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Date: Thu, 24 Mar 2011 04:33:06 +0000 (UTC)
From: "Jonathan Ellis (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (CASSANDRA-1954) Double-check or replace RRW
 memtable lock
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    [ https://issues.apache.org/jira/browse/CASSANDRA-1954?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13010549#comment-13010549 ] 

Jonathan Ellis commented on CASSANDRA-1954:
-------------------------------------------

I think we forgot a 3rd goal of the Big Lock: make sure that when a memtable is full, all the flush threads are busy, and the flush queue is full, we _want to_ block writes so we don't OOM from shoving more data into the heap before we can finish freeing what is being flushed.

Ideas?

> Double-check or replace RRW memtable lock
> -----------------------------------------
>
>                 Key: CASSANDRA-1954
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1954
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Stu Hood
>            Assignee: Sylvain Lebresne
>            Priority: Minor
>             Fix For: 0.8
>
>         Attachments: 0001-Double-check-in-maybeSwitchMemtable-to-minimize-writeL.txt, 0001-Remove-flusherLock-readLock.patch, 1954-0.7-v2.txt, 1954-v2.txt, 1954_trunk.patch
>
>   Original Estimate: 8h
>  Remaining Estimate: 8h
>
> {quote}...when a Memtable reaches its threshold, up to (all) N write threads will often notice, and race to acquire the writeLock in order to freeze the memtable. This means that we do way more writeLock acquisitions than we need to...{quote}
> See CASSANDRA-1930 for backstory, but adding double checking inside a read lock before trying to re-entrantly acquire the writelock would eliminate most of these excess writelock acquisitions.
> Alternatively, we should explore removing locking from these structures entirely, and replacing the writeLock acquisition with a per-memtable counter of active threads.

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