How to troubleshoot redis high CPU usage? And how to limit Redis CPU usage?

Question

We have inherited a system in which there is a central server for queuing operations. Redis is chosen as queuing agent.

Once in a while (like each 2-3 days) Redis service CPU usage goes even over 100%.

I tried to read the log to find the cause:

tail /var/log/redis/redis-server.log 

But it returns empty result.

I found this article that proposes some stuff. But Redis does not answer my commands.

I'm stuck at this point, on what to do and how to find the problem. Also, is there a way to limit the amount of CPU usage of redis?

Answer 1

I was also having problems with redis CPU usage. My case was many redis connections (clients) and linux default ulimit of 1024 open files.

High CPU usage could be caused by some CPU intensive operations like smembers.

You can also use the SLOWLOG redis command to log the long running commands.

The best you can do to trace the problem, when no log and SLOWLOG results, is to strace the redis process and see what is going on. For my case it was 'Too many open files' error, but it was only seen in strace output, which was solved by increasing default nr of files limit over 1024.

Answer 2

By default Redis doesn't impose any limitations on resource usage. So all depends of the power of the server. I would recommend you configure Redis with limitations:

1. Edit this file: /etc/redis.conf

2. Add the memory limitation (in bytes): maxmemory 536870912

   (512mb in my case, but you must use the limitation of ram that you want/need)

3. Also add this rule line: maxmemory-policy volatile-lru

With this rule activated, when the maxmemory is full Redis removes the less-used keys from the cache to give space to the new data.

If you want more info about this, everything is well commented in /etc/redis.conf.