Introduce Read amplification bitmap (read amp statistics)

Summary:
Add ReadOptions::read_amp_bytes_per_bit option which allow us to create a bitmap for every data block we read
the bitmap will contain (block_size / read_amp_bytes_per_bit) bits.

We will use this bitmap to mark which bytes have been used of the block so we can calculate the read amplification

Test Plan: added new tests

Reviewers: andrewkr, yhchiang, sdong

Reviewed By: sdong

Subscribers: yiwu, leveldb, march, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58707

db/db_test2.cc

@@ -1859,6 +1859,167 @@ TEST_F(DBTest2, MaxSuccessiveMergesInRecovery) {
   options.max_successive_merges = 3;
   Reopen(options);
 }
+
+size_t GetEncodedEntrySize(size_t key_size, size_t value_size) {
+  std::string buffer;
+
+  PutVarint32(&buffer, static_cast<uint32_t>(0));
+  PutVarint32(&buffer, static_cast<uint32_t>(key_size));
+  PutVarint32(&buffer, static_cast<uint32_t>(value_size));
+
+  return buffer.size() + key_size + value_size;
+}
+
+TEST_F(DBTest2, ReadAmpBitmap) {
+  Options options = CurrentOptions();
+  BlockBasedTableOptions bbto;
+  // Disable delta encoding to make it easier to calculate read amplification
+  bbto.use_delta_encoding = false;
+  // Huge block cache to make it easier to calculate read amplification
+  bbto.block_cache = NewLRUCache(1024 * 1024 * 1024);
+  bbto.read_amp_bytes_per_bit = 16;
+  options.table_factory.reset(NewBlockBasedTableFactory(bbto));
+  options.statistics = rocksdb::CreateDBStatistics();
+  DestroyAndReopen(options);
+
+  const size_t kNumEntries = 10000;
+
+  Random rnd(301);
+  for (size_t i = 0; i < kNumEntries; i++) {
+    ASSERT_OK(Put(Key(static_cast<int>(i)), RandomString(&rnd, 100)));
+  }
+  ASSERT_OK(Flush());
+
+  Close();
+  Reopen(options);
+
+  // Read keys/values randomly and verify that reported read amp error
+  // is less than 2%
+  uint64_t total_useful_bytes = 0;
+  std::set<int> read_keys;
+  std::string value;
+  for (size_t i = 0; i < kNumEntries * 5; i++) {
+    int key_idx = rnd.Next() % kNumEntries;
+    std::string k = Key(key_idx);
+    ASSERT_OK(db_->Get(ReadOptions(), k, &value));
+
+    if (read_keys.find(key_idx) == read_keys.end()) {
+      auto ik = InternalKey(k, 0, ValueType::kTypeValue);
+      total_useful_bytes += GetEncodedEntrySize(ik.size(), value.size());
+      read_keys.insert(key_idx);
+    }
+
+    double expected_read_amp =
+        static_cast<double>(total_useful_bytes) /
+        options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+    double read_amp =
+        static_cast<double>(options.statistics->getTickerCount(
+            READ_AMP_ESTIMATE_USEFUL_BYTES)) /
+        options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+    double error_pct = fabs(expected_read_amp - read_amp) * 100;
+    // Error between reported read amp and real read amp should be less than 2%
+    EXPECT_LE(error_pct, 2);
+  }
+
+  // Make sure we read every thing in the DB (which is smaller than our cache)
+  Iterator* iter = db_->NewIterator(ReadOptions());
+  for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
+    ASSERT_EQ(iter->value().ToString(), Get(iter->key().ToString()));
+  }
+  delete iter;
+
+  // Read amp is 100% since we read all what we loaded in memory
+  ASSERT_EQ(options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES),
+            options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES));
+}
+
+TEST_F(DBTest2, ReadAmpBitmapLiveInCacheAfterDBClose) {
+  if (dbname_.find("dev/shm") != std::string::npos) {
+    // /dev/shm dont support getting a unique file id, this mean that
+    // running this test on /dev/shm will fail because lru_cache will load
+    // the blocks again regardless of them being already in the cache
+    return;
+  }
+
+  std::shared_ptr<Cache> lru_cache = NewLRUCache(1024 * 1024 * 1024);
+  std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+
+  Options options = CurrentOptions();
+  BlockBasedTableOptions bbto;
+  // Disable delta encoding to make it easier to calculate read amplification
+  bbto.use_delta_encoding = false;
+  // Huge block cache to make it easier to calculate read amplification
+  bbto.block_cache = lru_cache;
+  bbto.read_amp_bytes_per_bit = 16;
+  options.table_factory.reset(NewBlockBasedTableFactory(bbto));
+  options.statistics = stats;
+  DestroyAndReopen(options);
+
+  const int kNumEntries = 10000;
+
+  Random rnd(301);
+  for (int i = 0; i < kNumEntries; i++) {
+    ASSERT_OK(Put(Key(i), RandomString(&rnd, 100)));
+  }
+  ASSERT_OK(Flush());
+
+  Close();
+  Reopen(options);
+
+  uint64_t total_useful_bytes = 0;
+  std::set<int> read_keys;
+  std::string value;
+  // Iter1: Read half the DB, Read even keys
+  // Key(0), Key(2), Key(4), Key(6), Key(8), ...
+  for (int i = 0; i < kNumEntries; i += 2) {
+    std::string k = Key(i);
+    ASSERT_OK(db_->Get(ReadOptions(), k, &value));
+
+    if (read_keys.find(i) == read_keys.end()) {
+      auto ik = InternalKey(k, 0, ValueType::kTypeValue);
+      total_useful_bytes += GetEncodedEntrySize(ik.size(), value.size());
+      read_keys.insert(i);
+    }
+  }
+
+  size_t total_useful_bytes_iter1 =
+      options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES);
+  size_t total_loaded_bytes_iter1 =
+      options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+  Close();
+  std::shared_ptr<Statistics> new_statistics = rocksdb::CreateDBStatistics();
+  // Destroy old statistics obj that the blocks in lru_cache are pointing to
+  options.statistics.reset();
+  // Use the statistics object that we just created
+  options.statistics = new_statistics;
+  Reopen(options);
+
+  // Iter2: Read half the DB, Read odd keys
+  // Key(1), Key(3), Key(5), Key(7), Key(9), ...
+  for (int i = 1; i < kNumEntries; i += 2) {
+    std::string k = Key(i);
+    ASSERT_OK(db_->Get(ReadOptions(), k, &value));
+
+    if (read_keys.find(i) == read_keys.end()) {
+      auto ik = InternalKey(k, 0, ValueType::kTypeValue);
+      total_useful_bytes += GetEncodedEntrySize(ik.size(), value.size());
+      read_keys.insert(i);
+    }
+  }
+
+  size_t total_useful_bytes_iter2 =
+      options.statistics->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES);
+  size_t total_loaded_bytes_iter2 =
+      options.statistics->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+  // We reached read_amp of 100% because we read all the keys in the DB
+  ASSERT_EQ(total_useful_bytes_iter1 + total_useful_bytes_iter2,
+            total_loaded_bytes_iter1 + total_loaded_bytes_iter2);
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {

