Redis is taking too long to respond

Question

Experiencing very high response latency with Redis, to the point of not being able to output information when using the info command through redis-cli.

This server handles requests from around 200 concurrent processes but it does not store too much information (at least to our knowledge). When the server is responsive, the info command reports used memory around 20 - 30 MB.

When running top on the server, during periods of high response latency, CPU usage hovers around 95 - 100%.

What are some possible causes for this kind of behavior?

Answer 1

It is difficult to propose an explanation only based on the provided data, but here is my guess. I suppose that you have already checked the obvious latency sources (the ones linked to persistence), that no Redis command is hogging the CPU in the slow log, and that the size of the job data pickled by Python-rq is not huge.

According to the documentation, Python-rq inserts the jobs into Redis as hash objects, and let Redis expires the related keys (500 seconds seems to be the default value) to get rid of the jobs. If you have some serious throughput, at a point, you will have many items in Redis waiting to be expired. Their number will be high compared to the pending jobs.

You can check this point by looking at the number of items to be expired in the result of the INFO command.

Redis expiration is based on a lazy mechanism (applied when a key is accessed), and a active mechanism based on key sampling, which is run in the event loop (in pseudo background mode, every 100 ms). The point is when the active expiration mechanism is running, no Redis command can be processed.

To avoid impacting the performance of the client applications too much, only a limited number of keys are processed each time the active mechanism is triggered (by default, 10 keys). However, if more than 25% keys are found to be expired, it tries to expire more keys and loops. This is the way this probabilistic algorithm automatically adapt its activity to the number of keys Redis has to expire.

When many keys are to be expired, this adaptive algorithm can impact the performance of Redis significantly though. You can find more information here.

My suggestion would be to try to prevent Python-rq to delegate item cleaning to Redis by setting expiration. This is a poor design for a queuing system anyway.