[KAFKA-5452] Aggressive log compaction ratio appears to have no negative effect on log-compacted topics

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.10.2.0, 0.10.2.1
• Component/s: config, core, log
• Labels: performance
• Environment: Ubuntu Trusty (14.04.5), Oracle JDK 8
• Assignee: Unassigned
• Reporter: Jeff Chao
• Created: 14/Jun/17 22:11
• Updated: 24/Feb/23 20:03
• Resolved: 24/Feb/23 20:03

Description

Some of our users are seeing unintuitive/unexpected behavior with log-compacted topics where they receive multiple records for the same key when consuming. This is a result of low throughput on log-compacted topics such that conditions (min.cleanable.dirty.ratio = 0.5, default) aren't met for compaction to kick in.

This prompted us to test and tune min.cleanable.dirty.ratio in our clusters. It appears that having more aggressive log compaction ratios don't have negative effects on CPU and memory utilization. If this is truly the case, we should consider changing the default from 0.5 to something more aggressive.

Setup:

1. 8 brokers
2. 5 zk nodes
3. 32 partitions on a topic
4. replication factor 3
5. log roll 3 hours
6. log segment bytes 1 GB
7. log retention 24 hours
8. all messages to a single key
9. all messages to a unique key
10. all messages to a bounded key range [0, 999]
11. min.cleanable.dirty.ratio per topic = 0, 0.5, and 1
12. 200 MB/s sustained, produce and consume traffic

Observations:

We were able to verify log cleaner threads were performing work by checking the logs and verifying the cleaner-offset-checkpoint file for all topics. We also observed the log cleaner's time-since-last-run-ms metric was normal, never going above the default of 15 seconds.

Under-replicated partitions stayed steady, same for replication lag.

Here's an example test run where we try out min.cleanable.dirty.ratio = 0, min.cleanable.dirty.ratio = 1, and min.cleanable.dirty.ratio = 0.5. Troughs in between the peaks represent zero traffic and reconfiguring of topics.

(200mbs-dirty-0-dirty1-dirty05.png attached)

Memory utilization is fine, but more interestingly, CPU doesn't appear to have much difference.

To get more detail, here is a flame graph (raw svg attached) of the run for min.cleanable.dirty.ratio = 0. The conservative and default ratio flame graphs are equivalent.

(flame-graph-200mbs-dirty0.png attached)

Notice that the majority of CPU is coming from:

1. SSL operations (on reads/writes)
2. KafkaApis::handleFetchRequest (ReplicaManager::fetchMessages)
3. KafkaApis::handleOffsetFetchRequest

We also have examples from small scale test runs which show similar behavior but with scaled down CPU usage.

It seems counterintuitive that there's no apparent difference in CPU whether it be aggressive or conservative compaction ratios, so we'd like to get some thoughts from the community.

We're looking for feedback on whether or not anyone else has experienced this behavior before as well or, if CPU isn't affected, has anyone seen something related instead.

If this is true, then we'd be happy to discuss further and provide a patch.