Bug 75001
| Summary: | Painting on a table with overflow: hidden cells is very slow | ||
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| Product: | WebKit | Reporter: | Julien Chaffraix <jchaffraix> |
| Component: | Layout and Rendering | Assignee: | Nobody <webkit-unassigned> |
| Status: | NEW | ||
| Severity: | Normal | CC: | dglazkov, eric, jamesr, simon.fraser, thorton, tomhudson, tonikitoo |
| Priority: | P2 | ||
| Version: | 528+ (Nightly build) | ||
| Hardware: | All | ||
| OS: | All | ||
| URL: | http://dglazkov.github.com/performance-tests/biggrid.html | ||
| See Also: | https://bugs.webkit.org/show_bug.cgi?id=73715 | ||
| Bug Depends on: | 88386, 89389, 89899, 85678, 88464, 88570, 88888 | ||
| Bug Blocks: | 73714, 92258 | ||
Julien Chaffraix
Seen on the benchmark: http://dglazkov.github.com/performance-tests/biggrid.html. If you toggle overflow: hidden and scroll on the table, you can see that painting takes several times more time that without overflow: hidden (for example by looking at the WebInspector timeline).
RenderTableSection implements an optimization during painting where it finds the dirtied cells to paint using a binary search on the rows and columns (see RenderTableSection::paintObject). When we toggle overflow: hidden, the painting goes through the RenderLayer path. However there is no such optimization and we spend a lot of time trying to repaint layers that are not dirty (or even worse off-screen). This bug may be related to bug 73715 but it looks like there is more to the other bug (composition is involved for example).
As a side note, hit testing has exactly the same issue.
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Julien Chaffraix
For the record, when scrolling on the page on a 10,000 * 100 table with overflow: hidden, the painting used to take ~255 ms (for 2048 x 256). We are now down to ~45 ms.
Painting has been sped up but the core issue still remains: we should have visit only the visible cells instead of all the cells in the table.
Julien Chaffraix
(In reply to comment #1)
> For the record, when scrolling on the page on a 10,000 * 100 table with overflow: hidden, the painting used to take ~255 ms (for 2048 x 256). We are now down to ~45 ms.
Bug 88888 landed another performance optimization. It makes us not visit any of the cells in biggrid.html particular case. This brings the paint time to ~6ms in the same conditions.
> Painting has been sped up but the core issue still remains: we should have visit only the visible cells instead of all the cells in the table.
The optimization is somewhat limited and could be improved upon. First hit-testing is still sluggish but also the optimization relies on having _no_ self-painting-descendants which means that if you have one, you don't reap any performance benefit.
I am keeping this bug open to track more of the fixes.
Eric Seidel (no email)
bug 92258 is another bug on the topic of this benchmark. Perhaps it should be duped against this one or both related to some more generic meta-bug.