[jira] [Created] (CASSANDRA-15367) Memtable memory allocations may deadlock

["Benedict Elliott Smith (Jira)" <ji...@apache.org>]

Benedict Elliott Smith created CASSANDRA-15367:
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             Summary: 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


* 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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