Hi dormando, thanks for your reply.
Thanks for confirming that the issues we thought we were seeing should
not have happened. It actually helped us to look for other possible
causes... :-)
It turns out that one of the memcached processes in our web server
cluster was running an old version. Whenever we tried to save a specific
piece of data, it was redirected there, and we got the "out of memory"
error. Now we've upgraded it as well, and everything seems to work just
fine.
Thanks!
Håkan Waara
Travelocity Nordic
dormando wrote:
Hey,
Are you running memcached with the -M option? What version are you
using? What clients?
The thing is, we don't have 100% control over the size of the data we
typically cache. It may change over time. The problem then of course
becomes that the slab classes are already populated, so things of
other sizes won't find a slab class that matches their size, and thus
cannot be cached at all.
This _should_ only happen when running in -M mode. What should
otherwise happen are cache evictions. So if there isn't enough memory
available in a slab class it would eject something else from the cache
first.
One common suggestion seems to be to "warm up" the cache. Would it be
a bad idea to warm up every slab class with a minimum of 1 item, just
to make sure they're all "open"? I guess that is something we'd like
to avoid.
Recent releases of memcached do this already, expressly to avoid that
issue.
Another way is to manually adjust the slab sizes over time, using
memcached-tool to move items around, or even restarting the daemons.
This seems a bit tedious though.
It might not work very well. The slab reassignment stuff is aligned
for a factor of 2.0, but the default slab class factor is now 1.2... I
don't fully grok how that affects everything yet, but noticed while
reading the code.
Is there any way to automatically adjust the slabs over time, as the
data stored changes? What's the current practice for dealing with
this problem, for other users of memcached?
Any advice appreciated - thanks!
No way as of yet, but you probably shouldn't be getting out of memory
errors outright, so there's a chance something else is holding you
back here.
-Dormando
