
 
+  size_t ApproximateMemoryUsage() const override;
+
   ~BlockBasedTable();
 
   bool TEST_filter_block_preloaded() const;

table/cuckoo_table_reader.cc

@@ -268,5 +268,8 @@ Iterator* CuckooTableReader::NewIterator(const ReadOptions&, Arena* arena) {
   }
   return iter;
 }
+
+size_t CuckooTableReader::ApproximateMemoryUsage() const { return 0; }
+
 }  // namespace rocksdb
 #endif

table/cuckoo_table_reader.h

@@ -47,6 +47,9 @@ class CuckooTableReader: public TableReader {
 
   Iterator* NewIterator(const ReadOptions&, Arena* arena = nullptr) override;
 
+  // Report an approximation of how much memory has been used.
+  size_t ApproximateMemoryUsage() const override;
+
   // Following methods are not implemented for Cuckoo Table Reader
   uint64_t ApproximateOffsetOf(const Slice& key) override { return 0; }
   void SetupForCompaction() override {}

table/filter_block.cc

@@ -184,4 +184,7 @@ bool FilterBlockReader::MayMatch(uint64_t block_offset, const Slice& entry) {
   return true;  // Errors are treated as potential matches
 }
 
+size_t FilterBlockReader::ApproximateMemoryUsage() const {
+  return num_ * 4 + 5 + (offset_ - data_);
+}
 }

table/filter_block.h

@@ -74,6 +74,7 @@ class FilterBlockReader {
     bool delete_contents_after_use = false);
   bool KeyMayMatch(uint64_t block_offset, const Slice& key);
   bool PrefixMayMatch(uint64_t block_offset, const Slice& prefix);
+  size_t ApproximateMemoryUsage() const;
 
  private:
   const FilterPolicy* policy_;

table/plain_table_reader.h

@@ -78,6 +78,10 @@ class PlainTableReader: public TableReader {
     return table_properties_;
   }
 
+  virtual size_t ApproximateMemoryUsage() const override {
+    return arena_.MemoryAllocatedBytes();
+  }
+
   PlainTableReader(const Options& options, unique_ptr<RandomAccessFile>&& file,
                    const EnvOptions& storage_options,
                    const InternalKeyComparator& internal_comparator,

table/table_reader.h

@@ -52,6 +52,9 @@ class TableReader {
   // Prepare work that can be done before the real Get()
   virtual void Prepare(const Slice& target) {}
 
+  // Report an approximation of how much memory has been used.
+  virtual size_t ApproximateMemoryUsage() const = 0;
+
   // Calls (*result_handler)(handle_context, ...) repeatedly, starting with
   // the entry found after a call to Seek(key), until result_handler returns
   // false, where k is the actual internal key for a row found and v as the