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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: 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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