[
https://issues.apache.org/jira/browse/CASSANDRA-6694?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13963619#comment-13963619
]
Benedict commented on CASSANDRA-6694:
-------------------------------------
bq. I'm getting mixed signals here, are you claiming that JVM does a bad job or
OOP is broken in general? Also CASSANDRA-6993 seems to point to a different
problem.
I'm saying performance critical code is impacted when you have virtual method
calls that cannot be optimised by the VM (i.e. those with multiple
implementations). I meant CASSANDRA-6553 and CASSANDRA-6934
bq. We can have a Cell separate implementation with multiple buffers as Thrift
allocates them anyway which we are going to be transformed to linear ones once
they get into memtable as we have to reallocate there.
Then what exactly do we win? We still have to have two hierarchies and the same
modularisation. Also the potential ease of optimisations for comparison
disappear, and we still have increased indirection and virtual method call
costs. If this is the suggestion, I am very -1, as the payoff is very small,
the work nontrivial and the negatives substantial.
> 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.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
