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Improve RocksDB "get" performance by computing merge result in memtable

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Summary:
Added an option (max_successive_merges) that can be used to specify the
maximum number of successive merge operations on a key in the memtable.
This can be used to improve performance of the "get" operation. If many
successive merge operations are performed on a key, the performance of "get"
operations on the key deteriorates, as the value has to be computed for each
"get" operation by applying all the successive merge operations.

FB Task ID: #3428853

Test Plan:
make all check
db_bench --benchmarks=readrandommergerandom
counter_stress_test

Reviewers: haobo, vamsi, dhruba, sdong

Reviewed By: haobo

CC: zshao

Differential Revision: https://reviews.facebook.net/D14991
This commit is contained in:
Schalk-Willem Kruger
2014-01-10 17:33:56 -08:00
parent 62197d28b6
commit a09ee1069d
7 changed files with 320 additions and 7 deletions
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58
db/write_batch.cc
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@@ -21,6 +21,7 @@
#include "rocksdb/write_batch.h"
#include "rocksdb/options.h"
#include "rocksdb/merge_operator.h"
#include "db/dbformat.h"
#include "db/db_impl.h"
#include "db/memtable.h"
@@ -203,7 +204,62 @@ class MemTableInserter : public WriteBatch::Handler {
sequence_++;
}
virtual void Merge(const Slice& key, const Slice& value) {
mem_->Add(sequence_, kTypeMerge, key, value);
bool perform_merge = false;
if (options_->max_successive_merges > 0 && db_ != nullptr) {
LookupKey lkey(key, sequence_);
// Count the number of successive merges at the head
// of the key in the memtable
size_t num_merges = mem_->CountSuccessiveMergeEntries(lkey);
if (num_merges >= options_->max_successive_merges) {
perform_merge = true;
}
}
if (perform_merge) {
// 1) Get the existing value
std::string get_value;
// Pass in the sequence number so that we also include previous merge
// operations in the same batch.
SnapshotImpl read_from_snapshot;
read_from_snapshot.number_ = sequence_;
ReadOptions read_options;
read_options.snapshot = &read_from_snapshot;
db_->Get(read_options, key, &get_value);
Slice get_value_slice = Slice(get_value);
// 2) Apply this merge
auto merge_operator = options_->merge_operator.get();
assert(merge_operator);
std::deque<std::string> operands;
operands.push_front(value.ToString());
std::string new_value;
if (!merge_operator->FullMerge(key,
&get_value_slice,
operands,
&new_value,
options_->info_log.get())) {
// Failed to merge!
RecordTick(options_->statistics.get(), NUMBER_MERGE_FAILURES);
// Store the delta in memtable
perform_merge = false;
} else {
// 3) Add value to memtable
mem_->Add(sequence_, kTypeValue, key, new_value);
}
}
if (!perform_merge) {
// Add merge operator to memtable
mem_->Add(sequence_, kTypeMerge, key, value);
}
sequence_++;
}
virtual void Delete(const Slice& key) {