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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Thu, 17 Apr 2014 23:01:19 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13973538#comment-13973538
] 

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

I don't think this is the right approach: with the changes we are making, we are pretty much
precluding doing anything fancy with GC (we'll have to rely on malloc for now). As such the
size is no longer providing any useful book keeping information to the NativeAllocator. It
should be dealt with entirely in the AbstractNativeCell - its concept of size is entirely
unique to it for now. This also, separately, makes packing structs of NativeCell a lot more
straight forward.

> 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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