serverCron() frequency is now a runtime parameter (was REDIS_HZ).
REDIS_HZ is the frequency our serverCron() function is called with. A more frequent call to this function results into less latency when the server is trying to handle very expansive background operations like mass expires of a lot of keys at the same time. Redis 2.4 used to have an HZ of 10. This was good enough with almost every setup, but the incremental key expiration algorithm was working a bit better under *extreme* pressure when HZ was set to 100 for Redis 2.6. However for most users a latency spike of 30 milliseconds when million of keys are expiring at the same time is acceptable, on the other hand a default HZ of 100 in Redis 2.6 was causing idle instances to use some CPU time compared to Redis 2.4. The CPU usage was in the order of 0.3% for an idle instance, however this is a shame as more energy is consumed by the server, if not important resources. This commit introduces HZ as a runtime parameter, that can be queried by INFO or CONFIG GET, and can be modified with CONFIG SET. At the same time the default frequency is set back to 10. In this way we default to a sane value of 10, but allows users to easily switch to values up to 500 for near real-time applications if needed and if they are willing to pay this small CPU usage penalty.
Please register or sign in to comment