1385120 - Speedometer Ember benchmark spends a lot of time in toString().

Status: NEW

(Core :: JavaScript Engine: JIT, defect, P2)

Description

[Sean Stangl [:sstangl]]

Profile here: https://perfht.ml/2w4g2x9

Running the Speedometer Ember benchmark, there is an Ion-heavy section that lasts for about 700ms. Within that section, we spend about 230ms just in the self-hosted toString().

toString() is mostly called in the context of the EmberJS function addObserverForContentKey(), reproduced below:

>  function addObserverForContentKey(content, keyName, proxy, idx, loc) {
>    while (--loc >= idx) {
>      var item = _emberRuntimeMixinsArray.objectAt(content, loc);
>      if (item) {
>        _emberMetalDebug.assert('When using @each to observe the array ' + content + ', the array must return an object', typeof item === 'object');
>        _emberMetalObserver._addBeforeObserver(item, keyName, proxy, 'contentKeyWillChange');
>        _emberMetalObserver.addObserver(item, keyName, proxy, >'contentKeyDidChange');
>      }
>    }
>  }

Note that the toString() call obviously happens in the context of a debug statement -- and that it's unfortunately converting an entire array to a String. Then it discards all that work.

The length of content begins at 1, then increases linearly to 100, at which point it remains at length 100 and gets converted to a string 10,000 times per benchmark iteration.

1) We should probably report this to Ember. They could avoid doing the Array.prototype.join() and toString() by just having assert() take a callback.

2) Since this is occurring in Ion, it's possible that we could observe that the toString() and join() are not effectful, inline assert(), and reorder that code into the taken branch.

Comment 1

[Sean Stangl [:sstangl]]

Also, the string built is just "[object Object],[object Object],[object Object],[object Object]," with "[object Object]," repeated 100 times.

Comment 2

[Jan de Mooij [:jandem]]

See also bug 1376948 and bug 1384562.