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https://issues.apache.org/jira/browse/CASSANDRA-12071?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15345631#comment-15345631
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Ariel Weisberg commented on CASSANDRA-12071:
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I am seeing this against a single table. I don't think the number of tables
matters. The memory pool is global for the process and shared by all tables.
The parallelism is reduced both for a single table and across tables. The issue
is that there is a single threaded executor kicking off flushes that is waiting
on the result of whatever amount of parallelism is available for a single
Memtable flush. [The parallelism for a single Memtable flush is set to the the
# of JBOD
disks.|https://github.com/apache/cassandra/commit/e2c6341898fa43b0e262ef031f267587050b8d0f#diff-f0a15c3588b56c5ce53ece7c48e325b5R262]
> Regression in flushing throughput under load after CASSANDRA-6696
> -----------------------------------------------------------------
>
> Key: CASSANDRA-12071
> URL: https://issues.apache.org/jira/browse/CASSANDRA-12071
> Project: Cassandra
> Issue Type: Bug
> Components: Local Write-Read Paths
> Reporter: Ariel Weisberg
> Assignee: Marcus Eriksson
>
> The way flushing used to work is that a ColumnFamilyStore could have multiple
> Memtables flushing at once and multiple ColumnFamilyStores could flush at the
> same time. The way it works now there can be only a single flush of any
> ColumnFamilyStore & Memtable running in the C* process, and the number of
> threads applied to that flush is bounded by the number of disks in JBOD.
> This works ok most of the time but occasionally flushing will be a little
> slower and ingest will outstrip it and then block on available memory. At
> this point you see several second stalls that cause timeouts.
> This is a problem for reasonable configurations that don't use JBOD but have
> access to a fast disk that can handle some IO queuing (RAID, SSD).
> You can reproduce on beefy hardware (12 cores 24 threads, 64 gigs of RAM,
> SSD) if you unthrottle compaction or set it to something like 64
> megabytes/second and run with 8 compaction threads and stress with the
> default write workload and a reasonable number of threads. I tested with 96.
> It started happening after about 60 gigabytes of data was loaded.
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