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https://issues.apache.org/jira/browse/KAFKA-5452?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Matthias J. Sax reopened KAFKA-5452:
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> Aggressive log compaction ratio appears to have no negative effect on
> log-compacted topics
> ------------------------------------------------------------------------------------------
>
> Key: KAFKA-5452
> URL: https://issues.apache.org/jira/browse/KAFKA-5452
> Project: Kafka
> Issue Type: Improvement
> Components: config, core, log
> Affects Versions: 0.10.2.0, 0.10.2.1
> Environment: Ubuntu Trusty (14.04.5), Oracle JDK 8
> Reporter: Jeff Chao
> Priority: Major
> Labels: performance
> Attachments: 200mbs-dirty0-dirty-1-dirty05.png,
> flame-graph-200mbs-dirty0.png, flame-graph-200mbs-dirty0.svg
>
>
> 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:
> # 8 brokers
> # 5 zk nodes
> # 32 partitions on a topic
> # replication factor 3
> # log roll 3 hours
> # log segment bytes 1 GB
> # log retention 24 hours
> # all messages to a single key
> # all messages to a unique key
> # all messages to a bounded key range [0, 999]
> # {{min.cleanable.dirty.ratio}} per topic = {{0}}, {{0.5}}, and {{1}}
> # 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)
> !200mbs-dirty0-dirty-1-dirty05.png|thumbnail!
> 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)
> !flame-graph-200mbs-dirty0.png|thumbnail!
> Notice that the majority of CPU is coming from:
> # SSL operations (on reads/writes)
> # KafkaApis::handleFetchRequest (ReplicaManager::fetchMessages)
> # 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.
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