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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-3578) Multithreaded commitlog
Date Thu, 28 Nov 2013 17:22:37 GMT


Benedict commented on CASSANDRA-3578:

Was browsing your changes and noticed your CL.getContext() comment clarification, which I
think probably doesn't go far enough - also realised its return value no longer needs to be
a Future (the callee also only uses immediateFuture). I think something like the following
is appropriate:

       * @return a ReplayPosition representing the current position of the commit log; 
       * any CL.add() that completed prior to this call will occur at an earlier ReplayPosition.

       * This makes no guarantees about persistence, only ordering
      public ReplayPosition getContext()
             return allocator.allocatingFrom().getContext()

> Multithreaded commitlog
> -----------------------
>                 Key: CASSANDRA-3578
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jonathan Ellis
>            Assignee: Benedict
>            Priority: Minor
>              Labels: performance
>         Attachments: 0001-CASSANDRA-3578.patch,, 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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