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From "graham sanderson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7546) AtomicSortedColumns.addAllWithSizeDelta has a spin lock that allocates memory
Date Tue, 15 Jul 2014 01:22:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7546?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14061514#comment-14061514
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graham sanderson commented on CASSANDRA-7546:
---------------------------------------------

The stateful learning behavior here seems like a good thing... attempting the first loop only
to fail, before doing any form of synchronization would mean a ratio of at least 2 in the
highly contended case.

>From looking at the numbers above, you'll see that this code:

- maintains a ratio of 1 as expected in the uncontended cases
- maintains a ratio of 1 in the highly contended cases, vs on this box as high as 17 in the
original code (which causes massive memory allocation)
- seems to max out at about 1.3 for cases in between, with that generally lower or very very
close to the original code.

> AtomicSortedColumns.addAllWithSizeDelta has a spin lock that allocates memory
> -----------------------------------------------------------------------------
>
>                 Key: CASSANDRA-7546
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7546
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: graham sanderson
>         Attachments: suggestion1.txt
>
>
> In order to preserve atomicity, this code attempts to read, clone/update, then CAS the
state of the partition.
> Under heavy contention for updating a single partition this can cause some fairly staggering
memory growth (the more cores on your machine the worst it gets).
> Whilst many usage patterns don't do highly concurrent updates to the same partition,
hinting today, does, and in this case wild (order(s) of magnitude more than expected) memory
allocation rates can be seen (especially when the updates being hinted are small updates to
different partitions which can happen very fast on their own) - see CASSANDRA-7545
> It would be best to eliminate/reduce/limit the spinning memory allocation whilst not
slowing down the very common un-contended case.



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