Bug 308946 – contain: layout causes ~8x regression in forced synchronous reflow on in-flow elements

• Component: Layout and Rendering
• Status: RESOLVED FIXED

Created attachment 478537 [details]
Repro

Adding contain: layout (or contain: strict, contain: content, contain: size layout) to in-flow position: relative elements causes a ~7-8x performance regression
during interleaved read/write DOM access patterns (forced synchronous reflow), compared to the same elements without containment.

overflow: hidden on in-flow elements exhibits the same regression.

Repro:

1. Open the attached HTML file
2. Observe the printed timing ratio

Expected: contain: layout should be equal or faster than baseline, since it restricts layout scope.

Actual: contain: layout is ~7-8x slower than baseline.

Test results on Safari
baseline (position:relative):  ~69ms
+ contain:layout:              ~539ms
ratio:                         ~7.8x

Chrome, same hardware & OS:
baseline (position:relative):  ~43ms
+ contain:layout:              ~32ms
ratio:                         ~0.75x  (faster, as expected)

Batched access (all writes then all reads) shows no regression — both ~4-6ms. The issue is specific to forced synchronous reflow triggered by interleaved
write-then-read patterns.

[Simon Fraser (smfr)]

Surprising!

[Radar WebKit Bug Importer]

<rdar://problem/171545381>

[Simon Fraser (smfr)]

(Look at the relative %, not the actual %):

Without contain:

Sample Count, Samples%, Normalized CPU %, Symbol
24, 26.7%, 1.2%, WebCore::Element::offsetHeight() (in WebCore)
24, 26.7%, 1.2%,     WebCore::Document::updateLayoutIfDimensionsOutOfDate(WebCore::Element&, WTF::OptionSet<WebCore::DimensionsCheck, (WTF::ConcurrencyTag)0>, WTF::OptionSet<WebCore::LayoutOptions, (WTF::ConcurrencyTag)0>) (in WebCore)
22, 24.4%, 1.1%,         WebCore::Document::updateLayout(WTF::OptionSet<WebCore::LayoutOptions, (WTF::ConcurrencyTag)0>, WebCore::Element const*) (in WebCore)
22, 24.4%, 1.1%,             WebCore::LocalFrameViewLayoutContext::layout(bool) (in WebCore)
22, 24.4%, 1.1%,                 WebCore::LocalFrameViewLayoutContext::performLayout(bool) (in WebCore)
13, 14.4%, 0.7%,                     WebCore::LocalFrameViewLayoutContext::flushUpdateLayerPositions() (in WebCore)
13, 14.4%, 0.7%,                         void WebCore::RenderLayer::recursiveUpdateLayerPositions<(WebCore::RenderLayer::UpdateLayerPositionsMode)0>(WTF::OptionSet<WebCore::RenderLayer::UpdateLayerPositionsFlag, (WTF::ConcurrencyTag)0>) (in WebCore)
9, 10.0%, 0.5%,                     WebCore::RenderView::layout() (in WebCore)
9, 10.0%, 0.5%,                         WebCore::RenderBlock::layout() (in WebCore)
9, 10.0%, 0.5%,                             WebCore::RenderBlockFlow::layoutBlock(WebCore::RelayoutChildren, WebCore::LayoutUnit) (in WebCore)
9, 10.0%, 0.5%,                                 WebCore::RenderBlockFlow::layoutBlockChildren(WebCore::RelayoutChildren, WebCore::LayoutUnit&) (in WebCore)
9, 10.0%, 0.5%,                                     WebCore::RenderBlockFlow::layoutBlockChild(WebCore::RenderBox&, WebCore::RenderBlockFlow::MarginInfo&, WebCore::LayoutUnit&, WebCore::LayoutUnit&) (in WebCore)
3, 3.3%, 0.2%,                                         WebCore::RenderBox::avoidsFloats() const (in WebCore)


With contain:

Sample Count, Samples%, Normalized CPU %, Symbol
304, 48.3%, 2.1%, WebCore::Element::offsetHeight() (in WebCore)
304, 48.3%, 2.1%,     WebCore::Document::updateLayoutIfDimensionsOutOfDate(WebCore::Element&, WTF::OptionSet<WebCore::DimensionsCheck, (WTF::ConcurrencyTag)0>, WTF::OptionSet<WebCore::LayoutOptions, (WTF::ConcurrencyTag)0>) (in WebCore)
299, 47.5%, 2.1%,         WebCore::Document::updateLayout(WTF::OptionSet<WebCore::LayoutOptions, (WTF::ConcurrencyTag)0>, WebCore::Element const*) (in WebCore)
299, 47.5%, 2.1%,             WebCore::LocalFrameViewLayoutContext::layout(bool) (in WebCore)
299, 47.5%, 2.1%,                 WebCore::LocalFrameViewLayoutContext::performLayout(bool) (in WebCore)
287, 45.6%, 2.0%,                     WebCore::RenderView::layout() (in WebCore)
287, 45.6%, 2.0%,                         WebCore::RenderBlock::layout() (in WebCore)
287, 45.6%, 2.0%,                             WebCore::RenderBlockFlow::layoutBlock(WebCore::RelayoutChildren, WebCore::LayoutUnit) (in WebCore)
287, 45.6%, 2.0%,                                 WebCore::RenderBlockFlow::layoutBlockChildren(WebCore::RelayoutChildren, WebCore::LayoutUnit&) (in WebCore)
287, 45.6%, 2.0%,                                     WebCore::RenderBlockFlow::layoutBlockChild(WebCore::RenderBox&, WebCore::RenderBlockFlow::MarginInfo&, WebCore::LayoutUnit&, WebCore::LayoutUnit&) (in WebCore)
209, 33.2%, 1.5%,                                         WebCore::RenderBlock::createsNewFormattingContext() const (in WebCore)
20, 3.2%, 0.1%,                                         WebCore::RenderBlock::layout() (in WebCore)
20, 3.2%, 0.1%,                                             WebCore::RenderBlockFlow::layoutBlock(WebCore::RelayoutChildren, WebCore::LayoutUnit) (in WebCore)
16, 2.5%, 0.1%,                                                 WebCore::RenderBlockFlow::layoutBlockChildren(WebCore::RelayoutChildren, WebCore::LayoutUnit&) (in WebCore)

[Yulun Wu]

Pull request: https://github.com/WebKit/WebKit/pull/61069

[EWS]

Committed 310173@main (541a1c2870a0): <https://commits.webkit.org/310173@main>

Reviewed commits have been landed. Closing PR #61069 and removing active labels.