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From "Jonathan Ellis (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-3344) Compaction throttling can be too slow
Date Fri, 14 Oct 2011 02:20:11 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-3344?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Jonathan Ellis updated CASSANDRA-3344:
--------------------------------------

          Component/s: Core
    Affects Version/s:     (was: 0.8.6)
                       0.8.0
    
> 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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