[CASSANDRA-10702] Statement concerning default ParallelGCThreads in jvm.options is not correct

• Type: Bug
• Status: Resolved
• Priority: Low
• Resolution: Fixed
• Fix Version/s: 3.0.1, 3.1
• Component/s: Local/Config
• Severity: Low
• Assignee: Nate McCall
• Reporter: Nate McCall

Authors

Nate McCall

Reviewers

Paulo Motta

• Created: 13/Nov/15 21:18
• Updated: 16/Apr/19 09:30
• Resolved: 20/Nov/15 13:55

Description

from jvm.options:

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:

[Parallel Time: 342.6 ms, GC Workers: 16]

on a 24 core system in this case.