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From Nick Kew <n...@webthing.com>
Subject Thundering herd patch for mod_cache
Date Sat, 09 Jan 2010 23:43:24 GMT
I've just been attempting to review Graham's proposed backport.
Looks interesting, but just a few points:

Trivial: needs r809335.
Trivial: indentation doesn't quite fit code style guidelines.

- In ap_cache_try_lock - do we really need the hashed directories
hardwired?  I thought most modern filesystems had abandoned
linear search, so that kind of thing is redundant!
At least make it optional - perhaps an additional arg to
the Cachelock directive.

- Why is ap_cache_remove_lock called from so many places
rather than just use a pool cleanup, and maybe in
cache_save_filter as the earliest possible point in the
normal processing path?

- Do we need to use lock files like this?  Not I think in
every case: with mod_disk_cache we already have files we
could lock (and create if they don't already exist).

- Your ELSE clause (cache_util, line 623) for requests
for a stale object that is being refreshed by another
request serves the stale object and adds a warning.
Is this the best thing to do?  What about waiting for
the file to be fetched?  Should still be quicker than
going to the backend in parallel with the other request.

-- 
Nick Kew


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