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Optimize DBIter::Prev() by reducing stack overhead

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Summary:
It looks like we are spending significant amount of time creating std::deque<std::string> every time we do Iterator::Prev()

{F921567}

By using merge_operands_ as a DBIter data member w create it once and reduce this overhead and see ~30% performance improvement when using Iterator::Prev() on hot data

Orignal performance

```
DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="readreverse" --db="/dev/shm/bench_prev_opt/" --use_existing_db --disable_auto_compactions
readreverse  :       0.713 micros/op 1402219 ops/sec;  155.1 MB/s
readreverse  :       0.609 micros/op 1641386 ops/sec;  181.6 MB/s
readreverse  :       0.684 micros/op 1461150 ops/sec;  161.6 MB/s
readreverse  :       0.629 micros/op 1589842 ops/sec;  175.9 MB/s
readreverse  :       0.647 micros/op 1544530 ops/sec;  170.9 MB/s
```

After optimization

```
DEBUG_LEVEL=0 make db_bench -j64 && ./db_bench --benchmarks="readreverse" --db="/dev/shm/bench_prev_opt/" --use_existing_db --disable_auto_compactions
readreverse  :       0.488 micros/op 2051189 ops/sec;  226.9 MB/s
readreverse  :       0.505 micros/op 1980892 ops/sec;  219.1 MB/s
readreverse  :       0.541 micros/op 1846971 ops/sec;  204.3 MB/s
readreverse  :       0.497 micros/op 2013612 ops/sec;  222.8 MB/s
readreverse  :       0.480 micros/op 2082665 ops/sec;  230.4 MB/s
```

Test Plan: make check -j64

Reviewers: sdong, anthony, rven, igor, yhchiang

Reviewed By: yhchiang

Subscribers: jkedgar, dhruba

Differential Revision: https://reviews.facebook.net/D52563
This commit is contained in:
Islam AbdelRahman 2016-01-07 07:59:14 -08:00
parent 73c31377bb
commit 8c71eb5afc
1 changed files with 11 additions and 8 deletions
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19
db/db_iter.cc
View file

@ -190,6 +190,8 @@ class DBIter: public Iterator {
IterKey prefix_start_;
bool prefix_same_as_start_;
bool iter_pinned_;
// List of operands for merge operator.
std::deque<std::string> merge_operands_;
// No copying allowed
DBIter(const DBIter&);
@ -493,8 +495,7 @@ void DBIter::PrevInternal() {
// saved_value_
bool DBIter::FindValueForCurrentKey() {
assert(iter_->Valid());
// Contains operands for merge operator.
std::deque<std::string> operands;
merge_operands_.clear();
// last entry before merge (could be kTypeDeletion, kTypeSingleDeletion or
// kTypeValue)
ValueType last_not_merge_type = kTypeDeletion;
@ -514,19 +515,19 @@ bool DBIter::FindValueForCurrentKey() {
last_key_entry_type = ikey.type;
switch (last_key_entry_type) {
case kTypeValue:
operands.clear();
merge_operands_.clear();
saved_value_ = iter_->value().ToString();
last_not_merge_type = kTypeValue;
break;
case kTypeDeletion:
case kTypeSingleDeletion:
operands.clear();
merge_operands_.clear();
last_not_merge_type = last_key_entry_type;
PERF_COUNTER_ADD(internal_delete_skipped_count, 1);
break;
case kTypeMerge:
assert(user_merge_operator_ != nullptr);
operands.push_back(iter_->value().ToString());
merge_operands_.push_back(iter_->value().ToString());
break;
default:
assert(false);
@ -548,8 +549,9 @@ bool DBIter::FindValueForCurrentKey() {
if (last_not_merge_type == kTypeDeletion) {
StopWatchNano timer(env_, statistics_ != nullptr);
PERF_TIMER_GUARD(merge_operator_time_nanos);
user_merge_operator_->FullMerge(saved_key_.GetKey(), nullptr, operands,
&saved_value_, logger_);
user_merge_operator_->FullMerge(saved_key_.GetKey(), nullptr,
merge_operands_, &saved_value_,
logger_);
RecordTick(statistics_, MERGE_OPERATION_TOTAL_TIME,
timer.ElapsedNanos());
} else {
@ -560,7 +562,8 @@ bool DBIter::FindValueForCurrentKey() {
StopWatchNano timer(env_, statistics_ != nullptr);
PERF_TIMER_GUARD(merge_operator_time_nanos);
user_merge_operator_->FullMerge(saved_key_.GetKey(), &temp_slice,
operands, &saved_value_, logger_);
merge_operands_, &saved_value_,
logger_);
RecordTick(statistics_, MERGE_OPERATION_TOTAL_TIME,
timer.ElapsedNanos());
}