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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 01:20:04 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7546?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14061511#comment-14061511
] 

graham sanderson commented on CASSANDRA-7546:
---------------------------------------------

The idea of the change here (which has three copies of the original loop which you could argue
is uglier and more complicated, but in some ways is also clearer), is to try to minimize the
number of loop attempts which allocate memory then fail the CAS

The first "raw" loop is largely identical to the old code
 - there is a check of a new non volatile Holder variable, but does no new memory barrier
requiring concurrency related checking
 - the CAS is not attempted (as part of the loop condition) if the loop continues due to early-out
 - I moved the deletioninfo stuff after the columns
 - I deferred the creation of the modified Holder until the last minute (mostly because in
the other loops the choice of the last constructor parameter is best made last, and I wanted
to keep all versions of the loop with the same structure)

The Holder instance, now tracks a state machine via the new contentionCount AtomicInteger
variable. The first loop is used when the  contentionCount is null. Any first loop thread,
which fails the first CAS, and then succeeds on a future first loop iteration will install
an AtomicInteger(0) value, directing future traffic to the second "count" loop

The "count" loop incudes a try/finally and tracks the number of concurrent calls in the contentionCount
variable... if the contention count is >1 traffic is directed to the third "sync" loop.
This loop will also on successful CAS deflate contentionCount to null, if it was at 1 (i.e.
there wasn't any current contention) just before the CAS

The "sync" loop is protected by a monitor (note choice here over j.u.c.Lock because of Lock
owner tracking overhead). This loop can still lose CAS, if another AtomicSortedColumn function
is doing the CAS, or another thread is CASing in either the "raw" or the "count" loop still.

The overall design is intended to be effectively free in the uncontended case, and to adapt
to effectively be completed synchronized in the highly contended case (for which the original
code was potentially really bad), and to be as good or better than the original code in between.

To that end, the debug code here today, tracks the ratio of loop iterations attempted (i.e.
they do some work before either failing CAS or doing an early out because they know the CAS
would fail)... Ideally this would be 1, and should remain close to that in all cases

> 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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