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From "Pavel Yaskevich (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Thu, 17 Apr 2014 22:57:16 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13973534#comment-13973534
] 

Pavel Yaskevich edited comment on CASSANDRA-6694 at 4/17/14 10:56 PM:
----------------------------------------------------------------------

Ok, hashCode and setPeer changes are now pushed to the same branch, AbstractNativeCell is
independent of NativeAllocation now because NativeAllocator returns aligned peer directly,
which allows "peer" field to be made final in AbstractNativeCell. Also I have pushed set/get
logic for data size associated with the pointer to the NativeAllocator as it's basically it's
metadata, IMO it's a bit cleaner comparing to how that is done in Benedict's branch where
NativeAllocation tracks pointer alignment to size (internalPeer() \{ return peer + 4; \})
but NativeAllocator takes care of allocating 4 additional bytes to requested size.


was (Author: xedin):
Ok, hashCode and setPeer changes are now pushed to the same branch, AbstractNativeCell is
independent of NativeAllocation now because NativeAllocator returns aligned peer directly,
which allows "peer" field to be made final in AbstractNativeCell. Also I have pushed set/get
logic for data size associated with the pointer to the NativeAllocator as it's basically it's
metadata, IMO it's a bit cleaner comparing to how that is done in Benedict's branch where
NativeAllocation tracks pointer alignment to size (internalPeer() { return peer + 4; }) but
NativeAllocator takes care of allocating 4 additional bytes to requested size.

> 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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