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From "Pavel Yaskevich (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-6694) Slightly More Off-Heap Memtables
Date Tue, 08 Apr 2014 07:53:15 GMT


Pavel Yaskevich commented on CASSANDRA-6694:

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:
>             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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