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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Thu, 17 Apr 2014 23:56:18 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13973582#comment-13973582
] 

Benedict edited comment on CASSANDRA-6694 at 4/17/14 11:55 PM:
---------------------------------------------------------------

The only reason we were assigning a size in NativeAllocator was to support moving the peer
around (in which case you need to know how much memory you're copying). 

NativeAllocation assuming it has (i.e. _being defined as having_) a size prefix is fine when
it is tightly coupled with NativeAllocator (like it is in my branch) - but once you have it
as a final field in another object, NativeAllocator should simply have no say in the matter.
It never needs to know the size of the allocation, so we should just redefine what our AbstractNativeCell
considers to be its size in its sizeOf() calculation, and have the NativeAllocator use that
unadulterated value.


was (Author: benedict):
The only reason it was happening in NativeAllocator was to support moving the peer around
(so you need to know how much memory you're copying). 

NativeAllocation assuming it has (i.e. _being defined as having_) a size prefix is fine when
it is tightly coupled with NativeAllocator (like it is in my branch) - but once you have it
as a final field in another object, NativeAllocator should simply have no say in the matter.
It never needs to know the size of the allocation, so we should just redefine what our AbstractNativeCell
considers to be its size in its sizeOf() calculation.

> 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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