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======================
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 <https://dxr.mozilla.org/mozilla-central/source/toolkit/components/telemetry/TelemetryStopwatch.jsm>`_.
``TelemetryStopwatch`` is a helper that simplifies recording elapsed time (in milliseconds) into histograms (plain or keyed).
API:
.. code-block:: js
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:
.. code-block:: js
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:
.. code-block:: cpp
// 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());
Example:
.. code-block:: cpp
{
Telemetry::AutoTimer<Telemetry::FIND_PLUGINS> telemetry;
// ... scan disk for plugins.
}
// When leaving the scope, AutoTimers destructor will record the time that passed.
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