Upgrade from 11 to 17 leads to busy CPU when idle

[Peter Karich]

Hi,

I'm sorry in advanced if this is the wrong mailing list, but I think my
issue might be related to ZGC.

The application ran under JDK 11.0.5+10 used -Xmx233g -Xms233g as jvm
parameters and was running fine and stable on Intel Xeon hardware with
256GB RAM and 6 cores. Note that most of the heap for this application
is static and the application itself is fine with a small amount of
memory like 3GB.

When upgrading to JDK 17.0.5+8 we observe that there was a steady load
(3 cores at 15-25%) although the application received no load. Maybe ZGC
tries to reduce the static heap somehow? I tried to tune the JVM with
-XX:+AlwaysPreTouch, -XX:ZFragmentationLimit=80 and/or -XX:-ZProactive
but I wasn't able to fix this. When using -Xloggc it heavily writes new
log entries and is frequently running "Garbage Collection (High Usage)".
See https://gist.github.com/karussell/7b9c2a94d5e86c927778421cead4ffd8

The only way to fix this was to increase -Xmx to 243g.

It is surely an unusual use case (large amount of read-only heap and
only a tiny free space), but requiring now nearly 10GB more RAM seems
unusual.

Kind Regards
Peter