include/rocksdb/statistics.h

@@ -197,6 +197,14 @@ enum Tickers : uint32_t {
   ROW_CACHE_HIT,
   ROW_CACHE_MISS,
 
+  // Read amplification statistics.
+  // Read amplification can be calculated using this formula
+  // (READ_AMP_TOTAL_READ_BYTES / READ_AMP_ESTIMATE_USEFUL_BYTES)
+  //
+  // REQUIRES: ReadOptions::read_amp_bytes_per_bit to be enabled
+  READ_AMP_ESTIMATE_USEFUL_BYTES,  // Estimate of total bytes actually used.
+  READ_AMP_TOTAL_READ_BYTES,       // Total size of loaded data blocks.
+
   TICKER_ENUM_MAX
 };
 
@@ -291,6 +299,8 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
     {FILTER_OPERATION_TOTAL_TIME, "rocksdb.filter.operation.time.nanos"},
     {ROW_CACHE_HIT, "rocksdb.row.cache.hit"},
     {ROW_CACHE_MISS, "rocksdb.row.cache.miss"},
+    {READ_AMP_ESTIMATE_USEFUL_BYTES, "rocksdb.read.amp.estimate.useful.bytes"},
+    {READ_AMP_TOTAL_READ_BYTES, "rocksdb.read.amp.total.read.bytes"},
 };
 
 /**

include/rocksdb/table.h

@@ -173,6 +173,29 @@ struct BlockBasedTableOptions {
   // algorithms.
   bool verify_compression = false;
 
+  // If used, For every data block we load into memory, we will create a bitmap
+  // of size ((block_size / `read_amp_bytes_per_bit`) / 8) bytes. This bitmap
+  // will be used to figure out the percentage we actually read of the blocks.
+  //
+  // When this feature is used Tickers::READ_AMP_ESTIMATE_USEFUL_BYTES and
+  // Tickers::READ_AMP_TOTAL_READ_BYTES can be used to calculate the
+  // read amplification using this formula
+  // (READ_AMP_TOTAL_READ_BYTES / READ_AMP_ESTIMATE_USEFUL_BYTES)
+  //
+  // value  =>  memory usage (percentage of loaded blocks memory)
+  // 1      =>  12.50 %
+  // 2      =>  06.25 %
+  // 4      =>  03.12 %
+  // 8      =>  01.56 %
+  // 16     =>  00.78 %
+  //
+  // Note: This number must be a power of 2, if not it will be sanitized
+  // to be the next lowest power of 2, for example a value of 7 will be
+  // treated as 4, a value of 19 will be treated as 16.
+  //
+  // Default: 0 (disabled)
+  uint32_t read_amp_bytes_per_bit = 0;
+
   // We currently have three versions:
   // 0 -- This version is currently written out by all RocksDB's versions by
   // default.  Can be read by really old RocksDB's. Doesn't support changing

table/block.cc

@@ -16,9 +16,11 @@
 #include <unordered_map>
 #include <vector>
 
+#include "port/port.h"
+#include "port/stack_trace.h"
 #include "rocksdb/comparator.h"
-#include "table/format.h"
 #include "table/block_prefix_index.h"
+#include "table/format.h"
 #include "util/coding.h"
 #include "util/logging.h"
 #include "util/perf_context_imp.h"
@@ -344,7 +346,8 @@ uint32_t Block::NumRestarts() const {
   return DecodeFixed32(data_ + size_ - sizeof(uint32_t));
 }
 
-Block::Block(BlockContents&& contents)
+Block::Block(BlockContents&& contents, size_t read_amp_bytes_per_bit,
+             Statistics* statistics)
     : contents_(std::move(contents)),
       data_(contents_.data.data()),
       size_(contents_.data.size()) {
@@ -359,10 +362,14 @@ Block::Block(BlockContents&& contents)
       size_ = 0;
     }
   }
+  if (read_amp_bytes_per_bit != 0 && statistics && size_ != 0) {
+    read_amp_bitmap_.reset(new BlockReadAmpBitmap(
+        restart_offset_, read_amp_bytes_per_bit, statistics));
+  }
 }
 
