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https://issues.apache.org/jira/browse/CASSANDRA-5661?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13701530#comment-13701530
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Ben Manes commented on CASSANDRA-5661:
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Weights are trivial to add and I wanted to avoid adding non-critical features
without more details. In your patch, it appears assumed that every queue as a
single entry with the same size buffer and privately Jonathan's description of
the problem stated 128KB per CRAR. If the weight is constant than they are
merely a convenience mapping as it really is the number of entries.
Uncontended CAS is cheap, short lived allocations are trivial, and Doug Lea
describes LTQ as lower overhead than CLQ (especially under load).
The use of weak references was an easy way to avoid race conditions when
flushing out the primary structure. It could be replaced with lazy clean-up
passes, which is what I originally started with. At this point it seemed unwise
to complicate things without more information so I simplified it. The number of
queues is probably going to be quite small, on the order of dozens, so the
reference cost in this case is quite small.
You're trying to compare approaches, which is valid but better oriented towards
discussing with Jonathan. The challenge presented to me is as described in the
class's JavaDoc: an multiway object pool bounded by the total number of
entries. I took a more general approach due to not knowing the trade-offs one
could make with context to Cassandra's behavior.
> Discard pooled readers for cold data
> ------------------------------------
>
> Key: CASSANDRA-5661
> URL: https://issues.apache.org/jira/browse/CASSANDRA-5661
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Affects Versions: 1.2.1
> Reporter: Jonathan Ellis
> Assignee: Pavel Yaskevich
> Fix For: 1.2.7
>
> Attachments: CASSANDRA-5661.patch, DominatorTree.png, Histogram.png
>
>
> Reader pooling was introduced in CASSANDRA-4942 but pooled
> RandomAccessReaders are never cleaned up until the SSTableReader is closed.
> So memory use is "the worst case simultaneous RAR we had open for this file,
> forever."
> We should introduce a global limit on how much memory to use for RAR, and
> evict old ones.
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