Slow log for performance analysis in Redis - Step-by-Step Execution

Concept Flow - Slow log for performance analysis

Command Executed

↓

Check Execution Time

↓

Is Time > Threshold?

No→Do Not Log

Yes↓

Add Entry to Slow Log

↓

Store Command, Time, Args

↓

Use SLOWLOG GET to Review Entries

When a Redis command runs, its execution time is checked. If it exceeds a set threshold, the command details are saved in the slow log for later review.

Execution Sample

Redis

SLOWLOG GET 3
SLOWLOG LEN
SLOWLOG RESET

Retrieve the last 3 slow log entries, check how many entries exist, then clear the slow log.

Execution Table

Step	Action	Execution Time (ms)	Threshold (ms)	Log Entry Added	Slow Log State
1	Execute SET key value	0.5	1	No	Empty
2	Execute GET key	0.8	1	No	Empty
3	Execute complex command	2.3	1	Yes	Entry 1: complex command logged
4	Execute another command	1.5	1	Yes	Entry 1 & 2: commands logged
5	Execute quick command	0.4	1	No	Entry 1 & 2: commands logged
6	SLOWLOG GET 3	-	-	-	Returns last 2 entries
7	SLOWLOG LEN	-	-	-	Returns 2
8	SLOWLOG RESET	-	-	-	Slow log cleared
9	SLOWLOG LEN	-	-	-	Returns 0

💡 Commands below threshold are not logged; slow log can be retrieved or reset by commands.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 8	Final
Slow Log Entries	Empty	1 entry	2 entries	2 entries	Cleared	Empty

Key Moments - 2 Insights

Why are some commands not added to the slow log even though they run?

Commands with execution time less than the threshold (1 ms in this example) are not logged, as shown in steps 1, 2, and 5 in the execution_table.

What happens when you run SLOWLOG RESET?

It clears all entries from the slow log, resetting it to empty, as shown in step 8 where the slow log state becomes cleared.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. Was the command logged to the slow log?

A Yes, but threshold was not checked

B Yes, because execution time was above threshold

C No, because execution time was below threshold

D No, because slow log was full

💡 Hint

Check the 'Log Entry Added' and 'Execution Time' columns at step 3 in execution_table.

At which step does the slow log get cleared?

A Step 6

B Step 7

C Step 8

D Step 9

💡 Hint

Look at the 'Action' and 'Slow Log State' columns in execution_table.

If the threshold was increased to 3 ms, how would the slow log entries change by step 4?

A No entries would be logged

B Only one entry would be logged

C Two entries would be logged as before

D All commands would be logged

💡 Hint

Compare execution times with threshold in execution_table rows 3 and 4.

Concept Snapshot

Redis Slow Log:
- Logs commands exceeding a time threshold
- Use SLOWLOG GET to view entries
- Use SLOWLOG LEN to count entries
- Use SLOWLOG RESET to clear log
- Threshold defaults to 10 ms but can be changed

Full Transcript

The Redis slow log records commands that take longer than a set threshold to execute. When a command runs, Redis checks its execution time. If it is above the threshold, the command details are saved in the slow log. You can retrieve recent slow commands with SLOWLOG GET, check how many entries exist with SLOWLOG LEN, and clear the log with SLOWLOG RESET. Commands running faster than the threshold are not logged. This helps analyze performance issues by focusing on slow commands.