 InternalIterator* Block::NewIterator(const Comparator* cmp, BlockIter* iter,
-                                     bool total_order_seek) {
+                                     bool total_order_seek, Statistics* stats) {
   if (size_ < 2*sizeof(uint32_t)) {
     if (iter != nullptr) {
       iter->SetStatus(Status::Corruption("bad block contents"));
@@ -385,10 +392,17 @@ InternalIterator* Block::NewIterator(const Comparator* cmp, BlockIter* iter,
 
     if (iter != nullptr) {
       iter->Initialize(cmp, data_, restart_offset_, num_restarts,
-                       prefix_index_ptr);
+                       prefix_index_ptr, read_amp_bitmap_.get());
     } else {
       iter = new BlockIter(cmp, data_, restart_offset_, num_restarts,
-                           prefix_index_ptr);
+                           prefix_index_ptr, read_amp_bitmap_.get());
+    }
+
+    if (read_amp_bitmap_) {
+      if (read_amp_bitmap_->GetStatistics() != stats) {
+        // DB changed the Statistics pointer, we need to notify read_amp_bitmap_
+        read_amp_bitmap_->SetStatistics(stats);
+      }
     }
   }
 

table/block.h

@@ -20,6 +20,7 @@
 #include "db/pinned_iterators_manager.h"
 #include "rocksdb/iterator.h"
 #include "rocksdb/options.h"
+#include "rocksdb/statistics.h"
 #include "table/block_prefix_index.h"
 #include "table/internal_iterator.h"
 
@@ -32,10 +33,119 @@ class Comparator;
 class BlockIter;
 class BlockPrefixIndex;
 
+// BlockReadAmpBitmap is a bitmap that map the rocksdb::Block data bytes to
+// a bitmap with ratio bytes_per_bit. Whenever we access a range of bytes in
+// the Block we update the bitmap and increment READ_AMP_ESTIMATE_USEFUL_BYTES.
+class BlockReadAmpBitmap {
+ public:
+  explicit BlockReadAmpBitmap(size_t block_size, size_t bytes_per_bit,
+                              Statistics* statistics)
+      : bitmap_(nullptr), bytes_per_bit_pow_(0), statistics_(statistics) {
+    assert(block_size > 0 && bytes_per_bit > 0);
+
+    // convert bytes_per_bit to be a power of 2
+    while (bytes_per_bit >>= 1) {
+      bytes_per_bit_pow_++;
+    }
+
+    // num_bits_needed = ceil(block_size / bytes_per_bit)
+    size_t num_bits_needed = (block_size >> bytes_per_bit_pow_) +
+                             (block_size % (1 << bytes_per_bit_pow_) != 0);
+
+    // bitmap_size = ceil(num_bits_needed / kBitsPerEntry)
+    size_t bitmap_size = (num_bits_needed / kBitsPerEntry) +
+                         (num_bits_needed % kBitsPerEntry != 0);
+
+    // Create bitmap and set all the bits to 0
+    bitmap_ = new std::atomic<uint32_t>[bitmap_size];
+    memset(bitmap_, 0, bitmap_size * kBytesPersEntry);
+
+    RecordTick(GetStatistics(), READ_AMP_TOTAL_READ_BYTES,
+               num_bits_needed << bytes_per_bit_pow_);
+  }
+
+  ~BlockReadAmpBitmap() { delete[] bitmap_; }
+
+  void Mark(uint32_t start_offset, uint32_t end_offset) {
+    assert(end_offset >= start_offset);
+
+    // Every new bit we set will bump this counter
+    uint32_t new_useful_bytes = 0;
+    // Index of first bit in mask (start_offset / bytes_per_bit)
+    uint32_t start_bit = start_offset >> bytes_per_bit_pow_;
+    // Index of last bit in mask (end_offset / bytes_per_bit)
+    uint32_t end_bit = end_offset >> bytes_per_bit_pow_;
+    // Index of middle bit (unique to this range)
+    uint32_t mid_bit = start_bit + 1;
+
+    // It's guaranteed that ranges sent to Mark() wont overlap, this mean that
+    // we dont need to set the middle bits, we can simply set only one bit of
+    // the middle bits, and check this bit if we want to know if the whole
+    // range is set or not.
+    if (mid_bit < end_bit) {
+      if (GetAndSet(mid_bit) == 0) {
+        new_useful_bytes += (end_bit - mid_bit) << bytes_per_bit_pow_;
+      } else {
+        // If the middle bit is set, it's guaranteed that start and end bits
+        // are also set
+        return;
+      }
+    } else {
+      // This range dont have a middle bit, the whole range fall in 1 or 2 bits
+    }
+
+    if (GetAndSet(start_bit) == 0) {
+      new_useful_bytes += (1 << bytes_per_bit_pow_);
+    }
+
+    if (GetAndSet(end_bit) == 0) {
+      new_useful_bytes += (1 << bytes_per_bit_pow_);
+    }
+
+    if (new_useful_bytes > 0) {
+      RecordTick(GetStatistics(), READ_AMP_ESTIMATE_USEFUL_BYTES,
+                 new_useful_bytes);
+    }
+  }
+
+  Statistics* GetStatistics() {
+    return statistics_.load(std::memory_order_relaxed);
+  }
+
+  void SetStatistics(Statistics* stats) { statistics_.store(stats); }
+
+  uint32_t GetBytesPerBit() { return 1 << bytes_per_bit_pow_; }
+
+ private:
+  // Get the current value of bit at `bit_idx` and set it to 1
+  inline bool GetAndSet(uint32_t bit_idx) {
+    const uint32_t byte_idx = bit_idx / kBitsPerEntry;
+    const uint32_t bit_mask = 1 << (bit_idx % kBitsPerEntry);
+
+    return bitmap_[byte_idx].fetch_or(bit_mask, std::memory_order_relaxed) &
+           bit_mask;
+  }
+
+  const uint32_t kBytesPersEntry = sizeof(uint32_t);   // 4 bytes
+  const uint32_t kBitsPerEntry = kBytesPersEntry * 8;  // 32 bits
+
+  // Bitmap used to record the bytes that we read, use atomic to protect
+  // against multiple threads updating the same bit
+  std::atomic<uint32_t>* bitmap_;
+  // (1 << bytes_per_bit_pow_) is bytes_per_bit. Use power of 2 to optimize
+  // muliplication and division
+  uint8_t bytes_per_bit_pow_;
+  // Pointer to DB Statistics object, Since this bitmap may outlive the DB
+  // this pointer maybe invalid, but the DB will update it to a valid pointer
+  // by using SetStatistics() before calling Mark()
+  std::atomic<Statistics*> statistics_;
+};
+
 class Block {
  public:
   // Initialize the block with the specified contents.
-  explicit Block(BlockContents&& contents);
+  explicit Block(BlockContents&& contents, size_t read_amp_bytes_per_bit = 0,
+                 Statistics* statistics = nullptr);
 
