On 2010-03-12 17:07:25 -0800, dormando wrote: > Now, it should be obvious that if a user session has reached a point where > it would be evicted early, it is because you did not have enough memory to > store *all active sessions anyway*. The odds of it evicting someone who > has visited your site *after* me are highly unlikely. The longer I stay > off the site, the higher the odds of it being evicted early due to lack of > memory. > > This does mean, by way of painfully describing how an LRU works, that the > odds of you finding sessions in memcached which have not been expired, but > are being evicted from the LRU earlier than expired sessions, is very > unlikely. [...] > The caveat is that memcached has one LRU per slab class. > > So, lets say your traffic ends up looking like: > > - For the first 10,000 sessions, they are all 200 kilobytes. This ends up > having memcached allocate all of its slab memory toward something that > will fit 200k items. > - You get linked from the frontpage of digg.com and suddenly you have a > bunch of n00bass users hitting your site. They have smaller sessions since > they are newbies. 10k items. > - Memcached has only reserved 1 megabyte toward 10k items. So now all of > your newbies share a 1 megabyte store for sessions, instead of 200 > megabytes.
There's another caveat (I think Martin may have been referring to this
scenario, but he wasn't very clear):
Suppose you have two kinds of entries in your memcached, with different
expire times. For example, in addition to your sessions with 3600s, you
have some alert box with an expiry time of 60s. By chance,
both items are approximately the same size and occupy the same slab
class(es).
You have enough memory to keep all sessions for 3600 seconds and enough
memory to keep all alert boxes for 60 seconds. But you don't have enough
memory to keep all alert boxes for 3600 seconds (why should you, they expire
after 60 seconds).
Now, when you walk the LRU chain, the search for expired items will only
return expired alert boxes which are about as old as your oldest session.
As soon as there are 50 (not yet expired) sessions older than the oldest
(expired) alert box, you will evict a session although you still have a
lot of expired alert boxes which you could reuse.
The only workaround for this problem I can see is to use different
memcached servers for items of (wildly) different expiration times.
> However the slab out of balance thing is a real fault of ours. It's a
> project on my plate to have automated slab rebalancing done in some usable
> fashion within the next several weeks. This means that if a slab is out of
> memory and under pressure, memcached will decide if it can pull memory
> from another slab class to satisfy that need. As the size of your items
> change over time, it will thus try to compensate.
That's good to hear.
hp
--
_ | Peter J. Holzer | Openmoko has already embedded
|_|_) | Sysadmin WSR | voting system.
| | | h...@hjp.at | Named "If you want it -- write it"
__/ | http://www.hjp.at/ | -- Ilja O. on commun...@lists.openmoko.org
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