[jira] [Commented] (CASSANDRA-14475) nodetool - Occasional high CPU on large, CPU capable machines

["Kurt Greaves (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-14475?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16493375#comment-16493375 ] 

Kurt Greaves commented on CASSANDRA-14475:
------------------------------------------

+1 from me. I can't see any valid reason why you would want to have more than 1 GC thread for nodetool. All the work is done within Cassandra, so there shouldn't be any adverse effects to only having 1 GC thread for the tool (famous last words).

> nodetool - Occasional high CPU on large, CPU capable machines
> -------------------------------------------------------------
>
>                 Key: CASSANDRA-14475
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14475
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Tools
>            Reporter: Thomas Steinmaurer
>            Priority: Major
>         Attachments: nodetool_gc_threads.patch, nodetool_highcpu_gcthreads1_cassandra_JIRA.png
>
>
> Periodically calling nodetool every 5 min results in increased CPU usage by nodetool only on a machine with 32 physical cores (64 vCPUs) according to our monitoring:
> !nodetool_highcpu_gcthreads1_cassandra_JIRA.png|width=600!
> Investigation and testing has shown that it is related to running with default number of parallel GC threads which is 43 on this particular machine. We see a System.gc() according to flight recorder but no real evidence from where it comes from. The nodetool call in question is simply gathering e.g. the version with "nodetool version".
> After explicitly setting the number of parallel GC threads to 1, the high CPU is entirely gone (see chart above), without impacting nodetool being executed successfully. 1 parallel GC thread should be sufficient for nodetool anyway I think.



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