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 | |
23 | namespace folly { |
24 | namespace 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 | */ |
39 | template <typename DigestT> |
40 | class 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 | |