Benedict commented on CASSANDRA-6694:

Oh, also, [~iamaleksey]: Your assertion about super columns and sparse 
composites appears to be broken by the CliTest somehow. I haven't investigated, 
but this is why I introduced that branch. I've stripped out the condition and 
just always take that branch if we fail to lookup in cfMetaData, to deal with 
names being dropped whilst we aren't expecting, so it no longer assumes this 
but also copes with it.

> 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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