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From "Nate McCall (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-10702) Statement concerning default ParallelGCThreads in jvm.options is not correct
Date Fri, 13 Nov 2015 21:19:10 GMT
Nate McCall created CASSANDRA-10702:
---------------------------------------

             Summary: Statement concerning default ParallelGCThreads in jvm.options is not
correct
                 Key: CASSANDRA-10702
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10702
             Project: Cassandra
          Issue Type: Bug
          Components: Configuration
            Reporter: Nate McCall
            Priority: Trivial


from {{jvm.options}}:

bq. The JVM maximum is 8 PGC threads and 1/4 of that for ConcGC.

This is not correct. If there are more than eight CPUs, the default becomes 5/8 of the number
of CPUs rounded up to the nearest even number (it seems - see below). See {{-XX:ParallelGCThreads=n}}
secion of http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html 

Pretty easy to test with > 16 cores (as 5/8 of such is 10): turn on GC logging, leave the
defaults, and the G1GC output will show something like:

{noformat}
[Parallel Time: 342.6 ms, GC Workers: 16]
{noformat}

on a 24 core system in this case. 



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