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From "Jeff Chao (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (KAFKA-5452) Aggressive log compaction ratio appears to have no negative effect on log-compacted topics
Date Fri, 25 Aug 2017 20:37:00 GMT

     [ https://issues.apache.org/jira/browse/KAFKA-5452?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Jeff Chao resolved KAFKA-5452.
    Resolution: Resolved

Following up after a long while. After talking offline with [~wushujames], the original thought
was to choose a sensible default in relation to disk I/O. I think it's best to leave this
default and prevent assumptions on the underlying infrastructure. That way, operators are
free to tune to their expectations. Closing this.

> 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:,
>         Environment: Ubuntu Trusty (14.04.5), Oracle JDK 8
>            Reporter: Jeff Chao
>              Labels: performance
>         Attachments: 200mbs-dirty0-dirty-1-dirty05.png, flame-graph-200mbs-dirty0.png,
> 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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