I believe @sam once said
redis was chucking an error causing a 500 on bgsave despite having a gig and a half of free ram.
It needed this overcommit flag on the kernel, I will be amending ./launcher to add a pre-req test for this setting cause it a rather bad failure mode for no legit reason.
Which is described thusly at http://redis.io/topics/faq
Background saving is failing with a fork() error under Linux even if I’ve a lot of free RAM!
echo 1 > /proc/sys/vm/overcommit_memory
And now the long one:
0: heuristic overcommit (this is the default)
1: always overcommit, never check
Redis background saving schema relies on the copy-on-write semantic of fork in modern operating systems: Redis forks (creates a child process) that is an exact copy of the parent. The child process dumps the DB on disk and finally exits. In theory the child should use as much memory as the parent being a copy, but actually thanks to the copy-on-write semantic implemented by most modern operating systems the parent and child process will share the common memory pages. A page will be duplicated only when it changes in the child or in the parent. Since in theory all the pages may change while the child process is saving, Linux can’t tell in advance how much memory the child will take, so if the overcommit_memory setting is set to zero fork will fail unless there is as much free RAM as required to really duplicate all the parent memory pages, with the result that if you have a Redis dataset of 3 GB and just 2 GB of free memory it will fail.
Setting overcommit_memory to 1 says Linux to relax and perform the fork in a more optimistic allocation fashion, and this is indeed what you want for Redis.
It generally won’t be an issue unless your Discourse is quite massive and thus Redis is storing a super huge number of key/value pairs, but we should fix it up.
(also it would be nice to not see that warning all the time, and get questions about it…)