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Benedict commented on CASSANDRA-6694:
-------------------------------------
bq. Can we decide if we actually want to have Cell (and derivatives) as this
patch set proposes (with static Impl static classes which is OOP unfriendly to
say the least) or do something else (question raised back in CASSANDRA-6689)?
Can we have something more concrete than "something else" as a suggestion?
bq. Is it essential to move everything to the separate package ".data" ?
No refactoring is "essential" - however it is much cleaner given all of the new
classes.
bq. Maybe there is a way which allows us to still have key/value/timestamp as
fields, so we should only change callers method/class signatures instead? In
general the idea would be to keep a single implementation of the Cell and add a
generic placeholder instead of ByteBuffer.
This seems to miss the entire purpose of this patch, which is to reduce the
heap consumption of each Cell. If we use another placeholder, we will no doubt
only *increase* the memory consumption, not decrease it; or, at best, reduce it
only fractionally for off-heap and increase it for on-heap implementations.
Neither are really acceptable, and would make this whole patch a bit worthless.
> 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.
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