[jira] [Commented] (CASSANDRA-1954) Double-check or replace RRW memtable lock

["Jonathan Ellis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-1954?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13014855#comment-13014855 ] 

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

Reverted until we can fix this (r1087919).

I also think we should measure the benefits of the volatile + writer count approach in smoothing latency.  If it is not reproducibly better let's stick with the approach that's been debugged longer. :)

I'm fine with leaving this until post-0.8 fwiw.

> 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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