   ~Block() = default;
 
@@ -70,7 +180,8 @@ class Block {
   // and prefix_index_ are null, so this option does not matter.
   InternalIterator* NewIterator(const Comparator* comparator,
                                 BlockIter* iter = nullptr,
-                                bool total_order_seek = true);
+                                bool total_order_seek = true,
+                                Statistics* stats = nullptr);
   void SetBlockPrefixIndex(BlockPrefixIndex* prefix_index);
 
   // Report an approximation of how much memory has been used.
@@ -82,6 +193,7 @@ class Block {
   size_t size_;                 // contents_.data.size()
   uint32_t restart_offset_;     // Offset in data_ of restart array
   std::unique_ptr<BlockPrefixIndex> prefix_index_;
+  std::unique_ptr<BlockReadAmpBitmap> read_amp_bitmap_;
 
   // No copying allowed
   Block(const Block&);
@@ -99,17 +211,22 @@ class BlockIter : public InternalIterator {
         restart_index_(0),
         status_(Status::OK()),
         prefix_index_(nullptr),
-        key_pinned_(false) {}
+        key_pinned_(false),
+        read_amp_bitmap_(nullptr),
+        last_bitmap_offset_(0) {}
 
   BlockIter(const Comparator* comparator, const char* data, uint32_t restarts,
-            uint32_t num_restarts, BlockPrefixIndex* prefix_index)
+            uint32_t num_restarts, BlockPrefixIndex* prefix_index,
+            BlockReadAmpBitmap* read_amp_bitmap)
       : BlockIter() {
-    Initialize(comparator, data, restarts, num_restarts, prefix_index);
+    Initialize(comparator, data, restarts, num_restarts, prefix_index,
+               read_amp_bitmap);
   }
 
   void Initialize(const Comparator* comparator, const char* data,
                   uint32_t restarts, uint32_t num_restarts,
-                  BlockPrefixIndex* prefix_index) {
+                  BlockPrefixIndex* prefix_index,
+                  BlockReadAmpBitmap* read_amp_bitmap) {
     assert(data_ == nullptr);           // Ensure it is called only once
     assert(num_restarts > 0);           // Ensure the param is valid
 
@@ -120,6 +237,8 @@ class BlockIter : public InternalIterator {
     current_ = restarts_;
     restart_index_ = num_restarts_;
     prefix_index_ = prefix_index;
+    read_amp_bitmap_ = read_amp_bitmap;
+    last_bitmap_offset_ = current_ + 1;
   }
 
   void SetStatus(Status s) {
@@ -134,6 +253,12 @@ class BlockIter : public InternalIterator {
   }
   virtual Slice value() const override {
     assert(Valid());
+    if (read_amp_bitmap_ && current_ < restarts_ &&
+        current_ != last_bitmap_offset_) {
+      read_amp_bitmap_->Mark(current_ /* current entry offset */,
+                             NextEntryOffset() - 1);
+      last_bitmap_offset_ = current_;
+    }
     return value_;
   }
 
@@ -164,6 +289,8 @@ class BlockIter : public InternalIterator {
 
   virtual bool IsValuePinned() const override { return true; }
 
+  size_t TEST_CurrentEntrySize() { return NextEntryOffset() - current_; }
+
  private:
   const Comparator* comparator_;
   const char* data_;       // underlying block contents
@@ -179,6 +306,11 @@ class BlockIter : public InternalIterator {
   BlockPrefixIndex* prefix_index_;
   bool key_pinned_;
 
+  // read-amp bitmap
+  BlockReadAmpBitmap* read_amp_bitmap_;
+  // last `current_` value we report to read-amp bitmp
+  mutable uint32_t last_bitmap_offset_;
+
   struct CachedPrevEntry {
     explicit CachedPrevEntry(uint32_t _offset, const char* _key_ptr,
                              size_t _key_offset, size_t _key_size, Slice _value)

