Measuring elapsed time

To make it easier to measure how long operations take, we have helpers for both JavaScript and C++. These helpers record the elapsed time into histograms, so you have to create suitable histograms for them first.

From JavaScript

JavaScript can measure elapsed time using TelemetryStopwatch.jsm.

TelemetryStopwatch is a helper that simplifies recording elapsed time (in milliseconds) into histograms (plain or keyed).

API:

TelemetryStopwatch = {
  // Start, cancel & finish recording elapsed time into a histogram.
  // |aObject| is optional. If specificied, the timer is associated with this
  // object, so multiple time measurements can be done concurrently.
  start(histogramId, aObject);
  cancel(histogramId, aObject);
  finish(histogramId, aObject);
  // Start, cancel & finished recording elapsed time into a keyed histogram.
  // |key| specificies the key to record into.
  // |aObject| is optional and used as above.
  startKeyed(histogramId, key, aObject);
  cancelKeyed(histogramId, key, aObject);
  finishKeyed(histogramId, key, aObject);
};

Example:

TelemetryStopwatch.start("SAMPLE_FILE_LOAD_TIME_MS");
// ... start loading file.
if (failedToOpenFile) {
  // Cancel this if the operation failed early etc.
  TelemetryStopwatch.cancel("SAMPLE_FILE_LOAD_TIME_MS");
  return;
}
// ... do more work.
TelemetryStopwatch.finish("SAMPLE_FILE_LOAD_TIME_MS");

From C++

API:

// This helper class is the preferred way to record elapsed time.
template<ID id, TimerResolution res = MilliSecond>
class AutoTimer {
  // Record into a plain histogram.
  explicit AutoTimer(TimeStamp aStart = TimeStamp::Now());
  // Record into a keyed histogram, with key |aKey|.
  explicit AutoTimer(const nsCString& aKey,
                     TimeStamp aStart = TimeStamp::Now());
};

void AccumulateTimeDelta(ID id, TimeStamp start, TimeStamp end = TimeStamp::Now());