DataBlockHashIndex: Remove the division from EstimateSize() (#4293)

Summary:
`BlockBasedTableBuilder::Add()` eventually calls
`DataBlockHashIndexBuilder::EstimateSize()`. The previous implementation
divides the `num_keys` by the `util_ratio_` to get an estimizted `num_buckets`.
Such division is expensive as it happens in every
`BlockBasedTableBuilder::Add()`.

This diff estimates the `num_buckets` by double addition instead of double
division. Specifically, in each `Add()`, we add `bucket_per_key_`
(inverse of `util_ratio_`) to the current `estimiated_num_buckets_`. The cost is
that we are gonna have the `estimated_num_buckets_` stored as one extra field
in the DataBlockHashIndexBuilder.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4293

Differential Revision: D9412472

Pulled By: fgwu

fbshipit-source-id: 2925c8509a401e7bd3c1ab1d9e9c7244755c277a

table/block_builder.cc

@@ -106,7 +106,6 @@ size_t BlockBuilder::EstimateSizeAfterKV(const Slice& key, const Slice& value)
     estimate += VarintLength(value.size());  // varint for value length.
   }
 
-  // TODO(fwu): add the delta of the DataBlockHashIndex
   return estimate;
 }
 

table/data_block_hash_index.cc

@@ -23,12 +23,13 @@ void DataBlockHashIndexBuilder::Add(const Slice& key,
   uint32_t hash_value = GetSliceHash(key);
   hash_and_restart_pairs_.emplace_back(hash_value,
                                        static_cast<uint8_t>(restart_index));
+  estimated_num_buckets_ += bucket_per_key_;
 }
 
 void DataBlockHashIndexBuilder::Finish(std::string& buffer) {
   assert(Valid());
-  uint16_t num_buckets = static_cast<uint16_t>(
-      static_cast<double>(hash_and_restart_pairs_.size()) / util_ratio_);
+  uint16_t num_buckets = static_cast<uint16_t>(estimated_num_buckets_);
+
   if (num_buckets == 0) {
     num_buckets = 1;  // sanity check
   }
@@ -66,8 +67,9 @@ void DataBlockHashIndexBuilder::Finish(std::string& buffer) {
 }
 
 void DataBlockHashIndexBuilder::Reset() {
-  hash_and_restart_pairs_.clear();
+  estimated_num_buckets_ = 0;
   valid_ = true;
+  hash_and_restart_pairs_.clear();
 }
 
 void DataBlockHashIndex::Initialize(const char* data, uint16_t size,

table/data_block_hash_index.h

@@ -72,23 +72,26 @@ const double kDefaultUtilRatio = 0.75;
 
 class DataBlockHashIndexBuilder {
  public:
-  DataBlockHashIndexBuilder() : util_ratio_(0), valid_(false) {}
+  DataBlockHashIndexBuilder()
+      : bucket_per_key_(-1 /*uninitialized marker*/),
+        estimated_num_buckets_(0),
+        valid_(false) {}
 
   void Initialize(double util_ratio) {
     if (util_ratio <= 0) {
       util_ratio = kDefaultUtilRatio;  // sanity check
     }
-    util_ratio_ = util_ratio;
+    bucket_per_key_ = 1 / util_ratio;
     valid_ = true;
   }
 
-  inline bool Valid() const { return valid_ && util_ratio_ > 0; }
+  inline bool Valid() const { return valid_ && bucket_per_key_ > 0; }
   void Add(const Slice& key, const size_t restart_index);
   void Finish(std::string& buffer);
   void Reset();
   inline size_t EstimateSize() const {
-    uint16_t estimated_num_buckets = static_cast<uint16_t>(
-        static_cast<double>(hash_and_restart_pairs_.size()) / util_ratio_);
+    uint16_t estimated_num_buckets =
+        static_cast<uint16_t>(estimated_num_buckets_);
 
     // Maching the num_buckets number in DataBlockHashIndexBuilder::Finish.
     estimated_num_buckets |= 1;
@@ -98,7 +101,8 @@ class DataBlockHashIndexBuilder {
   }
 
  private:
-  double util_ratio_;
+  double bucket_per_key_;  // is the multiplicative inverse of util_ratio_
+  double estimated_num_buckets_;
 
   // Now the only usage for `valid_` is to mark false when the inserted
   // restart_index is larger than supported. In this case HashIndex is not