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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] Updated: (CASSANDRA-1954) Double-check or replace RRW memtable lock
Date Wed, 16 Feb 2011 17:32:24 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-1954?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Sylvain Lebresne updated CASSANDRA-1954:
----------------------------------------

    Attachment: 0001-Remove-flusherLock-readLock.patch

Attached patch implements what's hinted in my previous comment. Mainly it keeps the flusher
lock to unsure we discard commit log segment in the order we read them, but remove the readLock
on writes, so flush don't block writes anymore.

It passes unit test but that doesn't say much.

> 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
>            Priority: Minor
>         Attachments: 0001-Double-check-in-maybeSwitchMemtable-to-minimize-writeL.txt,
0001-Remove-flusherLock-readLock.patch
>
>
> {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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