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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-5661) Discard pooled readers for cold data
Date Sun, 07 Jul 2013 05:39:50 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-5661?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13701511#comment-13701511
] 

Jonathan Ellis commented on CASSANDRA-5661:
-------------------------------------------

Perhaps it will help if I recap:

# CRAR pooling + LCS OOMs us at relatively small amounts of data (20GB in Jeremiah's example)
# worst case pool size proportional to sstables * concurrent readers means that memory usage
increases linearly with disk size; increasing sstable size decreases it linearly, so we expect
100MB sstables [which is on the high side of what is reasonable IMO] to get us to ~4TB of
space managed which is good but not "problem solved" territory. 
# thus, we need to bound CRAR pooling and not just say "it's working as designed, go use larger
sstables or STCS" [although that is probably adequate for 1.2, so I am tagging this for 2.0]
# The most common operations in an object pool are the borrow/return.  Ben's multiway pool
optimizes for this, with particular attention to possible contention from multiple reader
threads.
# The multiway pool supports expiring objects after they have been idle for a given time,
as well as a total pool size.

                
> 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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