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Date: Sat, 30 Nov 2013 18:51:35 +0000 (UTC)
From: "Jonathan Ellis (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
Message-ID: <JIRA.12533985.1323146554572.49702.1385837495588@arcas>
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Subject: [jira] [Commented] (CASSANDRA-3578) Multithreaded commitlog
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    [ https://issues.apache.org/jira/browse/CASSANDRA-3578?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13835864#comment-13835864 ] 

Jonathan Ellis commented on CASSANDRA-3578:
-------------------------------------------

Updated my github branch to address above and dig into CLS.

- We don't assume that segments we recycle are the "right" size, but I don't see anywhere that we actually extend a segment past the standard length.
- Looks like discardUnusedTail can be called before sync() in a couple ways.  This means that we won't have room to write a sync marker at the end of the allocated space which could confuse replay.
- Is there a race in removeCleanFromDirty where we compare the dirty position w/ the synced one, while someone dirties it with a higher position?


> Multithreaded commitlog
> -----------------------
>
>                 Key: CASSANDRA-3578
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3578
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jonathan Ellis
>            Assignee: Benedict
>            Priority: Minor
>              Labels: performance
>         Attachments: 0001-CASSANDRA-3578.patch, ComitlogStress.java, Current-CL.png, Multi-Threded-CL.png, TestEA.java, latency.svg, oprate.svg, parallel_commit_log_2.patch
>
>
> Brian Aker pointed out a while ago that allowing multiple threads to modify the commitlog simultaneously (reserving space for each with a CAS first, the way we do in the SlabAllocator.Region.allocate) can improve performance, since you're not bottlenecking on a single thread to do all the copying and CRC computation.
> Now that we use mmap'd CommitLog segments (CASSANDRA-3411) this becomes doable.
> (moved from CASSANDRA-622, which was getting a bit muddled.)


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