Analyzing the Slow Log
Use the Redis slow log to find commands that block the server, interpret entries, and tune the thresholds that capture them.
Analyzing the Slow Log is a free Redis Caching & Messaging (Pub/Sub, Streams) lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Redis Caching & Messaging (Pub/Sub, Streams) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Redis Is Single-Threaded
Command execution in Redis is effectively single-threaded. One slow command blocks every other client. The slow log records commands whose execution time exceeds a threshold, so you can hunt down these blockers.
What It Measures
The slow log times only command execution, not network or I/O wait. So a logged entry means Redis itself spent that long, usually on an expensive operation over many elements.
Configuring the Threshold
slowlog-log-slower-than sets the threshold in microseconds. A value of 10000 logs commands slower than 10ms. Set it to 0 to log everything (for debugging) or -1 to disable.
CONFIG SET slowlog-log-slower-than 10000Limiting Log Size
slowlog-max-len caps how many entries are retained; older ones are evicted. This keeps memory bounded.
CONFIG SET slowlog-max-len 128Reading the Log
SLOWLOG GET returns recent entries; pass a count to limit them. Each entry has an ID, timestamp, microsecond duration, the command and arguments, and the client address.
SLOWLOG GET 10Interpreting an Entry
Look at the command and its duration. A KEYS * or a large SMEMBERS over a million-element set will show big durations, pinpointing the culprit and the calling client.
# 1) id 14
# 2) timestamp 1716900000
# 3) microseconds 23000
# 4) ["KEYS", "*"]Length and Reset
SLOWLOG LEN gives the current entry count, and SLOWLOG RESET clears the log, handy before reproducing an issue.
SLOWLOG LEN
SLOWLOG RESETCommon Offenders
Frequent slow-log culprits:
KEYSon large keyspaces (useSCAN)- Big
SMEMBERS/HGETALL/LRANGE 0 -1 - Unbounded
SORT - Large
DEL(useUNLINK)
SCAN 0 MATCH user:* COUNT 100
UNLINK biglistFrom Detection to Fix
Once the slow log names a command, fix it: paginate with cursors, model data to avoid huge single keys, or move heavy work off the hot path. Re-check the log to confirm the durations drop.
Continuous Monitoring
Sample the slow log periodically from a monitoring job and alert when slow commands appear, so you catch regressions before users feel them.
redis-cli SLOWLOG GET 5Threshold Tuning
Start with a moderate threshold (e.g. 10ms) in production. Lower it temporarily when investigating, then restore it, because logging everything adds a tiny per-command cost.
Quick Check
Test your understanding of the slow log.
Recap
You learned to use the slow log: configure slowlog-log-slower-than and slowlog-max-len, read entries with SLOWLOG GET, interpret durations, and fix common offenders like KEYS and huge single-key reads. Monitor it continuously to catch performance regressions early.
Frequently asked questions
Is the “Analyzing the Slow Log” lesson free?
Yes — the full text of “Analyzing the Slow Log” is free to read here on the web, and the Redis Caching & Messaging (Pub/Sub, Streams) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Redis Caching & Messaging (Pub/Sub, Streams) course, upgrade to CoddyKit PRO.
What will I learn in “Analyzing the Slow Log”?
Use the Redis slow log to find commands that block the server, interpret entries, and tune the thresholds that capture them. You practise Redis Caching & Messaging (Pub/Sub, Streams) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Redis Caching & Messaging (Pub/Sub, Streams)?
No prior experience is required. Redis Caching & Messaging (Pub/Sub, Streams) on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Analyzing the Slow Log” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Redis Caching & Messaging (Pub/Sub, Streams) lesson?
Yes. Every Redis Caching & Messaging (Pub/Sub, Streams) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Redis Monitoring Tools
- Diagnosing Performance Issues
- Performance Tuning & Optimization
- Analyzing the Slow Log