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Apply fractional cascading in ForwardIterator::Seek()

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Summary:
Use search hint to reduce FindFile range thus avoid comparison
For a small DB with 50M keys, perf_context counter shows it reduces
comparison from 2B to 1.3B for a 15-minute run. No perf change was
observed for 1 seek thread, but quite good improvement was seen for 32
seek threads, when CPU was busy.
will post detail results when ready

Test Plan: db_bench and db_test

Reviewers: haobo, sdong, dhruba, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18879
This commit is contained in:
Lei Jin 2014-07-08 11:40:42 -07:00
parent 1b95bf734d
commit b278ae8e50
1 changed files with 58 additions and 3 deletions
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61
db/forward_iterator.cc
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@ -189,6 +189,10 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
}
}
Slice user_key;
if (!seek_to_first) {
user_key = ExtractUserKey(internal_key);
}
auto* files = sv_->current->files_;
for (uint32_t i = 0; i < files[0].size(); ++i) {
if (seek_to_first) {
@ -196,7 +200,7 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
} else {
// If the target key passes over the larget key, we are sure Next()
// won't go over this file.
if (user_comparator_->Compare(ExtractUserKey(internal_key),
if (user_comparator_->Compare(user_key,
files[0][i]->largest.user_key()) > 0) {
continue;
}
@ -206,16 +210,67 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
immutable_min_heap_.push(l0_iters_[i]);
}
}
int32_t search_left_bound = 0;
int32_t search_right_bound = FileIndexer::kLevelMaxIndex;
for (int32_t level = 1; level < sv_->current->NumberLevels(); ++level) {
if (files[level].empty()) {
search_left_bound = 0;
search_right_bound = FileIndexer::kLevelMaxIndex;
continue;
}
assert(level_iters_[level - 1] != nullptr);
uint32_t f_idx = 0;
if (!seek_to_first) {
f_idx = FindFileInRange(
files[level], internal_key, 0, files[level].size());
// TODO(ljin): remove before committing
// f_idx = FindFileInRange(
// files[level], internal_key, 0, files[level].size());
if (search_left_bound == search_right_bound) {
f_idx = search_left_bound;
} else if (search_left_bound < search_right_bound) {
f_idx = FindFileInRange(
files[level], internal_key, search_left_bound,
search_right_bound == FileIndexer::kLevelMaxIndex ?
files[level].size() : search_right_bound);
} else {
// search_left_bound > search_right_bound
// There are only 2 cases this can happen:
// (1) target key is smaller than left most file
// (2) target key is larger than right most file
assert(search_left_bound == (int32_t)files[level].size() ||
search_right_bound == -1);
if (search_right_bound == -1) {
assert(search_left_bound == 0);
f_idx = 0;
} else {
sv_->current->file_indexer_.GetNextLevelIndex(
level, files[level].size() - 1,
1, 1, &search_left_bound, &search_right_bound);
continue;
}
}
// Prepare hints for the next level
if (f_idx < files[level].size()) {
int cmp_smallest = user_comparator_->Compare(
user_key, files[level][f_idx]->smallest.user_key());
int cmp_largest = -1;
if (cmp_smallest >= 0) {
cmp_smallest = user_comparator_->Compare(
user_key, files[level][f_idx]->smallest.user_key());
}
sv_->current->file_indexer_.GetNextLevelIndex(level, f_idx,
cmp_smallest, cmp_largest,
&search_left_bound, &search_right_bound);
} else {
sv_->current->file_indexer_.GetNextLevelIndex(
level, files[level].size() - 1,
1, 1, &search_left_bound, &search_right_bound);
}
}
// Seek
if (f_idx < files[level].size()) {
level_iters_[level - 1]->SetFileIndex(f_idx);
seek_to_first ? level_iters_[level - 1]->SeekToFirst() :