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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Tue, 08 Apr 2014 15:50:17 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13963100#comment-13963100
] 

Benedict commented on CASSANDRA-6694:
-------------------------------------

Yes, but I've gutted those classes into interfaces and introduced new sister classes. And
modelling those (in my head, at least) is very difficult when it's not easy to see what classes
relate to each other at a glance, and the "db" package is overpopulated as it is. So anything
I need to do to make it possible to think about whilst writing it, I assume is going to be
helpful for anybody else reading it.

But if you both want to change that bit, I can rebase again. Since I've written it, it doesn't
matter so much to me now.

> Slightly More Off-Heap Memtables
> --------------------------------
>
>                 Key: CASSANDRA-6694
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6694
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>              Labels: performance
>             Fix For: 2.1 beta2
>
>
> The Off Heap memtables introduced in CASSANDRA-6689 don't go far enough, as the on-heap
overhead is still very large. It should not be tremendously difficult to extend these changes
so that we allocate entire Cells off-heap, instead of multiple BBs per Cell (with all their
associated overhead).
> The goal (if possible) is to reach an overhead of 16-bytes per Cell (plus 4-6 bytes per
cell on average for the btree overhead, for a total overhead of around 20-22 bytes). This
translates to 8-byte object overhead, 4-byte address (we will do alignment tricks like the
VM to allow us to address a reasonably large memory space, although this trick is unlikely
to last us forever, at which point we will have to bite the bullet and accept a 24-byte per
cell overhead), and 4-byte object reference for maintaining our internal list of allocations,
which is unfortunately necessary since we cannot safely (and cheaply) walk the object graph
we allocate otherwise, which is necessary for (allocation-) compaction and pointer rewriting.
> The ugliest thing here is going to be implementing the various CellName instances so
that they may be backed by native memory OR heap memory.



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