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Date: Thu, 28 Nov 2013 13:16:37 +0000 (UTC)
From: "Benedict (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
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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=13834803#comment-13834803 ] 

Benedict commented on CASSANDRA-3578:
-------------------------------------

Thinking a little more on the PCLE guarantees about syncing, I think perhaps a better scheme is to attempt to sync() twice every "poll interval" (or once every 0.49*interval), and to only update the heartbeat at the end of a sync(); at the moment I update before and after, to deal with the fact that the blockInterval and pollInterval are approximately the same (so we don't simply block every time we start a sync()). This means you could lose upto 2x interval commits. I addressed this in a previous comment, but didn't deal with it in the code.

It also occurs to me that pollInterval is a really bad name, and it should be called maxSyncInterval or something.

> 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, 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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