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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Fri, 21 Feb 2014 17:08:21 GMT


Benedict commented on CASSANDRA-6694:

bq. Not sure I follow, I only see static methods sizeOf and construct in NativeCell for instance.

These are in a subclass called Impl within each *Cell interface, so it is shared between Buffer*Cell
and Native*Cell

bq. My preferred solution would be, stop extracting the name so often by itself. Spot checking
the code, it seems we usually do this just to "simplify" a comparison, so this could in principle
just be done with the Cell object rather than just the name.

I'll have a closer look and see how easy this would be. As it happens, I've made (but not
yet published) some changes to FastByteComparisons that might be extensible to make this work
without object instantiation. If we eliminated allocation for name comparisons we probably
would get _most_ of any benefit, so this might be workable.

> Slightly More Off-Heap Memtables
> --------------------------------
>                 Key: CASSANDRA-6694
>                 URL:
>             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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