What does Redis do when it runs out of memory?

Question

This might be easy question but I am having a hard time finding the answer. How does Redis 2.0 handle running out of maximum allocated memory? How does it decide which data to remove or which data to keep in memory?

Answer 1

If you have virtual memory functionality turned on (EDIT: now deprecated), then Redis starts to store the "not-so-frequently-used" data to disk when memory runs out.

If virtual memory in Redis is disabled (the default) and the maxmemory parameter is set (the default), Redis will not use any more memory than maxmemory allows. If you turn maxmemory off, Redis will start using virtual memory (i.e. swap), and performance will drop tremendously.

Newer versions of Redis have various policies when maxmemory is reached:

• volatile-lru - remove a key among the ones with an expire set, trying to remove keys not recently used.
• volatile-ttl - remove a key among the ones with an expire set, trying to remove keys with short remaining time to live.
• volatile-random - remove a random key among the ones with an expire set.
• allkeys-lru - like volatile-lru, but will remove every kind of key, both normal keys or keys with an expire set.
• allkeys-random - like volatile-random, but will remove every kind of keys, both normal keys and keys with an expire set.

If you pick a policy that only removes keys with an EXPIRE set, then when Redis runs out of memory, it looks like the program just aborts the malloc() operation. That is, if you try to store more data, the write operation simply fails.

Some links for more info:

• http://antirez.com/post/redis-as-LRU-cache.html
• http://eli.thegreenplace.net/2009/10/30/handling-out-of-memory-conditions-in-c/