​ 本讲是Android Camera性能分析专题的第27讲 ​，我们介绍CameraServer Request Latency，包括如下内容：

1. Request Latency是什么
2. Request Latency配置
3. Request Latency实战

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Request Latency是什么

Request Latency用于衡量HAL执行processBatchCaptureRequests的耗时。

影响:

• 如果该耗时太长，会拉低CaptureRequest的帧率，需要从HAL角度分析

Request Latency配置

Request Latency直方图配置

• 分成10份（默认值）， 间隔为40ms

• static const int32_t kRequestLatencyBinSize = 40; // in ms

添加Sample的时机

• 在每次调用HAL的processCaptureRequest接口时，会统计其耗时，然后作为一个Sample添加到CameraLatencyHistogram

Request Latency实战

• 代码讲解

nsecs_t tRequestStart = systemTime(SYSTEM_TIME_MONOTONIC);
submitRequestSuccess = sendRequestsBatch();nsecs_t tRequestEnd = systemTime(SYSTEM_TIME_MONOTONIC);
mRequestLatency.add(tRequestStart, tRequestEnd);

• 实战演示：在CameraLatencyHistogram增加Samples总的平均耗时

void CameraLatencyHistogram::add(nsecs_t start, nsecs_t end) {nsecs_t duration = end - start;int32_t durationMs = static_cast(duration / 1000000LL);int32_t binIndex = durationMs / mBinSizeMs;if (binIndex < 0) {binIndex = 0;} else if (binIndex >= mBinCount) {binIndex = mBinCount-1;}mBins[binIndex]++;mTotalCount++;mTotoalDuration += duration;
}

打印Log时：

void CameraLatencyHistogram::log(const char* fmt, ...) {if (mTotalCount == 0) {return;}va_list args;va_start(args, fmt);String8 histogramName = String8::formatV(fmt, args);ALOGI("%s (%" PRId64 ") samples, avg(%.2f ms):", histogramName.string(), mTotalCount, (float)(mTotoalDuration/1e6) / mTotalCount);va_end(args);String8 lineBins, lineBinCounts;formatHistogramText(lineBins, lineBinCounts);ALOGI("%s", lineBins.c_str());ALOGI("%s", lineBinCounts.c_str());
}