Redis Performance Debugging

http://lzone.de/blog/Redis%20Performance%20Debugging?mkt_tok=3RkMMJWWfF9wsRons67KZKXonjHpfsX56uolX6KxlMI%2F0ER3fOvrPUfGjI4DT8tgI%2BSLDwEYGJlv6SgFQ7HAMa5m3rgMWRk%3D

Monitoring Live Redis Queries

Run the "monitor" command to see queries as they are sent against an Redis instance. Do not use on high traffic instance!

redis-cli monitor

The output looks like this

redis 127.0.0.1:6379> MONITOR
OK
1371241093.375324 "monitor"
1371241109.735725 "keys" "*"
1371241152.344504 "set" "testkey" "1"
1371241165.169184 "get" "testkey"

Analyzing Slow Commands

When there are too many queries better use "slowlog" to see the top slow queries running against your Redis instance:

slowlog get 25		# print top 25 slow queries
slowlog len		
slowlog reset

Debugging Latency

If you suspect latency to be an issue use "redis-cli" built-in support for latency measuring. First measure system latency on your Redis server with

redis-cli --intrinsic-latency 100

and then sample from your Redis clients with

redis-cli --latency -h <host> -p <port>

If you have problems with high latency check if transparent huge pages are disabled. Disable it with

echo never > /sys/kernel/mm/transparent_hugepage/enabled

Check Background Save Settings

If your instance seemingly freezes peridiocally you probably have background dumping enabled.

grep ^save /etc/redis/redis.conf

Comment out all save lines and setup a cron job to do dumping or a Redis slave who can dump whenever he wants to. 

Alternatively you can try to mitigate the effect using the "no-appendfsync-on-rewrite" option (set to "yes") in redis.conf.

Check fsync Setting

Per default Redis runs fsync() every 1s. Other possibilities are "always" and "no".

grep ^appendfsync /etc/redis/redis.conf

So if you do not care about DB corruption you might want to set "no" here.