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Pavel Yaskevich commented on CASSANDRA-6694:
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If I had something more concrete you would see a patch for it, but here I am
trying to start a discussion, I think [~jbellis] mentioned that it might be
better to reduce usage of the column names instead of merging cell with column
name (if I remember correctly). Regarding the moving stuff around - if it's not
essential then we can do it at the very last stage once we done with all more
important changes which are plenty. Regarding placeholders idea, if we allocate
contiguous region for the whole cell we can just have memory object + 1 int (or
was it even short?...) field which marks the end of the column name at that
buffer, as column timestamp is a fixed size long we know exactly where column
value ends, that also helps with spatial locality in most of the cases.
> 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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