Fix a perf regression that caused every key to go through upper bound check (#7209)

Summary:
https://github.com/facebook/rocksdb/pull/5289 introduces a performance regression that caused an upper bound check within every BlockBasedTableIterator::Next(). This is unnecessary if we've checked the boundary key for current block and it is within upper bound.

Fix the bug. Also rename the boolean to a enum so that the code is slightly better readable. The original regression was probably to fix a bug that the block upper bound check status is not reset after a new block is created. Fix it bug so that the regression can be avoided without hitting the bug.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7209

Test Plan: Run all existing tests. Will run atomic black box crash test for a while.

Reviewed By: anand1976

Differential Revision: D22859246

fbshipit-source-id: cbdad1f5e656c55fd8b71726d5a4f6cb53ff9140

HISTORY.md

@@ -1,5 +1,8 @@
 # Rocksdb Change Log
 ## Unreleased
+### Bug fixes
+* Fix a performance regression introduced in 6.4 that makes a upper bound check for every Next() even if keys are within a data block that is within the upper bound.
+
 
 ## 6.12 (2020-07-28)
 ### Public API Change

table/block_based/block_based_table_iterator.cc

@@ -300,7 +300,8 @@ void BlockBasedTableIterator::FindBlockForward() {
     // Whether next data block is out of upper bound, if there is one.
     const bool next_block_is_out_of_bound =
         read_options_.iterate_upper_bound != nullptr &&
-        block_iter_points_to_real_block_ && !data_block_within_upper_bound_;
+        block_iter_points_to_real_block_ &&
+        block_upper_bound_check_ == BlockUpperBound::kUpperBoundInCurBlock;
     assert(!next_block_is_out_of_bound ||
            user_comparator_.CompareWithoutTimestamp(
                *read_options_.iterate_upper_bound, /*a_has_ts=*/false,
@@ -358,7 +359,9 @@ void BlockBasedTableIterator::FindKeyBackward() {
 }
 
 void BlockBasedTableIterator::CheckOutOfBound() {
-  if (read_options_.iterate_upper_bound != nullptr && Valid()) {
+  if (read_options_.iterate_upper_bound != nullptr &&
+      block_upper_bound_check_ != BlockUpperBound::kUpperBoundBeyondCurBlock &&
+      Valid()) {
     is_out_of_bound_ =
         user_comparator_.CompareWithoutTimestamp(
             *read_options_.iterate_upper_bound, /*a_has_ts=*/false, user_key(),
@@ -369,11 +372,12 @@ void BlockBasedTableIterator::CheckOutOfBound() {
 void BlockBasedTableIterator::CheckDataBlockWithinUpperBound() {
   if (read_options_.iterate_upper_bound != nullptr &&
       block_iter_points_to_real_block_) {
-    data_block_within_upper_bound_ =
-        (user_comparator_.CompareWithoutTimestamp(
-             *read_options_.iterate_upper_bound, /*a_has_ts=*/false,
-             index_iter_->user_key(),
-             /*b_has_ts=*/true) > 0);
+    block_upper_bound_check_ = (user_comparator_.CompareWithoutTimestamp(
+                                    *read_options_.iterate_upper_bound,
+                                    /*a_has_ts=*/false, index_iter_->user_key(),
+                                    /*b_has_ts=*/true) > 0)
+                                   ? BlockUpperBound::kUpperBoundBeyondCurBlock
+                                   : BlockUpperBound::kUpperBoundInCurBlock;
   }
 }
 }  // namespace ROCKSDB_NAMESPACE

table/block_based/block_based_table_iterator.h

@@ -104,7 +104,8 @@ class BlockBasedTableIterator : public InternalIteratorBase<Slice> {
 
   inline bool MayBeOutOfUpperBound() override {
     assert(Valid());
-    return !data_block_within_upper_bound_;
+    return block_upper_bound_check_ !=
+           BlockUpperBound::kUpperBoundBeyondCurBlock;
   }
 
   void SetPinnedItersMgr(PinnedIteratorsManager* pinned_iters_mgr) override {
@@ -134,6 +135,7 @@ class BlockBasedTableIterator : public InternalIteratorBase<Slice> {
       block_iter_.Invalidate(Status::OK());
       block_iter_points_to_real_block_ = false;
     }
+    block_upper_bound_check_ = BlockUpperBound::kUnknown;
   }
 
   void SavePrevIndexValue() {
@@ -149,6 +151,34 @@ class BlockBasedTableIterator : public InternalIteratorBase<Slice> {
     kForward,
     kBackward,
   };
+  // This enum indicates whether the upper bound falls into current block
+  // or beyond.
+  //   +-------------+
+  //   |  cur block  |       <-- (1)
+  //   +-------------+
+  //                         <-- (2)
+  //  --- <boundary key> ---
+  //                         <-- (3)
+  //   +-------------+
+  //   |  next block |       <-- (4)
+  //        ......
+  //
+  // When the block is smaller than <boundary key>, kUpperBoundInCurBlock
+  // is the value to use. The examples are (1) or (2) in the graph. It means
+  // all keys in the next block or beyond will be out of bound. Keys within
+  // the current block may or may not be out of bound.
+  // When the block is larger or equal to <boundary key>,
+  // kUpperBoundBeyondCurBlock is to be used. The examples are (3) and (4)
+  // in the graph. It means that all keys in the current block is within the
+  // upper bound and keys in the next block may or may not be within the uppder
+  // bound.
+  // If the boundary key hasn't been checked against the upper bound,
+  // kUnknown can be used.
+  enum class BlockUpperBound {
+    kUpperBoundInCurBlock,
+    kUpperBoundBeyondCurBlock,
+    kUnknown,
+  };
 
   const BlockBasedTable* table_;
   const ReadOptions& read_options_;
@@ -169,8 +199,9 @@ class BlockBasedTableIterator : public InternalIteratorBase<Slice> {
   bool block_iter_points_to_real_block_;
   // See InternalIteratorBase::IsOutOfBound().
   bool is_out_of_bound_ = false;
-  // Whether current data block being fully within iterate upper bound.
-  bool data_block_within_upper_bound_ = false;
+  // How current data block's boundary key with the next block is compared with
+  // iterate upper bound.
+  BlockUpperBound block_upper_bound_check_ = BlockUpperBound::kUnknown;
   // True if we're standing at the first key of a block, and we haven't loaded
   // that block yet. A call to PrepareValue() will trigger loading the block.
   bool is_at_first_key_from_index_ = false;