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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.