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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Thu, 17 Apr 2014 09:44:15 GMT


Benedict commented on CASSANDRA-6694:

bq. the scope is introduce native implementations -as optimized as possible-

Otherwise we need to do a lot more than the changes you are suggesting :)

bq.  Also I'm fine if methods in both implementations are going to return constant values
like serializationFlags or isMarkedForDeleted

Well, these are still duplication - it is not clear as a result where the definition of these
behaviours live. If the semantics change in future, it may introduce errors unnecessarily.
Either way equals(),  reconcile() and validateFields() will still be issues. You don't seem
to have implemented most of these methods yet (looks like your code doesn't actually compile).
These methods are each non-trivial amounts of code duplication, equals() especially so is
we optimise it as you want to. CounterCell.diff() will also need to be duplicated.

But, like I said, I can probably live with all of this if we address the setPeer() issue.
equals() should probably still end up in a shared static method, at the very least, though.

> Slightly More Off-Heap Memtables
> --------------------------------
>                 Key: CASSANDRA-6694
>                 URL:
>             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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