[jira] [Created] (CASSANDRA-6694) Slightly More Off-Heap Memtables

["Benedict (JIRA)" <ji...@apache.org>]

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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