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From "graham sanderson (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7546) AtomicSortedColumns.addAllWithSizeDelta has a spin loop that allocates memory
Date Tue, 22 Jul 2014 07:37:40 GMT


graham sanderson commented on CASSANDRA-7546:

However whether it is one-way or not is somewhat unimportant for me. This flip would only
last the lifetime of a memtable, which is not super lengthy (under heavily load probably only
a few minutes), and would not have dramatically negative consequences if it got it slightly
Cool, that's what I was asking/thinking.

As for the tree size/rebalancing, I have no particular proof... when things go wrong we are
hinting massively, and so maybe there are hundreds of hint mutation threads each with their
own in progress rebalance, pinning a lot of nodes across young GC. That said, the memory allocation
rate is truly spectacular, even given the excessive hinting, so I have to suspect the spinning
(and as you say probably some of the in arena allocation it does too) - though that would
also be surprising since these are hint updates which are a single cell update

Anyway... we can track cost in the Holder I guess to avoid any atomic operations, and maybe
factor in the tree size there too.

Note as an aside, we are partly to blame for this issue (best practices to be learned, and
ways we can mitigate) but the result is surprising enough (because things go bad at random,
and usually when we are inserting 100s of times less data than we can easily handle) that
others might easily get bitten. I would describe everything that I think is going on in the
snowballing of problems, but it is a bit of a comedy of errors.

> 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, 7546.20_2.txt, 7546.20_alt.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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