Redis Slowlog

thapabishwa | Sep 10, 2022

What is Redis SLOWLOG?

The Redis SLOWLOG command configures and displays the content of a log of slow queries detected by Redis. On most Redis deployments, you’re able to get this log from the Redis command line. If you are experiencing slow query execution or high CPU usage in your Redis server, this is the tool to use.

It’s worth remembering that slow logs are concerned with only the execution time of the command. The I/O involved in processing the command itself is not measured, only the time spent doing the actual work of the command. The I/O is multithreaded so it can take place alongside other work, but the execution happens on a single thread, blocking other commands from running, which is why it’s the critical thing to measure from the database’s point of view. Another thing to keep in mind is that the Slow log is transient; there’s no persistence for it so in the case of failover, the slow log is lost. If you are looking to rely on a persistent slow log, you’ll be wanting to reconsider your design choices.

How does SLOWLOG work?

Once a query is issued, the server keeps track of the time elapsed when executing the command. If the command exceeds the allocated time, it is logged using the slowlog system. There are two configurable settings, slow-log-slower-than and slowlog-max-len. These directives are used to filter slow events and control the size of the slowlog queue respectively.

slowlog-log-slower-than

The other way to capture that tricky slow event is to filter out all the slow, but not that slow, log events. That’s where slowlog-log-slower-than comes in. It sets the threshold on what qualifies as a slow event.

The default value is 10000 microseconds, and when you are trying to track down a slow event you should begin with this default. You can reduce the value all the way down to 0 if you want - as a result everything will be logged as it all takes more than zero microseconds. To disable slow logging completely set it to -1.

If you are trying to track down a particularly slow event, it’s then worth raising the slowlog-log-slower-than to isolate those events in the log. In the example below, it is set to 10 to catch more slow queries.

slowlog-max-len

The slow log is actually a queue of slow log events. You can control the size of that queue with slowlog-max-len. The bigger you make it, the more memory it consumes. Ideally, it should be big enough for you to catch your problematic slow commands, but not so big that it becomes an issue itself. The default is 128 and you should start with that until you are certain you need to increase it.

SLOWLOG in action

Fetching the parameters

Using the config set command, you can also configure the slowlog parameters at runtime.

 
127.0.0.1:6379> config get slow*
1) "slowlog-log-slower-than"
2) "10000"
3) "slowlog-max-len"
4) "128"

Modifying the parameters

Using the config set command, you can configure the log parameters at runtime.

 
127.0.0.1:6379> config set slowlog-log-slower-than 10
OK

127.0.0.1:6379> config set slowlog-max-len 100
OK

Fetching SLOWLOG Entries

To fetch all the entries in the Redis Slow log, run the SLOWLOG GET command.

Syntax

 
redis 127.0.0.1:6379> SLOWLOG subcommand [argument]

Sample Output

 
127.0.0.1:6379> slowlog get
 1) 1) (integer) 2         
    2) (integer) 1662782518
    3) (integer) 17
    4) 1) "exists"
       2) "keyabc"
    5) "127.0.0.1:54172"
    6) ""

 2) 1) (integer) 1
    2) (integer) 1662782520
    3) (integer) 155
    4) 1) "expire"
       2) "keyxyz"
       3) "0"
    5) "127.0.0.1:54172"
    6) ""

Components of a log entry

Each Slow Log entry is comprised of 6 main parts.

 
1) (integer) 1              # unique identifier for the log entry.
2) (integer) 1662782518     # unix timestamp denoting the time at which the entry was added
3) (integer) 17             # time taken by the query in microseconds.
4) 1) "exists"              # array of the command used by the client and arguments passed
   2) "keyabc"
5) "127.0.0.1:54172"        # client address and port that issued the command.
6) ""                       # client name as specified by the client setname command.

Resetting SLOWLOG entries

To clean up the slow log entries, use the SLOWLOG RESET command.

 
127.0.0.1:6379> slowlog reset
OK