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.

Practice

1. What is the primary purpose of the Redis slow log?

easy

A. To clear the Redis cache automatically

B. To store all commands executed in Redis

C. To record commands that take longer than a set time to execute

D. To backup Redis data periodically

Solution

1. Step 1: Understand what slow log tracks

   The slow log records commands that exceed a certain execution time threshold.

2. Step 2: Differentiate from other Redis features

   It does not store all commands, clear cache, or backup data; it only logs slow commands.

3. Final Answer:

   To record commands that take longer than a set time to execute -> Option C

4. Quick Check:

   Slow log = records slow commands [OK]

Hint: Slow log tracks only slow commands, not all commands [OK]

Common Mistakes:

• Thinking slow log stores all commands
• Confusing slow log with cache clearing
• Assuming slow log is for backups

2. Which Redis command correctly retrieves the slow log entries?

easy

A. SLOWLOG LIST

B. SLOWLOG FETCH

C. SLOWLOG SHOW

D. SLOWLOG GET

Solution

1. Step 1: Recall the Redis slow log retrieval command

   The official command to get slow log entries is SLOWLOG GET.

2. Step 2: Verify other options

   Commands like SLOWLOG FETCH, SLOWLOG SHOW, and SLOWLOG LIST do not exist in Redis.

3. Final Answer:

   SLOWLOG GET -> Option D

4. Quick Check:

   Retrieve slow log = SLOWLOG GET [OK]

Hint: Use SLOWLOG GET to view slow log entries [OK]

Common Mistakes:

• Using non-existent commands like SLOWLOG FETCH
• Confusing with LIST or SHOW commands
• Misspelling the command

3. Given the following Redis commands executed in order:

SLOWLOG RESET
SLOWLOG LEN
SLOWLOG GET 2

What will be the output of SLOWLOG LEN immediately after the reset?

medium

A. 0

B. 2

C. An error message

D. The last 2 slow log entries

Solution

1. Step 1: Understand SLOWLOG RESET effect

   The SLOWLOG RESET command clears all slow log entries, so the log becomes empty.

2. Step 2: Check SLOWLOG LEN after reset

   Since the log is empty, SLOWLOG LEN returns 0, indicating no entries.

3. Final Answer:

   0 -> Option A

4. Quick Check:

   Reset clears log, length = 0 [OK]

Hint: Reset clears log; length after reset is zero [OK]

Common Mistakes:

• Expecting old entries after reset
• Confusing SLOWLOG LEN with SLOWLOG GET output
• Thinking reset causes error

4. You run SLOWLOG GET but receive an empty list even though you expect slow commands. What is the most likely cause?

medium

A. The slow log threshold is set too high, so no commands qualify

B. The Redis server is down

C. You need to restart Redis to enable slow log

D. The slow log is full and cannot record more entries

Solution

1. Step 1: Understand slow log threshold role

   Redis only logs commands slower than the configured threshold (default 10ms). If threshold is too high, no commands get logged.

2. Step 2: Eliminate other causes

   Redis server down would cause connection errors, not empty logs. Restart is not needed to enable slow log. Slow log size is limited but overwrites old entries, so it won't be empty due to fullness.

3. Final Answer:

   The slow log threshold is set too high, so no commands qualify -> Option A

4. Quick Check:

   High threshold = no slow commands logged [OK]

Hint: Check slow log threshold if no entries appear [OK]

Common Mistakes:

• Assuming Redis must restart to enable slow log
• Thinking slow log fills up and stops logging
• Confusing server down with empty slow log

5. You want to analyze slow commands that took longer than 50 milliseconds and keep only the last 5 entries. Which sequence of Redis commands achieves this?

hard

A. CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

B. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

C. CONFIG SET slowlog-log-slower-than 50000
SLOWLOG RESET
SLOWLOG GET 5

D. CONFIG SET slowlog-max-len 5
CONFIG SET slowlog-log-slower-than 50
SLOWLOG RESET
SLOWLOG GET 5

Solution

1. Step 1: Set threshold to 50 milliseconds

   Redis uses microseconds, so 50 ms = 50000 μs. Use CONFIG SET slowlog-log-slower-than 50000. A and C correct; B and D use 50 μs (too low).

2. Step 2: Limit slow log length to 5 entries

   Use CONFIG SET slowlog-max-len 5. Only A and D do this; B and C do not.

3. Step 3: Reset slow log and get last 5 entries

   SLOWLOG RESET clears old entries; SLOWLOG GET 5 retrieves last 5. All options include this.

4. Step 4: Choose correct option

   Only A sets both max-len to 5 and threshold to 50000 μs (50 ms).

5. Final Answer:

   CONFIG SET slowlog-max-len 5 CONFIG SET slowlog-log-slower-than 50000 SLOWLOG RESET SLOWLOG GET 5 -> Option B

6. Quick Check:

   Threshold 50ms = 50000μs, max len 5, reset, get 5 [OK]

Hint: Set threshold in microseconds and max length before reset [OK]

Common Mistakes:

• Confusing milliseconds with microseconds in threshold
• Not setting max length to limit entries
• Forgetting to reset slow log before analysis