diff -r 000000000000 -r 6474c204b198 mobile/android/base/gfx/DrawTimingQueue.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mobile/android/base/gfx/DrawTimingQueue.java	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,95 @@
+/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; 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/. */
+
+package org.mozilla.gecko.gfx;
+
+import android.os.SystemClock;
+
+/**
+ * A custom-built data structure to assist with measuring draw times.
+ *
+ * This class maintains a fixed-size circular buffer of DisplayPortMetrics
+ * objects and associated timestamps. It provides only three operations, which
+ * is all we require for our purposes of measuring draw times. Note
+ * in particular that the class is designed so that even though it is
+ * accessed from multiple threads, it does not require synchronization;
+ * any concurrency errors that result from this are handled gracefully.
+ *
+ * Assuming an unrolled buffer so that mTail is greater than mHead, the data
+ * stored in the buffer at entries [mHead, mTail) will never be modified, and
+ * so are "safe" to read. If this reading is done on the same thread that
+ * owns mHead, then reading the range [mHead, mTail) is guaranteed to be safe
+ * since the range itself will not shrink.
+ */
+final class DrawTimingQueue {
+    private static final String LOGTAG = "GeckoDrawTimingQueue";
+    private static final int BUFFER_SIZE = 16;
+
+    private final DisplayPortMetrics[] mMetrics;
+    private final long[] mTimestamps;
+
+    private int mHead;
+    private int mTail;
+
+    DrawTimingQueue() {
+        mMetrics = new DisplayPortMetrics[BUFFER_SIZE];
+        mTimestamps = new long[BUFFER_SIZE];
+        mHead = BUFFER_SIZE - 1;
+        mTail = 0;
+    }
+
+    /**
+     * Add a new entry to the tail of the queue. If the buffer is full,
+     * do nothing. This must only be called from the Java UI thread.
+     */
+    boolean add(DisplayPortMetrics metrics) {
+        if (mHead == mTail) {
+            return false;
+        }
+        mMetrics[mTail] = metrics;
+        mTimestamps[mTail] = SystemClock.uptimeMillis();
+        mTail = (mTail + 1) % BUFFER_SIZE;
+        return true;
+    }
+
+    /**
+     * Find the timestamp associated with the given metrics, AND remove
+     * all metrics objects from the start of the queue up to and including
+     * the one provided. Note that because of draw coalescing, the metrics
+     * object passed in here may not be the one at the head of the queue,
+     * and so we must iterate our way through the list to find it.
+     * This must only be called from the compositor thread.
+     */
+    long findTimeFor(DisplayPortMetrics metrics) {
+        // keep a copy of the tail pointer so that we ignore new items
+        // added to the queue while we are searching. this is fine because
+        // the one we are looking for will either have been added already
+        // or will not be in the queue at all.
+        int tail = mTail;
+        // walk through the "safe" range from mHead to tail; these entries
+        // will not be modified unless we change mHead.
+        int i = (mHead + 1) % BUFFER_SIZE;
+        while (i != tail) {
+            if (mMetrics[i].fuzzyEquals(metrics)) {
+                // found it, copy out the timestamp to a local var BEFORE
+                // changing mHead or add could clobber the timestamp.
+                long timestamp = mTimestamps[i];
+                mHead = i;
+                return timestamp;
+            }
+            i = (i + 1) % BUFFER_SIZE;
+        }
+        return -1;
+    }
+
+    /**
+     * Reset the buffer to empty.
+     * This must only be called from the compositor thread.
+     */
+    void reset() {
+        // we can only modify mHead on this thread.
+        mHead = (mTail + BUFFER_SIZE - 1) % BUFFER_SIZE;
+    }
+}