table/block_based_table_reader.cc

@@ -66,14 +66,16 @@ namespace {
 Status ReadBlockFromFile(RandomAccessFileReader* file, const Footer& footer,
                          const ReadOptions& options, const BlockHandle& handle,
                          std::unique_ptr<Block>* result,
-                         const ImmutableCFOptions &ioptions,
-                         bool do_uncompress, const Slice& compression_dict,
-                         const PersistentCacheOptions& cache_options) {
+                         const ImmutableCFOptions& ioptions, bool do_uncompress,
+                         const Slice& compression_dict,
+                         const PersistentCacheOptions& cache_options,
+                         size_t read_amp_bytes_per_bit) {
   BlockContents contents;
   Status s = ReadBlockContents(file, footer, options, handle, &contents, ioptions,
                                do_uncompress, compression_dict, cache_options);
   if (s.ok()) {
-    result->reset(new Block(std::move(contents)));
+    result->reset(new Block(std::move(contents), read_amp_bytes_per_bit,
+                            ioptions.statistics));
   }
 
   return s;
@@ -188,7 +190,8 @@ class BinarySearchIndexReader : public IndexReader {
     std::unique_ptr<Block> index_block;
     auto s = ReadBlockFromFile(file, footer, ReadOptions(), index_handle,
                                &index_block, ioptions, true /* decompress */,
-                               Slice() /*compression dict*/, cache_options);
+                               Slice() /*compression dict*/, cache_options,
+                               0 /* read_amp_bytes_per_bit */);
 
     if (s.ok()) {
       *index_reader = new BinarySearchIndexReader(
@@ -227,17 +230,20 @@ class BinarySearchIndexReader : public IndexReader {
 // key.
 class HashIndexReader : public IndexReader {
  public:
-  static Status Create(
-      const SliceTransform* hash_key_extractor, const Footer& footer,
-      RandomAccessFileReader* file, const ImmutableCFOptions &ioptions,
-      const Comparator* comparator, const BlockHandle& index_handle,
-      InternalIterator* meta_index_iter, IndexReader** index_reader,
+  static Status Create(const SliceTransform* hash_key_extractor,
+                       const Footer& footer, RandomAccessFileReader* file,
+                       const ImmutableCFOptions& ioptions,
+                       const Comparator* comparator,
+                       const BlockHandle& index_handle,
+                       InternalIterator* meta_index_iter,
+                       IndexReader** index_reader,
                        bool hash_index_allow_collision,
                        const PersistentCacheOptions& cache_options) {
     std::unique_ptr<Block> index_block;
     auto s = ReadBlockFromFile(file, footer, ReadOptions(), index_handle,
                                &index_block, ioptions, true /* decompress */,
-                               Slice() /*compression dict*/, cache_options);
+                               Slice() /*compression dict*/, cache_options,
+                               0 /* read_amp_bytes_per_bit */);
 
     if (!s.ok()) {
       return s;
@@ -791,7 +797,7 @@ Status BlockBasedTable::ReadMetaBlock(Rep* rep,
       rep->file.get(), rep->footer, ReadOptions(),
       rep->footer.metaindex_handle(), &meta, rep->ioptions,
       true /* decompress */, Slice() /*compression dict*/,
-      rep->persistent_cache_options);
+      rep->persistent_cache_options, 0 /* read_amp_bytes_per_bit */);
 
   if (!s.ok()) {
     Log(InfoLogLevel::ERROR_LEVEL, rep->ioptions.info_log,
@@ -811,7 +817,7 @@ Status BlockBasedTable::GetDataBlockFromCache(
     Cache* block_cache, Cache* block_cache_compressed,
     const ImmutableCFOptions& ioptions, const ReadOptions& read_options,
     BlockBasedTable::CachableEntry<Block>* block, uint32_t format_version,
-    const Slice& compression_dict) {
+    const Slice& compression_dict, size_t read_amp_bytes_per_bit) {
   Status s;
   Block* compressed_block = nullptr;
   Cache::Handle* block_cache_compressed_handle = nullptr;
@@ -861,7 +867,8 @@ Status BlockBasedTable::GetDataBlockFromCache(
 
   // Insert uncompressed block into block cache
   if (s.ok()) {
-    block->value = new Block(std::move(contents));  // uncompressed block
+    block->value = new Block(std::move(contents), read_amp_bytes_per_bit,
+                             statistics);  // uncompressed block
     assert(block->value->compression_type() == kNoCompression);
     if (block_cache != nullptr && block->value->cachable() &&
         read_options.fill_cache) {
@@ -888,7 +895,7 @@ Status BlockBasedTable::PutDataBlockToCache(
     Cache* block_cache, Cache* block_cache_compressed,
     const ReadOptions& read_options, const ImmutableCFOptions& ioptions,
     CachableEntry<Block>* block, Block* raw_block, uint32_t format_version,
-    const Slice& compression_dict) {
+    const Slice& compression_dict, size_t read_amp_bytes_per_bit) {
   assert(raw_block->compression_type() == kNoCompression ||
          block_cache_compressed != nullptr);
 
@@ -906,7 +913,8 @@ Status BlockBasedTable::PutDataBlockToCache(
   }
 
   if (raw_block->compression_type() != kNoCompression) {
-    block->value = new Block(std::move(contents));  // uncompressed block
+    block->value = new Block(std::move(contents), read_amp_bytes_per_bit,
+                             statistics);  // compressed block
   } else {
     block->value = raw_block;
     raw_block = nullptr;
@@ -1206,8 +1214,9 @@ InternalIterator* BlockBasedTable::NewDataBlockIterator(
     }
 
     s = GetDataBlockFromCache(
-        key, ckey, block_cache, block_cache_compressed, rep->ioptions, ro, &block,
-        rep->table_options.format_version, compression_dict);
+        key, ckey, block_cache, block_cache_compressed, rep->ioptions, ro,
+        &block, rep->table_options.format_version, compression_dict,
+        rep->table_options.read_amp_bytes_per_bit);
 
