第26讲 DequeueBuffer Latency - Android Camera性能分析|极客笔记

本讲是Android Camera性能分析专题的第26讲，我们介绍DequeueBuffer Latency，包括如下内容：

DequeueBuffer Latency是什么
DequeueBuffer Latency配置
DequeeuBuffer Latency实战

资源	描述
在线课程	极客笔记在线课程
知识星球	星球名称：深入浅出Android Camera 星球ID: 17296815
Wechat	极客笔记圈

DequeueBuffer Latency是什么

DequeueBuffer Latency 用于衡量/记录从BufferQueue里面获取Buffer的耗时。

影响：

如果该耗时太长，会拉低CaptureRequest的帧率，具体体现在送给HAL的CaptureRequest速度变慢。

每一个Camera3OutputStream都会统计DequeueBuffer Latency。

这里的Output/Input Stream是相对HAL而言，HAL输出的流是Output Stream，输入给HAL的流是Input Stream

DequeueBuffer：

通过GraphicBufferProducer从BufferQueue里面获取一张未使用的Buffer

DequeueBuffer Latency是什么

DequeueBuffer Latency配置

DequeueBuffer Latency直方图配置

分成10份（默认值），间隔为5ms
static const int32_t kDequeueLatencyBinSize = 5; // in ms

添加Sample的时机

在每个CaptureRequest的prepareHalRequests中会调用getBuffer，进而触发到对BufferQueue的DequeueBuffer操作。
Camera3OutputStream会对每次dequeueBuffer操作耗时做记录，然后作为一个Sample添加到CameraLatencyHistogram。

DequeueBuffer Latency配置

DequeeuBuffer Latency实战

代码讲解

nsecs_t dequeueStart = systemTime(SYSTEM_TIME_MONOTONIC);

size_t batchSize = mBatchSize.load();
if (batchSize == 1) {
  sp anw = consumer;
  res = anw->dequeueBuffer(anw.get(), anb, fenceFd);
} else {
  std::unique_lock batchLock(mBatchLock);
  res = OK;
  if (mBatchedBuffers.size() == 0) {
      if (remainingBuffers == 0) {
          ALOGE("%s: cannot get buffer while all buffers are handed out", __FUNCTION__);
          return INVALID_OPERATION;
      }
      if (batchSize > remainingBuffers) {
          batchSize = remainingBuffers;
      }
      batchLock.unlock();
      // Refill batched buffers
      std::vector batchedBuffers;
      batchedBuffers.resize(batchSize);
      res = consumer->dequeueBuffers(&batchedBuffers);
      batchLock.lock();
      if (res != OK) {
          ALOGE("%s: batch dequeueBuffers call failed! %s (%d)",
                  __FUNCTION__, strerror(-res), res);
      } else {
          mBatchedBuffers = std::move(batchedBuffers);
      }
  }

  if (res == OK) {
      // Dispatch batch buffers
      *anb = mBatchedBuffers.back().buffer;
      *fenceFd = mBatchedBuffers.back().fenceFd;
      mBatchedBuffers.pop_back();
  }
}

nsecs_t dequeueEnd = systemTime(SYSTEM_TIME_MONOTONIC);
mDequeueBufferLatency.add(dequeueStart, dequeueEnd);