https://bugzilla.wikimedia.org/show_bug.cgi?id=55981
Web browser: ---
Bug ID: 55981
Summary: Redis can fill with non-expiring data from wgMemc
Product: MediaWiki-Vagrant
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: Unprioritized
Component: General
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
Mobile Platform: ---
We use 'redis' for wgMainCacheType. That means there are several places where
data is stored to the Redis cache without any expiration, using either wgMemc
or CACHE_ANYTHING. These include:
ResourceLoader -
https://git.wikimedia.org/blob/mediawiki%2Fcore.git/d987f43b24a24a6f3fcd5886fedaf483fa3c6891/includes%2Fresourceloader%2FResourceLoader.php#L180
User cache -
https://git.wikimedia.org/blob/mediawiki%2Fcore.git/d987f43b24a24a6f3fcd5886fedaf483fa3c6891/includes%2FUser.php#L368
The size of the Vagrant Redis database is rather low (16 MB), even considering
the default size of the box is 768 MB. Since Redis is also set to persist, the
16 MB can fill up.
Redis allows multiple policies for expiration
(https://raw.github.com/antirez/redis/2.6/redis.conf). We don't override this
currently, so it uses the default of 'volatile-lru'. This means, when max
memory is hit, it will try to pick a key with an expiration using an
approximated least recently used algorithm (this is further configurable with
maxmemory-samples).
The problem is that as stated above, a lot of keys don't have an expiration
set. My Redis database actually says none do currently, which is probably
because it has been at maxmemory for a while and thus has evicted all the ones
that could expire.
I'm not sure the ideal solution, and it would be helpful for someone more
familiar with MW's caching to weigh in. However, so far it seems like I think
increasing it to 32 MB (still less than 5% of the overall RAM) and using
allkeys-lru (remove any key, using least recently used) should be an
improvement (at least then it won't just stay filled and block all writes).
However, there might be a better way that makes it less likely to evict
important stuff like ongoing sessions (MediaWiki-Vagrant also uses Redis for
sessions).
Production uses 'memcached-pecl' for the main cache. I'm not sure exactly how
that expiration is configured, but prod doesn't currently deal with Redis here.
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