1/*
2 * Copyright 2018-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#pragma once
18
19#include <memory>
20
21#include <folly/SpinLock.h>
22
23namespace folly {
24namespace detail {
25
26/*
27 * Stat digests, such as TDigest, can be expensive to merge. It is faster to
28 * buffer writes and merge them in larger chunks. DigestBuilder buffers writes
29 * to improve performance.
30 *
31 * Values are stored in a cpu local buffer. Hot stats will merge the cpu local
32 * buffer into a cpu-local digest when the buffer size is reached.
33 *
34 * All methods in this class are thread safe, but it probably doesn't make sense
35 * for multiple threads to call build simultaneously. A typical usage is to
36 * buffer writes for a period of time, and then have one thread call build to
37 * merge the buffer into some other DigestT instance.
38 */
39template <typename DigestT>
40class DigestBuilder {
41 public:
42 explicit DigestBuilder(size_t bufferSize, size_t digestSize);
43
44 /*
45 * Builds a DigestT from the buffer. All values used to build the DigestT are
46 * removed from the buffer.
47 */
48 DigestT build();
49
50 /*
51 * Adds a value to the buffer.
52 */
53 void append(double value);
54
55 private:
56 struct alignas(hardware_destructive_interference_size) CpuLocalBuffer {
57 public:
58 mutable SpinLock mutex;
59 std::vector<double> buffer;
60 std::unique_ptr<DigestT> digest;
61 };
62
63 std::vector<CpuLocalBuffer> cpuLocalBuffers_;
64 size_t bufferSize_;
65 size_t digestSize_;
66};
67
68} // namespace detail
69} // namespace folly
70