Bug 125944 – REGRESSION(r156214): 11-13% regressions on Dromaeo/jslib-traverse-jquery

• Status: NEW
• Component: JavaScriptCore

[Ryosuke Niwa]

See https://perf.webkit.org/#mode=charts&chartList=%5B%5B%22mac-mountainlion%22%2C%22Dromaeo%2Fjslib-traverse-jquery%3ARuns%22%5D%2C%5B%22mac-mountainlion%22%2C%22Dromaeo%2Fjslib-traverse-prototype%3ARuns%22%5D%2C%5B%22mac-lion%22%2C%22Dromaeo%2Fjslib-traverse-jquery%3ARuns%22%5D%5D&days=218&zoom=%5B1379161460728.1538%2C1380252801171.3845%5D

http://trac.webkit.org/log/?rev=156215&stop_rev=156206&verbose=on

[Radar WebKit Bug Importer]

<rdar://problem/15710364>

[Ryosuke Niwa]

I've bisected locally.  This regression is caused by http://trac.webkit.org/changeset/156214.

[Ryosuke Niwa]

Created attachment 219817 [details]
A/B test result

[Ryosuke Niwa]

*** Bug 125953 has been marked as a duplicate of this bug. ***

[Mark Hahnenberg]

The original bug was that we would be in the middle of copying, see and hole and we realize we should have done something else, and then we start copying all over again.

The fix was a simple one: first check if there's a hole, then do the copy. So you're always doing two passes over the array.

If we want to fix this regression then we need a way to start copying and then gracefully fail when we see a hole without forgetting that we had already started copying.

[Mark Hahnenberg]

(In reply to comment #5)
> The original bug was that we would be in the middle of copying, see and hole and we realize we should have done something else, and then we start copying all over again.
> 
..see a hole, realize we should...

Jeez.

[Ryosuke Niwa]

I think we need to come up with some strategy to improve the performance of jslib-traverse-jquery because we've regressed this particular test thrice:

https://perf.webkit.org/#mode=charts&chartList=%5B%5B%22mac-mountainlion%22%2C%22Dromaeo%2Fjslib-traverse-jquery%3ARuns%22%5D%5D&days=200

We used to get 22 runs/s but now we're getting 17 runs/s, a hopping 30% regression in the total :(

Perhaps we can improve elsewhere to compensate?