     if (block.value == nullptr && !no_io && ro.fill_cache) {
       std::unique_ptr<Block> raw_block;
@@ -1216,14 +1225,15 @@ InternalIterator* BlockBasedTable::NewDataBlockIterator(
         s = ReadBlockFromFile(rep->file.get(), rep->footer, ro, handle,
                               &raw_block, rep->ioptions,
                               block_cache_compressed == nullptr,
-                              compression_dict, rep->persistent_cache_options);
+                              compression_dict, rep->persistent_cache_options,
+                              rep->table_options.read_amp_bytes_per_bit);
       }
 
       if (s.ok()) {
-        s = PutDataBlockToCache(key, ckey, block_cache, block_cache_compressed,
-                                ro, rep->ioptions, &block, raw_block.release(),
-                                rep->table_options.format_version,
-                                compression_dict);
+        s = PutDataBlockToCache(
+            key, ckey, block_cache, block_cache_compressed, ro, rep->ioptions,
+            &block, raw_block.release(), rep->table_options.format_version,
+            compression_dict, rep->table_options.read_amp_bytes_per_bit);
       }
     }
   }
@@ -1242,7 +1252,8 @@ InternalIterator* BlockBasedTable::NewDataBlockIterator(
     std::unique_ptr<Block> block_value;
     s = ReadBlockFromFile(rep->file.get(), rep->footer, ro, handle,
                           &block_value, rep->ioptions, true /* compress */,
-                          compression_dict, rep->persistent_cache_options);
+                          compression_dict, rep->persistent_cache_options,
+                          rep->table_options.read_amp_bytes_per_bit);
     if (s.ok()) {
       block.value = block_value.release();
     }
@@ -1251,7 +1262,8 @@ InternalIterator* BlockBasedTable::NewDataBlockIterator(
   InternalIterator* iter;
   if (s.ok()) {
     assert(block.value != nullptr);
-    iter = block.value->NewIterator(&rep->internal_comparator, input_iter);
+    iter = block.value->NewIterator(&rep->internal_comparator, input_iter, true,
+                                    rep->ioptions.statistics);
     if (block.cache_handle != nullptr) {
       iter->RegisterCleanup(&ReleaseCachedEntry, block_cache,
           block.cache_handle);
@@ -1607,12 +1619,12 @@ bool BlockBasedTable::TEST_KeyInCache(const ReadOptions& options,
                   handle, cache_key_storage);
   Slice ckey;
 
-  s = GetDataBlockFromCache(cache_key, ckey, block_cache, nullptr,
-                            rep_->ioptions, options, &block,
+  s = GetDataBlockFromCache(
+      cache_key, ckey, block_cache, nullptr, rep_->ioptions, options, &block,
       rep_->table_options.format_version,
-                            rep_->compression_dict_block
-                                ? rep_->compression_dict_block->data
-                                : Slice());
+      rep_->compression_dict_block ? rep_->compression_dict_block->data
+                                   : Slice(),
+      0 /* read_amp_bytes_per_bit */);
   assert(s.ok());
   bool in_cache = block.value != nullptr;
   if (in_cache) {

table/block_based_table_reader.h

@@ -192,7 +192,7 @@ class BlockBasedTable : public TableReader {
       Cache* block_cache, Cache* block_cache_compressed,
       const ImmutableCFOptions& ioptions, const ReadOptions& read_options,
       BlockBasedTable::CachableEntry<Block>* block, uint32_t format_version,
-      const Slice& compression_dict);
+      const Slice& compression_dict, size_t read_amp_bytes_per_bit);
 
   // Put a raw block (maybe compressed) to the corresponding block caches.
   // This method will perform decompression against raw_block if needed and then
@@ -209,7 +209,7 @@ class BlockBasedTable : public TableReader {
       Cache* block_cache, Cache* block_cache_compressed,
       const ReadOptions& read_options, const ImmutableCFOptions& ioptions,
       CachableEntry<Block>* block, Block* raw_block, uint32_t format_version,
-      const Slice& compression_dict);
+      const Slice& compression_dict, size_t read_amp_bytes_per_bit);
 
   // Calls (*handle_result)(arg, ...) repeatedly, starting with the entry found
   // after a call to Seek(key), until handle_result returns false.

table/block_test.cc

@@ -4,7 +4,11 @@
 //  of patent rights can be found in the PATENTS file in the same directory.
 //
 #include <stdio.h>
+#include <algorithm>
+#include <set>
 #include <string>
+#include <unordered_set>
+#include <utility>
 #include <vector>
 
 #include "db/dbformat.h"
@@ -216,6 +220,259 @@ TEST_F(BlockTest, IndexHashWithSharedPrefix) {
   CheckBlockContents(std::move(contents), kMaxKey, keys, values);
 }
 
