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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-1181) kinder gentler compaction
Date Fri, 25 Jun 2010 21:18:51 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1181?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12882717#action_12882717
] 

Jonathan Ellis commented on CASSANDRA-1181:
-------------------------------------------

it's expected that everything runs at normal priority w/o TPP, even as root.  

normally, you would set TPP=1 to allow thread priorities to be set in the jvm.  the reason
42 is a trick is that the jvm will not let you set it to 1 if you are not running as root,
but the only reason you have to be root as far as linux is concerned is to *raise* thread
priority.  lowering it is always ok.  so setting it to 42 fools the "is tpp 0" check on setPriority,
without having the jvm's excessively stringent santity-checking hose you.

source diving at http://stackoverflow.com/questions/1662185/do-linux-jvms-actually-implement-thread-priorities

> kinder gentler compaction
> -------------------------
>
>                 Key: CASSANDRA-1181
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1181
>             Project: Cassandra
>          Issue Type: Task
>          Components: Core
>            Reporter: Jonathan Ellis
>            Assignee: Jonathan Ellis
>             Fix For: 0.6.3
>
>         Attachments: 1181.txt, CompactionManager.java
>
>
> I suggested this in a ML thread but it seems that nobody actually tried it.  I think
it's worth following up on:
> You could try setting the compaction thread to a lower priority.  You could add a thread
priority to NamedThreadPool, and pass that up from CompactionExecutor constructor.  According
to http://www.javamex.com/tutorials/threads/priority_what.shtml you have to run as root and
add a JVM option to get this to work.
> In particular, Brandon saw stress.py read latencies spike to 100ms during [anti]compaction
on a 2 core machine.  I'd like to see if this can mitigate that.

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