[jira] [Commented] (CASSANDRA-13299) Potential OOMs and lock contention in write path streams

["Benjamin Roth (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-13299?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15896460#comment-15896460 ] 

Benjamin Roth commented on CASSANDRA-13299:
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Relating to CASSANDRA-11670 this would also allow to write all streamed mutations to commitlog without problems.
I also propose to do so with small streams (see CASSANDRA-13290). Writing small streams (e.g. < 100KB) to commitlog does not require a flush at the end of stream receive. This avoids tons of flushes if tons of tiny streams are sent during a repair session.

These are maybe apples and oranges but fixing all these ends makes the whole process less error prone and probably perform better.

> Potential OOMs and lock contention in write path streams
> --------------------------------------------------------
>
>                 Key: CASSANDRA-13299
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13299
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Benjamin Roth
>
> I see a potential OOM, when a stream (e.g. repair) goes through the write path as it is with MVs.
> StreamReceiveTask gets a bunch of SSTableReaders. These produce rowiterators and they again produce mutations. So every partition creates a single mutation, which in case of (very) big partitions can result in (very) big mutations. Those are created on heap and stay there until they are processed.
> I don't think it is necessary to create a single mutation for each partition. Why don't we implement a PartitionUpdateGeneratorIterator that takes a UnfilteredRowIterator and a max size and spits out PartitionUpdates to be used to create and apply mutations?
> The max size should be something like min(reasonable_absolute_max_size, max_mutation_size, commitlog_segment_size / 2). reasonable_absolute_max_size could be like 16M or sth.
> A mutation shouldn't be too large as it also affects MV partition locking. As longer a MV partition is locked during a stream, the higher chances are that WTE's occur during streams.
> I could also imagine that a max number of updates per mutation regardless of size in bytes could make sense to avoid lock contention.
> Love to get feedback and suggestions, incl. naming suggestions.



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