ChangeLog

2011-02-18  Tony Gentilcore  <tonyg@chromium.org>

        Reviewed by NOBODY (OOPS!).

        Let the parser yield for layout before running scripts

        https://bugs.webkit.org/show_bug.cgi?id=54355

        Prior to this patch, the parser would yield to perform a layout/paint before running a

        script only if the script or a stylesheet blocking the script is not loaded yet. Since we

        don't preload scan into the body while parsing the head, typically we'll block on a script

        early in the body that causes us to yield to do the first paint within a reasonable time.

        However, I'm planning to change the PreloadScanner to scan into the body from the head.

        That significantly improves overall load time, but would hurt first paint time because

        fewer scripts would be blocked during parsing and thus wouldn't yield.

        This change causes us to yield before running scripts if we haven't painted yet (regardless

        of whether or not the script is loaded). In addition to allowing the above mentioned

        PreloadScanner change to be implemented without regressing first paint time, this also

        improves first paint time by itself.

        I tested Alexa's top 45 websites using Web Page Replay to control the content and simulate

        bandwidth. This patch improved average first paint time by 1% over an unlimited connection,

        6% over a 1Mbps connection and 11% over a 5Mbps connection. There was no statistically

        signifcant change in page load time.

        Within the pages tested, 33 had no statistically significant change in time to first paint,

        12 improved, and none regressed. Of the improved, some of the standouts from the 1Mbps set

        are: 20% on youtube, 37% on wiki, 27% on ebay, 13% on cnn, 16% on espn, 74% on sohu.

        * html/parser/HTMLDocumentParser.cpp:

        (WebCore::HTMLDocumentParser::canTakeNextToken): This is the new yield point.

        (WebCore::HTMLDocumentParser::pumpTokenizer): Remove ASSERT that we are not paused. isPaused

        means that we are waiting for a script. Bug 54574 changed pumpTokenizer() so that it does

        the right thing whether we are just before a token or waiting for a script. Now that we may

        yield before a token or before a script, this may be called while paused.

        * html/parser/HTMLParserScheduler.cpp:

        (WebCore::isLayoutTimerActive): Added a FIXME because r52919 changed minimumLayoutDelay()

        to return m_extraLayoutDelay instead of 0 as a minimum. So checking !minimumLayoutDelay()

        no longer works. The fix is to change it to check minimumLayoutDelay() ==

        m_extraLayoutDelay. But this is all the more reason to move this method onto Document. I'll

        do this in a follow up.

        (WebCore::HTMLParserScheduler::checkForYieldBeforeScript): Added.

        * html/parser/HTMLParserScheduler.h:

        (WebCore::HTMLParserScheduler::checkForYieldBeforeToken):

        * page/FrameView.h:

        (WebCore::FrameView::hasEverPainted): Added.

2011-02-16  Tony Gentilcore  <tonyg@chromium.org>

        Reviewed by Eric Seidel.

Source/WebCore/html/parser/HTMLDocumentParser.cpp

@@bool HTMLDocumentParser::canTakeNextToken(SynchronousMode mode, PumpSession& ses

    // The parser will pause itself when waiting on a script to load or run.

    if (m_treeBuilder->isPaused()) {

        if (mode == AllowYield && m_parserScheduler->checkForYieldBeforeScript(session))

            return false;

        // If we're paused waiting for a script, we try to execute scripts before continuing.

        bool shouldContinueParsing = runScriptsForPausedTreeBuilder();

        m_treeBuilder->setPaused(!shouldContinueParsing);

@@bool HTMLDocumentParser::canTakeNextToken(SynchronousMode mode, PumpSession& ses

        && document()->frame() && document()->frame()->navigationScheduler()->locationChangePending())

        return false;

227 if (mode == AllowYield)

228 ~~m_p~~ar~~serScheduler->checkForYieldBeforeToken(session)~~;

230 if (mode == AllowYield && m_parserScheduler->checkForYieldBeforeToken(session))

231 return false;

    return true;

}

@@bool HTMLDocumentParser::canTakeNextToken(SynchronousMode mode, PumpSession& ses

void HTMLDocumentParser::pumpTokenizer(SynchronousMode mode)

{

    ASSERT(!isStopped());

236 ASSERT(!m_treeBuilder->isPaused());

    ASSERT(!isScheduledForResume());

    // ASSERT that this object is both attached to the Document and protected.

    ASSERT(refCount() >= 2);

@@void HTMLDocumentParser::pumpTokenizer(SynchronousMode mode)

    InspectorInstrumentationCookie cookie = InspectorInstrumentation::willWriteHTML(document(), m_input.current().length(), m_tokenizer->lineNumber());

    PumpSession session;

249 while (canTakeNextToken(mode, session) ~~&& !session.needsYield~~) {

251 while (canTakeNextToken(mode, session)) {

        m_sourceTracker.start(m_input, m_token);

        if (!m_tokenizer->nextToken(m_input.current(), m_token))

            break;

Source/WebCore/html/parser/HTMLParserScheduler.cpp

@@HTMLParserScheduler::~HTMLParserScheduler()

static bool isLayoutTimerActive(Document* doc)

{

    ASSERT(doc);

81 // FIXME: This is broken on Android because minimumLayoutDelay is never 0.

    return doc->view() && doc->view()->layoutPending() && !doc->minimumLayoutDelay();

}

@@void HTMLParserScheduler::continueNextChunkTimerFired(Timer<HTMLParserScheduler>

    m_parser->resumeParsingAfterYield();

}

bool HTMLParserScheduler::checkForYieldBeforeScript(PumpSession& session)

{

    // If we've never painted before and a layout is pending, yield prior to running

    // scripts to give the page a chance to paint earlier.

    Document* document = m_parser->document();

    bool needsFirstPaint = document->view() && !document->view()->hasEverPainted();

    if (needsFirstPaint && isLayoutTimerActive(document)) {

        session.needsYield = true;

        return true;

    }

    return false;

}

void HTMLParserScheduler::scheduleForResume()

{

    m_continueNextChunkTimer.startOneShot(0);

Source/WebCore/html/parser/HTMLParserScheduler.h

@@public:

    ~HTMLParserScheduler();

    // Inline as this is called after every token in the parser.

60 v~~oid~~ checkForYieldBeforeToken(PumpSession& session)

60 bool checkForYieldBeforeToken(PumpSession& session)

    {

        if (session.processedTokens > m_parserChunkSize) {

            session.processedTokens = 0;

            double elapsedTime = currentTime() - session.startTime;

65 if (elapsedTime > m_parserTimeLimit)

65 if (elapsedTime > m_parserTimeLimit) {

6666 session.needsYield = true;

67 return true;

68 }

6769 }

6870 ++session.processedTokens;

71 return false;

6972 }

73 bool checkForYieldBeforeScript(PumpSession&);

    void scheduleForResume();

    bool isScheduledForResume() const { return m_isSuspendedWithActiveTimer || m_continueNextChunkTimer.isActive(); }

Source/WebCore/ChangeLog

Source/WebCore/html/parser/HTMLDocumentParser.cpp

Source/WebCore/html/parser/HTMLParserScheduler.cpp

Source/WebCore/html/parser/HTMLParserScheduler.h

Source/WebCore/page/FrameView.h