http://redis.io/topics/faq
What happens if Redis runs out of memory?
With modern operating
systems malloc() returning NULL is not common, usually the server will
start swapping and Redis performances will degrade so you'll probably
notice there is something wrong. The INFO command will report the
amount of memory Redis is using so you can write scripts that monitor
your Redis servers checking for critical conditions. Alternatively can
use the "maxmemory" option in the config file to put a limit to the
memory Redis can use. If this limit is reached Redis will start to
reply with an error to write commands (but will continue to accept
read-only commands), or you can configure it to evict keys when the
max memory limit is reached in the case you are using Redis for
caching.
Don't do anything...? At worst you get a serious performance degradation from swapping but that's it? You have error handling in your application anyways, don't you?