Using memcached -k to prevent paging

February 27, 2011 at 11:51 PM

Memcached is a powerful brute. Since it's basically just a giant in-memory hash map, you can safely run it on just about any machine that has spare memory. But be sure to know the sort of memory profile your machine has.

If something causes memcached to go to swap, performance degrades significantly. Paging can block all queries to the given server, which can in turn block all rendering for every page of your site. The performance degradation I observed was on the order of 30+ seconds, so this is enough for any number of layers in your application stack to time out inscrutably.

Fortunately, memcached provides the -k switch to indicate that its memory should be locked from paging. You'll wind up running like this:

brendan@ishmael:~$ sudo memcached -m 1024 -p 11211 -u memcache -l -k
warning: -k invalid, mlockall() failed: Cannot allocate memory

Oh, wait. Cannot allocate memory?

brendan@ishmael:~$ ulimit -l

Ah, I see. We're limited to 64k of locked memory by default. Fortunately, this can be changed with per-user granularity by adding some lines to /etc/security/limits.conf:

root            -       memlock         1048576

That takes the root user's limit up quite a bit. The memcached help output indicates that you should up the limit for the user that launches the program, not for the user that memcached eventually runs under.

Of course, even better than all this would be organizing the system architecture in such a way that the memcached machine never needs to swap.