1348761 - Console hangs the browser when attempting to view a printed object

Status: RESOLVED FIXED

(DevTools :: Console, defect, P2)

Description

[Jukka Jylänki]

STR:

1. Visit http://mzl.la/webassemblydemo
2. After the page loads, open Console devtool and type in "Module" and press enter.
3. The Module object gets printed to console. Click on the word "Module" to attempt to view that object in the console's "object explorer" view.

Observed:

The browser tab hangs. Tested to reproduce on FF 52 stable on OS Xand current FF Nightly on Windows 10.

Comment 1

[Brian Grinstead [:bgrins]]

Obviously we shouldn't be hanging, but note that there's work upcoming to drop the variables view in favor of expanding the object inline (Bug 1308566).

We'll need to test this page with the new expansion ui and make sure that it doesn't hang there either.  I did a quick test in the new Debugger (which is using the same upcoming expansion ui) by pausing and then setting 'Module' as a Watch Expression and attempting to expand, and it seems to have a similar problem.  I filed https://github.com/devtools-html/debugger.html/issues/2671 for this.

Comment 2

[Alexandre Poirot [:ochameau] [PTO back August 21]]

Here is a profile:
  https://perfht.ml/2pHcwGK

Most of the time is spent in grip function:
  http://searchfox.org/mozilla-central/source/devtools/server/actors/object.js#79-143
Then about 70% of that time is spent in:
  g.ownPropertyLength = this.obj.getOwnPropertyNames().length;

Which is weird as it seems to be fast if you call Object.getOwnPropertyNames(Module).length.
May be the Debugger API getOwnPropertyNames is slower??

The rest of the time, a bit less than 30% is spent in TypedArray:
http://searchfox.org/mozilla-central/source/devtools/server/actors/object.js#1472-1504
Itself spending most of its time on Cu.cloneInto.

Comment 3

[Alexandre Poirot [:ochameau] [PTO back August 21]]

Note that it is easier to test against:
  http://www.hellorust.com/emscripten/sha1/
which doesn't introduce massive computation and slow down the profiler.

Comment 4

[Alexandre Poirot [:ochameau] [PTO back August 21]]

Oh, it looks like it isn't against Module object itself, but Module.HEAP* attributes which are big typed array.
We end up retrieving grips for all attributes of Module, this is what takes time

Object.getOwnPropertyNames(Module.HEAP8).length = 16777216
And takes more than a second to compute.

Comment 5

[Alexandre Poirot [:ochameau] [PTO back August 21]]

Attachment: Bug 1348761 - Fix object inspection performances against objects with large Typed Arrays.

Review commit: https://reviewboard.mozilla.org/r/132128/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/132128/

Comment 6

[Brian Grinstead [:bgrins]]

Comment on attachment 8860114 [details]
Bug 1348761 - Fix object inspection performances against objects with large Typed Arrays.

https://reviewboard.mozilla.org/r/132128/#review134984

Great find - this is much better!  I think this will also fix https://github.com/devtools-html/debugger.html/issues/2671

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/934221c412fc
Fix object inspection performances against objects with large Typed Arrays. r=bgrins

Comment 8

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/934221c412fc