All, I've posted the results of my cache usage survey to
http://todesschaf.org/posts/2012/07/25/cache-usage-results.html

The short version of the story is that it appears we can
*significantly* reduce the max size of the disk cache without causing
problems related to unnecessary churn in the cache. I propose we do
the following:

(1) Cut the default max size from 1GiB to 512MiB for Firefox 17. Watch
telemetry for hit rate to make sure there are no unexpected bad
effects. Also, watch telemetry for lock wait time to see if this helps
at all with that (in parallel with other lock contention reducing
work). Assuming no objections here and a quick r+, a patch for this
could be landed tomorrow.
(2) When development starts on Firefox 18, cut the default max size
further to 350MiB (similar to chrome's number). Again, watch relevant
telemetry for any contraindications.

Why the 2-phase approach? To help mitigate the (once per version)
effect of slowing down startup by evicting a bunch of entries. It also
gives us a longer window to keep an eye on things, and having multiple
steps can help us see if the win by dropping to 350MiB is really more
significant from dropping to 512MiB (this of course assumes we see a
significant win from either of these actions).

Anyone have any concerns, or should we go ahead and give this a shot?
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