commit 91d8370969ef72232b8ed35b1c6e147142059249
author rniwa@webkit.org <rniwa@webkit.org@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Wed Aug 23 21:40:41 2017 +0000
committer rniwa@webkit.org <rniwa@webkit.org@268f45cc-cd09-0410-ab3c-d52691b4dbfc> Wed Aug 23 21:40:41 2017 +0000
tree 573ea5c3b3429ffa0df941dfb91d17211b1ecd4a
parent e2acc410d90589072928e72d912714428fda7522

Speedometer 2.0: Async time is not always capturing layout time
https://bugs.webkit.org/show_bug.cgi?id=175871

Reviewed by Simon Fraser.

Speedometer harness was designed to capture the time browser engine spends relayouting and repainting the content
after DOM mutations this in its async time measurement, which is the time between each test case's code had finished
running and when a newly scheduled 0s timer is fired immediately afterwards.

It turns out that modern web browsers defer this reflow and repaint work until the next animation frame is requested.
This results in Speedometer harness measuring reflow and repaint cost only sometimes depending on when each test case
had finished running relative to the next frame request.

While such a behavior makes sense and might be desirable for a modern browser engine, we would like to capture it in
the async time for the purpose of Speedometer. Unfortunately, there isn't an interoperable API for browsers to report
the total layout and repaint time, and relying on 16ms-granularity requestAnimationFrame is too coarse for Speedometer.

This patch works around these limitations by manually forcing the layout in async time measurement by calling
getBoundingClientRect() in iframe's document. Since the height of the document depends on the number of todo items,
this should cause browser engines to do most if not all of the work needed to reflow the document at least for now.

Note that even new async time doesn't always capture painting time but there isn't a good cross-browser mechanism
to measure paint time in the granurality we need for Speedometer at the moment anyway. (Should such a mechanism exist,
that could be a huge timing attack surface so it's probably best that we don't have one.)

* Speedometer/resources/benchmark-runner.js:
(BenchmarkRunner.prototype._runTest):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@221106 268f45cc-cd09-0410-ab3c-d52691b4dbfc