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https://issues.apache.org/jira/browse/CASSANDRA-3344?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13127247#comment-13127247
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Jonathan Ellis commented on CASSANDRA-3344:
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+1 v2
> Compaction throttling can be too slow
> -------------------------------------
>
> Key: CASSANDRA-3344
> URL: https://issues.apache.org/jira/browse/CASSANDRA-3344
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Affects Versions: 0.8.0
> Reporter: Fabien Rousseau
> Priority: Minor
> Fix For: 0.8.8, 1.0.1
>
> Attachments: 001-CASSANDRA-3344.patch, 3344.patch, 3344_v2.patch
>
>
> Compaction throttling needs to know how many active compactions are running
> (to divide bandwith for each active compaction).
> The way active compaction is counted can be broken because it counts the
> number of active threads in the executor BUT the thread starts by acquiring a
> lock.
> If the lock can't be acquired immediately : the thread is seen as "active"
> but does not participate in IO operations.
> The case can happen when major compaction are triggered (major compaction
> acquire a write lock, while minor compactions acquire a read lock).
> Having compaction througput to 16Mb/s, we observed is the following (two
> times) :
> - only 1 active compaction (a long one for a few hours) starting at 16Mb/s,
> then after some time running at 2Mb/s, thus taking a very long time to
> complete
> - many pending compactions
> Using JMX and monitoring the stack trace of the compaction threads showed
> that :
> - 1 thread was effectively compacting
> - 1 thread was waiting to acquire the write lock (due to a major compaction)
> - 6 threads were waiting to acquire the read lock (probably due to the
> thread above trying to acquire the write lock)
> Attached is a proposed patch (very simple, not yet tested) which counts only
> active compactions.
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