Benedict created CASSANDRA-6694:
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Summary: 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
Fix For: 2.1
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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