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From "Michael Shuler (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Thu, 01 May 2014 14:42:19 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13986642#comment-13986642
] 

Michael Shuler commented on CASSANDRA-6694:
-------------------------------------------

Comparing the default 2.1 dtest runs with offheap dtest runs shows the same results:
http://cassci.datastax.com/job/cassandra-2.1_dtest/132/
http://cassci.datastax.com/job/cassandra-2.1_offheap_dtest/4/

The 2.1 unit tests got a default config with this commit for "memtable_allocation_type: offheap_objects"
and they appear pretty stable. (some new unit test errors have come up since CASSANDRA-6855,
but no big regression appears to have come from this commit)

> 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
>
>         Attachments: 6694.fix1.txt
>
>
> 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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