+// A slow and accurate version of BlockReadAmpBitmap that simply store
+// all the marked ranges in a set.
+class BlockReadAmpBitmapSlowAndAccurate {
+ public:
+  void Mark(size_t start_offset, size_t end_offset) {
+    assert(end_offset >= start_offset);
+
+    marked_ranges_.emplace(end_offset, start_offset);
+  }
+
+  // Return true if any byte in this range was Marked
+  bool IsAnyInRangeMarked(size_t start_offset, size_t end_offset) {
+    auto it = marked_ranges_.lower_bound(std::make_pair(start_offset, 0));
+    if (it == marked_ranges_.end()) {
+      return false;
+    }
+    return start_offset <= it->first && end_offset >= it->second;
+  }
+
+ private:
+  std::set<std::pair<size_t, size_t>> marked_ranges_ = {};
+};
+
+TEST_F(BlockTest, BlockReadAmpBitmap) {
+  std::vector<size_t> block_sizes = {
+      1,                 // 1 byte
+      32,                // 32 bytes
+      61,                // 61 bytes
+      64,                // 64 bytes
+      512,               // 0.5 KB
+      1024,              // 1 KB
+      1024 * 4,          // 4 KB
+      1024 * 10,         // 10 KB
+      1024 * 50,         // 50 KB
+      1024 * 1024,       // 1 MB
+      1024 * 1024 * 4,   // 4 MB
+      1024 * 1024 * 50,  // 10 MB
+      777,
+      124653,
+  };
+  const size_t kBytesPerBit = 64;
+
+  Random rnd(301);
+  for (size_t block_size : block_sizes) {
+    std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+    BlockReadAmpBitmap read_amp_bitmap(block_size, kBytesPerBit, stats.get());
+    BlockReadAmpBitmapSlowAndAccurate read_amp_slow_and_accurate;
+
+    size_t needed_bits = (block_size / kBytesPerBit);
+    if (block_size % kBytesPerBit != 0) {
+      needed_bits++;
+    }
+    size_t bitmap_size = needed_bits / 32;
+    if (needed_bits % 32 != 0) {
+      bitmap_size++;
+    }
+    size_t bits_in_bitmap = bitmap_size * 32;
+
+    ASSERT_EQ(stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES),
+              needed_bits * kBytesPerBit);
+
+    // Generate some random entries
+    std::vector<size_t> random_entry_offsets;
+    for (int i = 0; i < 1000; i++) {
+      random_entry_offsets.push_back(rnd.Next() % block_size);
+    }
+    std::sort(random_entry_offsets.begin(), random_entry_offsets.end());
+    auto it =
+        std::unique(random_entry_offsets.begin(), random_entry_offsets.end());
+    random_entry_offsets.resize(
+        std::distance(random_entry_offsets.begin(), it));
+
+    std::vector<std::pair<uint32_t, uint32_t>> random_entries;
+    for (size_t i = 0; i < random_entry_offsets.size(); i++) {
+      size_t entry_start = random_entry_offsets[i];
+      size_t entry_end;
+      if (i + 1 < random_entry_offsets.size()) {
+        entry_end = random_entry_offsets[i + 1] - 1;
+      } else {
+        entry_end = block_size - 1;
+      }
+      random_entries.emplace_back(entry_start, entry_end);
+    }
+
+    for (size_t i = 0; i < random_entries.size(); i++) {
+      auto &current_entry = random_entries[rnd.Next() % random_entries.size()];
+
+      read_amp_bitmap.Mark(current_entry.first, current_entry.second);
+      read_amp_slow_and_accurate.Mark(current_entry.first,
+                                      current_entry.second);
+
+      size_t total_bits = 0;
+      for (size_t bit_idx = 0; bit_idx < bits_in_bitmap; bit_idx++) {
+        size_t start_rng = bit_idx * kBytesPerBit;
+        size_t end_rng = (start_rng + kBytesPerBit) - 1;
+
+        total_bits +=
+            read_amp_slow_and_accurate.IsAnyInRangeMarked(start_rng, end_rng);
+      }
+      size_t expected_estimate_useful = total_bits * kBytesPerBit;
+      size_t got_estimate_useful =
+          stats->getTickerCount(READ_AMP_ESTIMATE_USEFUL_BYTES);
+
+      ASSERT_EQ(expected_estimate_useful, got_estimate_useful);
+    }
+  }
+}
+
+TEST_F(BlockTest, BlockWithReadAmpBitmap) {
+  Random rnd(301);
+  Options options = Options();
+  std::unique_ptr<InternalKeyComparator> ic;
+  ic.reset(new test::PlainInternalKeyComparator(options.comparator));
+
+  std::vector<std::string> keys;
+  std::vector<std::string> values;
+  BlockBuilder builder(16);
+  int num_records = 10000;
+
+  GenerateRandomKVs(&keys, &values, 0, num_records, 1);
+  // add a bunch of records to a block
+  for (int i = 0; i < num_records; i++) {
+    builder.Add(keys[i], values[i]);
+  }
+
+  Slice rawblock = builder.Finish();
+  const size_t kBytesPerBit = 8;
+
+  // Read the block sequentially using Next()
+  {
+    std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+
+    // create block reader
+    BlockContents contents;
+    contents.data = rawblock;
+    contents.cachable = true;
+    Block reader(std::move(contents), kBytesPerBit, stats.get());
+
+    // read contents of block sequentially
+    size_t read_bytes = 0;
+    BlockIter *iter = static_cast<BlockIter *>(
+        reader.NewIterator(options.comparator, nullptr, true, stats.get()));
+    for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
+      iter->value();
+      read_bytes += iter->TEST_CurrentEntrySize();
+
+      double semi_acc_read_amp =
+          static_cast<double>(read_bytes) / rawblock.size();
+      double read_amp = static_cast<double>(stats->getTickerCount(
+                            READ_AMP_ESTIMATE_USEFUL_BYTES)) /
+                        stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+      // Error in read amplification will be less than 1% if we are reading
+      // sequentially
+      double error_pct = fabs(semi_acc_read_amp - read_amp) * 100;
+      EXPECT_LT(error_pct, 1);
+    }
+
+    delete iter;
+  }
+
+  // Read the block sequentially using Seek()
+  {
+    std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+
+    // create block reader
+    BlockContents contents;
+    contents.data = rawblock;
+    contents.cachable = true;
+    Block reader(std::move(contents), kBytesPerBit, stats.get());
+
+    size_t read_bytes = 0;
+    BlockIter *iter = static_cast<BlockIter *>(
+        reader.NewIterator(options.comparator, nullptr, true, stats.get()));
+    for (int i = 0; i < num_records; i++) {
+      Slice k(keys[i]);
+
+      // search in block for this key
+      iter->Seek(k);
+      iter->value();
+      read_bytes += iter->TEST_CurrentEntrySize();
+
+      double semi_acc_read_amp =
+          static_cast<double>(read_bytes) / rawblock.size();
+      double read_amp = static_cast<double>(stats->getTickerCount(
+                            READ_AMP_ESTIMATE_USEFUL_BYTES)) /
+                        stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+      // Error in read amplification will be less than 1% if we are reading
+      // sequentially
+      double error_pct = fabs(semi_acc_read_amp - read_amp) * 100;
+      EXPECT_LT(error_pct, 1);
+    }
+    delete iter;
+  }
+
+  // Read the block randomly
+  {
+    std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+
+    // create block reader
+    BlockContents contents;
+    contents.data = rawblock;
+    contents.cachable = true;
+    Block reader(std::move(contents), kBytesPerBit, stats.get());
+
+    size_t read_bytes = 0;
+    BlockIter *iter = static_cast<BlockIter *>(
+        reader.NewIterator(options.comparator, nullptr, true, stats.get()));
+    std::unordered_set<int> read_keys;
+    for (int i = 0; i < num_records; i++) {
+      int index = rnd.Uniform(num_records);
+      Slice k(keys[index]);
+
+      iter->Seek(k);
+      iter->value();
+      if (read_keys.find(index) == read_keys.end()) {
+        read_keys.insert(index);
+        read_bytes += iter->TEST_CurrentEntrySize();
+      }
+
+      double semi_acc_read_amp =
+          static_cast<double>(read_bytes) / rawblock.size();
+      double read_amp = static_cast<double>(stats->getTickerCount(
+                            READ_AMP_ESTIMATE_USEFUL_BYTES)) /
+                        stats->getTickerCount(READ_AMP_TOTAL_READ_BYTES);
+
+      double error_pct = fabs(semi_acc_read_amp - read_amp) * 100;
+      // Error in read amplification will be less than 2% if we are reading
+      // randomly
+      EXPECT_LT(error_pct, 2);
+    }
+    delete iter;
+  }
+}
+
+TEST_F(BlockTest, ReadAmpBitmapPow2) {
+  std::shared_ptr<Statistics> stats = rocksdb::CreateDBStatistics();
+  ASSERT_EQ(BlockReadAmpBitmap(100, 1, stats.get()).GetBytesPerBit(), 1);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 2, stats.get()).GetBytesPerBit(), 2);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 4, stats.get()).GetBytesPerBit(), 4);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 8, stats.get()).GetBytesPerBit(), 8);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 16, stats.get()).GetBytesPerBit(), 16);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 32, stats.get()).GetBytesPerBit(), 32);
+
+  ASSERT_EQ(BlockReadAmpBitmap(100, 3, stats.get()).GetBytesPerBit(), 2);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 7, stats.get()).GetBytesPerBit(), 4);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 11, stats.get()).GetBytesPerBit(), 8);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 17, stats.get()).GetBytesPerBit(), 16);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 33, stats.get()).GetBytesPerBit(), 32);
+  ASSERT_EQ(BlockReadAmpBitmap(100, 35, stats.get()).GetBytesPerBit(), 32);
+}
+
 }  // namespace rocksdb
 
