rocksdb/util/kv_map.h
Siying Dong 0bb555630f Consolidate hash function used for non-persistent data in a new function (#5155)
Summary:
Create new function NPHash64() and GetSliceNPHash64(), which are currently
implemented using murmurhash.
Replace the current direct call of murmurhash() to use the new functions
if the hash results are not used in on-disk format.
This will make it easier to try out or switch to alternative functions
in the uses where data format compatibility doesn't need to be considered.
This part shouldn't have any performance impact.

Also, the sharded cache hash function is changed to the new format, because
it falls into this categoery. It doesn't show visible performance impact
in db_bench results. CPU showed by perf is increased from about 0.2% to 0.4%
in an extreme benchmark setting (4KB blocks, no-compression, everything
cached in block cache). We've known that the current hash function used,
our own Hash() has serious hash quality problem. It can generate a lots of
conflicts with similar input. In this use case, it means extra lock contention
for reads from the same file. This slight CPU regression is worthy to me
to counter the potential bad performance with hot keys. And hopefully this
will get further improved in the future with a better hash function.

cache_test's condition is relaxed a little bit to. The new hash is slightly
more skewed in this use case, but I manually checked the data and see
the hash results are still in a reasonable range.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5155

Differential Revision: D14834821

Pulled By: siying

fbshipit-source-id: ec9a2c0a2f8ae4b54d08b13a5c2e9cc97aa80cb5
2019-04-08 13:32:06 -07:00

34 lines
880 B
C++

// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <map>
#include <string>
#include "rocksdb/comparator.h"
#include "rocksdb/slice.h"
#include "util/coding.h"
namespace rocksdb {
namespace stl_wrappers {
struct LessOfComparator {
explicit LessOfComparator(const Comparator* c = BytewiseComparator())
: cmp(c) {}
bool operator()(const std::string& a, const std::string& b) const {
return cmp->Compare(Slice(a), Slice(b)) < 0;
}
bool operator()(const Slice& a, const Slice& b) const {
return cmp->Compare(a, b) < 0;
}
const Comparator* cmp;
};
typedef std::map<std::string, std::string, LessOfComparator> KVMap;
}
}