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).


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);


// ... start loading file.
if (failedToOpenFile) {
  // Cancel this if the operation failed early etc.
// ... do more work.

// Periodically, it's necessary to attempt to finish a
// TelemetryStopwatch that's already been canceled or
// finished. Normally, that throws a warning to the
// console. If the TelemetryStopwatch being possibly
// cancelled or finished is expected behaviour, the
// warning can be suppressed by passing the optional
// aCanceledOkay argument.

// ... suppress warning on a previously finished
// TelemetryStopwatch
TelemetryStopwatch.finish("SAMPLE_FILE_LOAD_TIME_MS", null,
                          true /* aCanceledOkay */);

From C++


// 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());


  Telemetry::AutoTimer<Telemetry::FIND_PLUGINS> telemetry;
  // ... scan disk for plugins.
// When leaving the scope, AutoTimers destructor will record the time that passed.