1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mobile/android/base/gfx/DrawTimingQueue.java Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,95 @@
1.4 +/* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-
1.5 + * This Source Code Form is subject to the terms of the Mozilla Public
1.6 + * License, v. 2.0. If a copy of the MPL was not distributed with this file,
1.7 + * You can obtain one at http://mozilla.org/MPL/2.0/. */
1.8 +
1.9 +package org.mozilla.gecko.gfx;
1.10 +
1.11 +import android.os.SystemClock;
1.12 +
1.13 +/**
1.14 + * A custom-built data structure to assist with measuring draw times.
1.15 + *
1.16 + * This class maintains a fixed-size circular buffer of DisplayPortMetrics
1.17 + * objects and associated timestamps. It provides only three operations, which
1.18 + * is all we require for our purposes of measuring draw times. Note
1.19 + * in particular that the class is designed so that even though it is
1.20 + * accessed from multiple threads, it does not require synchronization;
1.21 + * any concurrency errors that result from this are handled gracefully.
1.22 + *
1.23 + * Assuming an unrolled buffer so that mTail is greater than mHead, the data
1.24 + * stored in the buffer at entries [mHead, mTail) will never be modified, and
1.25 + * so are "safe" to read. If this reading is done on the same thread that
1.26 + * owns mHead, then reading the range [mHead, mTail) is guaranteed to be safe
1.27 + * since the range itself will not shrink.
1.28 + */
1.29 +final class DrawTimingQueue {
1.30 + private static final String LOGTAG = "GeckoDrawTimingQueue";
1.31 + private static final int BUFFER_SIZE = 16;
1.32 +
1.33 + private final DisplayPortMetrics[] mMetrics;
1.34 + private final long[] mTimestamps;
1.35 +
1.36 + private int mHead;
1.37 + private int mTail;
1.38 +
1.39 + DrawTimingQueue() {
1.40 + mMetrics = new DisplayPortMetrics[BUFFER_SIZE];
1.41 + mTimestamps = new long[BUFFER_SIZE];
1.42 + mHead = BUFFER_SIZE - 1;
1.43 + mTail = 0;
1.44 + }
1.45 +
1.46 + /**
1.47 + * Add a new entry to the tail of the queue. If the buffer is full,
1.48 + * do nothing. This must only be called from the Java UI thread.
1.49 + */
1.50 + boolean add(DisplayPortMetrics metrics) {
1.51 + if (mHead == mTail) {
1.52 + return false;
1.53 + }
1.54 + mMetrics[mTail] = metrics;
1.55 + mTimestamps[mTail] = SystemClock.uptimeMillis();
1.56 + mTail = (mTail + 1) % BUFFER_SIZE;
1.57 + return true;
1.58 + }
1.59 +
1.60 + /**
1.61 + * Find the timestamp associated with the given metrics, AND remove
1.62 + * all metrics objects from the start of the queue up to and including
1.63 + * the one provided. Note that because of draw coalescing, the metrics
1.64 + * object passed in here may not be the one at the head of the queue,
1.65 + * and so we must iterate our way through the list to find it.
1.66 + * This must only be called from the compositor thread.
1.67 + */
1.68 + long findTimeFor(DisplayPortMetrics metrics) {
1.69 + // keep a copy of the tail pointer so that we ignore new items
1.70 + // added to the queue while we are searching. this is fine because
1.71 + // the one we are looking for will either have been added already
1.72 + // or will not be in the queue at all.
1.73 + int tail = mTail;
1.74 + // walk through the "safe" range from mHead to tail; these entries
1.75 + // will not be modified unless we change mHead.
1.76 + int i = (mHead + 1) % BUFFER_SIZE;
1.77 + while (i != tail) {
1.78 + if (mMetrics[i].fuzzyEquals(metrics)) {
1.79 + // found it, copy out the timestamp to a local var BEFORE
1.80 + // changing mHead or add could clobber the timestamp.
1.81 + long timestamp = mTimestamps[i];
1.82 + mHead = i;
1.83 + return timestamp;
1.84 + }
1.85 + i = (i + 1) % BUFFER_SIZE;
1.86 + }
1.87 + return -1;
1.88 + }
1.89 +
1.90 + /**
1.91 + * Reset the buffer to empty.
1.92 + * This must only be called from the compositor thread.
1.93 + */
1.94 + void reset() {
1.95 + // we can only modify mHead on this thread.
1.96 + mHead = (mTail + BUFFER_SIZE - 1) % BUFFER_SIZE;
1.97 + }
1.98 +}
