DigestBuilder.h

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/*
 * Copyright 2018-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <memory>

#include <folly/SpinLock.h>

namespace folly {
namespace detail {

/*
 * Stat digests, such as TDigest, can be expensive to merge. It is faster to
 * buffer writes and merge them in larger chunks. DigestBuilder buffers writes
 * to improve performance.
 *
 * Values are stored in a cpu local buffer. Hot stats will merge the cpu local
 * buffer into a cpu-local digest when the buffer size is reached.
 *
 * All methods in this class are thread safe, but it probably doesn't make sense
 * for multiple threads to call build simultaneously. A typical usage is to
 * buffer writes for a period of time, and then have one thread call build to
 * merge the buffer into some other DigestT instance.
 */
template <typename DigestT>
class DigestBuilder {
 public:
  explicit DigestBuilder(size_t bufferSize, size_t digestSize);

  /*
   * Builds a DigestT from the buffer. All values used to build the DigestT are
   * removed from the buffer.
   */
  DigestT build();

  /*
   * Adds a value to the buffer.
   */
  void append(double value);

 private:
  struct alignas(hardware_destructive_interference_size) CpuLocalBuffer {
   public:
    mutable SpinLock mutex;
    std::vector<double> buffer;
    std::unique_ptr<DigestT> digest;
  };

  std::vector<CpuLocalBuffer> cpuLocalBuffers_;
  size_t bufferSize_;
  size_t digestSize_;
};

} // namespace detail
} // namespace folly