1 files changed, 216 insertions, 0 deletions
diff --git a/toolkit/components/telemetry/GCTelemetry.jsm b/toolkit/components/telemetry/GCTelemetry.jsm
new file mode 100644
index 000000000..43a4ea9ca
--- /dev/null
+++ b/toolkit/components/telemetry/GCTelemetry.jsm
@@ -0,0 +1,216 @@
+/* -*- js-indent-level: 2; indent-tabs-mode: nil -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+/**
+ * This module records detailed timing information about selected
+ * GCs. The data is sent back in the telemetry session ping. To avoid
+ * bloating the ping, only a few GCs are included. There are two
+ * selection strategies. We always save the five GCs with the worst
+ * max_pause time. Additionally, five collections are selected at
+ * random. If a GC runs for C milliseconds and the total time for all
+ * GCs since the session began is T milliseconds, then the GC has a
+ * 5*C/T probablility of being selected (the factor of 5 is because we
+ * save 5 of them).
+ *
+ * GCs from both the main process and all content processes are
+ * recorded. The data is cleared for each new subsession.
+ */
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm", this);
+
+this.EXPORTED_SYMBOLS = ["GCTelemetry"];
+
+// Names of processes where we record GCs.
+const PROCESS_NAMES = ["main", "content"];
+
+// Should be the time we started up in milliseconds since the epoch.
+const BASE_TIME = Date.now() - Services.telemetry.msSinceProcessStart();
+
+// Records selected GCs. There is one instance per process type.
+class GCData {
+  constructor(kind) {
+    let numRandom = {main: 0, content: 2};
+    let numWorst = {main: 2, content: 2};
+
+    this.totalGCTime = 0;
+    this.randomlySelected = Array(numRandom[kind]).fill(null);
+    this.worst = Array(numWorst[kind]).fill(null);
+  }
+
+  // Turn absolute timestamps (in microseconds since the epoch) into
+  // milliseconds since startup.
+  rebaseTimes(data) {
+    function fixup(t) {
+      return t / 1000.0 - BASE_TIME;
+    }
+
+    data.timestamp = fixup(data.timestamp);
+
+    for (let i = 0; i < data.slices.length; i++) {
+      let slice = data.slices[i];
+      slice.start_timestamp = fixup(slice.start_timestamp);
+      slice.end_timestamp = fixup(slice.end_timestamp);
+    }
+  }
+
+  // Records a GC (represented by |data|) in the randomlySelected or
+  // worst batches depending on the criteria above.
+  record(data) {
+    this.rebaseTimes(data);
+
+    let time = data.total_time;
+    this.totalGCTime += time;
+
+    // Probability that we will replace any one of our
+    // current randomlySelected GCs with |data|.
+    let prob = time / this.totalGCTime;
+
+    // Note that we may replace multiple GCs in
+    // randomlySelected. It's easier to reason about the
+    // probabilities this way, and it's unlikely to have any effect in
+    // practice.
+    for (let i = 0; i < this.randomlySelected.length; i++) {
+      let r = Math.random();
+      if (r <= prob) {
+        this.randomlySelected[i] = data;
+      }
+    }
+
+    // Save the 5 worst GCs based on max_pause. A GC may appear in
+    // both worst and randomlySelected.
+    for (let i = 0; i < this.worst.length; i++) {
+      if (!this.worst[i]) {
+        this.worst[i] = data;
+        break;
+      }
+
+      if (this.worst[i].max_pause < data.max_pause) {
+        this.worst.splice(i, 0, data);
+        this.worst.length--;
+        break;
+      }
+    }
+  }
+
+  entries() {
+    return {
+      random: this.randomlySelected.filter(e => e !== null),
+      worst: this.worst.filter(e => e !== null),
+    };
+  }
+}
+
+// If you adjust any of the constants here (slice limit, number of keys, etc.)
+// make sure to update the JSON schema at:
+// https://github.com/mozilla-services/mozilla-pipeline-schemas/blob/master/telemetry/main.schema.json
+// You should also adjust browser_TelemetryGC.js.
+const MAX_GC_KEYS = 25;
+const MAX_SLICES = 4;
+const MAX_SLICE_KEYS = 15;
+const MAX_PHASES = 65;
+
+function limitProperties(obj, count) {
+  // If there are too many properties, just delete them all. We don't
+  // expect this ever to happen.
+  if (Object.keys(obj).length > count) {
+    for (let key of Object.keys(obj)) {
+      delete obj[key];
+    }
+  }
+}
+
+function limitSize(data) {
+  // Store the number of slices so we know if we lost any at the end.
+  data.num_slices = data.slices.length;
+
+  data.slices.sort((a, b) => b.pause - a.pause);
+
+  if (data.slices.length > MAX_SLICES) {
+    // Make sure we always keep the first slice since it has the
+    // reason the GC was started.
+    let firstSliceIndex = data.slices.findIndex(s => s.slice == 0);
+    if (firstSliceIndex >= MAX_SLICES) {
+      data.slices[MAX_SLICES - 1] = data.slices[firstSliceIndex];
+    }
+
+    data.slices.length = MAX_SLICES;
+  }
+
+  data.slices.sort((a, b) => a.slice - b.slice);
+
+  limitProperties(data, MAX_GC_KEYS);
+
+  for (let slice of data.slices) {
+    limitProperties(slice, MAX_SLICE_KEYS);
+    limitProperties(slice.times, MAX_PHASES);
+  }
+
+  limitProperties(data.totals, MAX_PHASES);
+}
+
+let processData = new Map();
+for (let name of PROCESS_NAMES) {
+  processData.set(name, new GCData(name));
+}
+
+var GCTelemetry = {
+  initialized: false,
+
+  init() {
+    if (this.initialized) {
+      return false;
+    }
+
+    this.initialized = true;
+    Services.obs.addObserver(this, "garbage-collection-statistics", false);
+
+    if (Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_DEFAULT) {
+      Services.ppmm.addMessageListener("Telemetry:GCStatistics", this);
+    }
+
+    return true;
+  },
+
+  shutdown() {
+    if (!this.initialized) {
+      return;
+    }
+
+    Services.obs.removeObserver(this, "garbage-collection-statistics");
+
+    if (Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_DEFAULT) {
+      Services.ppmm.removeMessageListener("Telemetry:GCStatistics", this);
+    }
+    this.initialized = false;
+  },
+
+  observe(subject, topic, arg) {
+    let data = JSON.parse(arg);
+
+    limitSize(data);
+
+    if (Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_DEFAULT) {
+      processData.get("main").record(data);
+    } else {
+      Services.cpmm.sendAsyncMessage("Telemetry:GCStatistics", data);
+    }
+  },
+
+  receiveMessage(msg) {
+    processData.get("content").record(msg.data);
+  },
+
+  entries(kind, clear) {
+    let result = processData.get(kind).entries();
+    if (clear) {
+      processData.set(kind, new GCData(kind));
+    }
+    return result;
+  },
+};