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From "Benedict (JIRA)" <>
Subject [jira] [Updated] (CASSANDRA-7546) AtomicSortedColumns.addAllWithSizeDelta has a spin loop that allocates memory
Date Mon, 21 Jul 2014 14:18:39 GMT


Benedict updated CASSANDRA-7546:

    Attachment: 7546.20.txt

Attaching my suggested alternative approach, which I think is a little simpler to reason about:
if we loop for any reason we immediately take the monitor, if unsafe is available to us; if
it isn't we don't benefit from the optimisation. We don't change the logic for updating, as
taking the monitor doesn't guarantee we have exclusive access, it only guarantees we have
exclusive access versus those writes that have themselves spun once, thereby capping the amount
of competition.

For 2.1 I suggest sticking with the same approach, then in 3.0/.1 we'll take a look at the
more complex approach of making writes lazy under competition.

> AtomicSortedColumns.addAllWithSizeDelta has a spin loop that allocates memory
> -----------------------------------------------------------------------------
>                 Key: CASSANDRA-7546
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>            Reporter: graham sanderson
>            Assignee: graham sanderson
>         Attachments: 7546.20.txt, suggestion1.txt, suggestion1_21.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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