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https://issues.apache.org/jira/browse/CASSANDRA-15367?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17023133#comment-17023133
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Blake Eggleston commented on CASSANDRA-15367:
---------------------------------------------
Just so I understand, the scenario you’re describing could be described like
this?
* there are 2 tables, T1 & T2, writing against OpGroup 1 (OP[1])
* a write comes in for T1 and is assigned to op group 1 (W1[1])
* T2 starts to flush, OpGroup is bumped to OP[2]. T2 is now waiting on W1[1]
* a write comes in for T1 and is assigned to op group 2 (W2[2])
* W2[2] acquires a lock, but is unable to allocate memory until T2 flushes
* W1[0] is blocking the T2 flush, and is unable to acquire the lock
* deadlock
> Memtable memory allocations may deadlock
> ----------------------------------------
>
> Key: CASSANDRA-15367
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15367
> Project: Cassandra
> Issue Type: Bug
> Components: Local/Commit Log, Local/Memtable
> Reporter: Benedict Elliott Smith
> Assignee: Benedict Elliott Smith
> Priority: Normal
> Fix For: 4.0, 2.2.x, 3.0.x, 3.11.x
>
>
> * Under heavy contention, we guard modifications to a partition with a mutex,
> for the lifetime of the memtable.
> * Memtables block for the completion of all {{OpOrder.Group}} started before
> their flush began
> * Memtables permit operations from this cohort to fall-through to the
> following Memtable, in order to guarantee a precise commitLogUpperBound
> * Memtable memory limits may be lifted for operations in the first cohort,
> since they block flush (and hence block future memory allocation)
> With very unfortunate scheduling
> * A contended partition may rapidly escalate to a mutex
> * The system may reach memory limits that prevent allocations for the new
> Memtable’s cohort (C2)
> * An operation from C2 may hold the mutex when this occurs
> * Operations from a prior Memtable’s cohort (C1), for a contended partition,
> may fall-through to the next Memtable
> * The operations from C1 may execute after the above is encountered by those
> from C2
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