1358709 - Avoid burning CPU before showing the slow script infobar for callstacks from minified scripts by calling PCToLineNumber with no good reason

Status: RESOLVED FIXED

(Core :: DOM: Core & HTML, enhancement)

Description

[(no longer active)]

See the profile in bug 1330231 (https://clptr.io/2jC9bsJ).  We spend 15 seconds calling PCLineToNumber() in the content process before showing the slow script info bar.  I looked into where this data goes as we don't show it in the UI, and it is currently stored in HangMonitoredProcess::mHangData's lineno member, but nobody reads from it.  Basically the consumer is dead code.  So we can just not compute this in the first place!

Comment 1

[(no longer active)]

Attachment: Part 1: Remove nsIHangReport::GetScriptLineNo

This is dead code, and removing it is harmless.

Comment 2

[(no longer active)]

Attachment: Part 2: Remove the SlowScriptData.lineno IPDL field and the glue code

Comment 3

[(no longer active)]

Attachment: Part 3: Avoid calling PCToLineNumber before showing the slow script info bar in the content process

Comment 4

[Bill McCloskey [inactive unless it's an emergency] (:billm)]

Comment on attachment 8860595 [details] [diff] [review]
Part 3: Avoid calling PCToLineNumber before showing the slow script info bar in the content process

Review of attachment 8860595 [details] [diff] [review]:
-----------------------------------------------------------------

::: dom/base/nsGlobalWindow.cpp
@@ +11592,5 @@
> +  // parent process, so we avoid computing it elsewhere.  This gives us most of
> +  // the wins we are interested in, since the source of the slowness here is
> +  // minified scripts which is more common in Web content that is loaded in the
> +  // content process.
> +  unsigned* linenumber = XRE_IsParentProcess() ? &lineno : nullptr;

I think linenop would be a better variable name. That's how we do it in the JS engine at least.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/09fa7049871d
Part 1: Remove nsIHangReport::GetScriptLineNo; r=billm
https://hg.mozilla.org/integration/mozilla-inbound/rev/298e509e123b
Part 2: Remove the SlowScriptData.lineno IPDL field and the glue code; r=bill
https://hg.mozilla.org/integration/mozilla-inbound/rev/ee3a2d1d6cbf
Part 3: Avoid calling PCToLineNumber before showing the slow script info bar in the content process; r=billm

Comment 6

[Iris Hsiao [:ihsiao]]

https://hg.mozilla.org/mozilla-central/rev/09fa7049871d
https://hg.mozilla.org/mozilla-central/rev/298e509e123b
https://hg.mozilla.org/mozilla-central/rev/ee3a2d1d6cbf