 int main(int argc, char **argv) {

tools/db_bench_tool.cc

@@ -384,6 +384,10 @@ DEFINE_int32(index_block_restart_interval,
              "Number of keys between restart points "
              "for delta encoding of keys in index block.");
 
+DEFINE_int32(read_amp_bytes_per_bit,
+             rocksdb::BlockBasedTableOptions().read_amp_bytes_per_bit,
+             "Number of bytes per bit to be used in block read-amp bitmap");
+
 DEFINE_int64(compressed_cache_size, -1,
              "Number of bytes to use as a cache of compressed data.");
 
@@ -2805,6 +2809,7 @@ class Benchmark {
       block_based_options.skip_table_builder_flush =
           FLAGS_skip_table_builder_flush;
       block_based_options.format_version = 2;
+      block_based_options.read_amp_bytes_per_bit = FLAGS_read_amp_bytes_per_bit;
       options.table_factory.reset(
           NewBlockBasedTableFactory(block_based_options));
     }

util/options_helper.h

@@ -558,7 +558,10 @@ static std::unordered_map<std::string, OptionTypeInfo>
           OptionType::kUInt32T, OptionVerificationType::kNormal}},
         {"verify_compression",
          {offsetof(struct BlockBasedTableOptions, verify_compression),
-          OptionType::kBoolean, OptionVerificationType::kNormal}}};
+          OptionType::kBoolean, OptionVerificationType::kNormal}},
+        {"read_amp_bytes_per_bit",
+         {offsetof(struct BlockBasedTableOptions, read_amp_bytes_per_bit),
+          OptionType::kSizeT, OptionVerificationType::kNormal}}};
 
 static std::unordered_map<std::string, OptionTypeInfo> plain_table_type_info = {
     {"user_key_len",

util/options_settable_test.cc

@@ -159,7 +159,7 @@ TEST_F(OptionsSettableTest, BlockBasedTableOptionsAllFieldsSettable) {
       "filter_policy=bloomfilter:4:true;whole_key_filtering=1;"
       "skip_table_builder_flush=1;format_version=1;"
       "hash_index_allow_collision=false;"
-      "verify_compression=true;",
+      "verify_compression=true;read_amp_bytes_per_bit=0",
       new_bbto));
 
   ASSERT_EQ(unset_bytes_base,