Australis update

Justin Dolske dolske at
Wed Nov 13 22:47:35 UTC 2013

On 11/13/13 2:03 PM, Matt Woodrow wrote:
> I agree that this shouldn't block the australis landing.
> I don't think that we need to rush into a backout just yet though.
> Reverting the changing is incredibly simple, and it's very unlikely that
> we'll regress anything on the existing path (since all other platforms
> are still using it).
> I'd much prefer to leave it in for now, so that we can get more user
> coverage of the changes and potentially find out more information on the
> effects the change has. We already missed one regression on
> mozilla-central, it's fairly plausible that there are other issues not
> covered by tests on any branch.

It should be backed out (or pref'd off) as soon as possible, for a few 

* Tree policy is pretty clear about the need to back out regressions. 
This is a bit of a special circumstance, but had the order been reversed 
(with Australis landing a few days before your patch) there would be no 
question about it.

* Our bug/perf tracking tools are... not great, and are geared around 
detecting regressions when they land. Not for making sure that future 
improvements restore previous performance. (Part of the reason for the 
backout policy.) This only gets more complicated the longer things stay 
in the tree.

* When Australis was discovered to regress TART back in August -- 
coincidentally, also around 40% -- there was extreme pushback from all 
quarters stressing the importance of not regressing performance. Even 
though I said that would mean we'd have to redo a bunch of work and blow 
out the schedule by months (which is exactly what happened). So the 
tolerance for regression has already been set, and should apply here as 


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