Running into Allocation Stalls during class unloading

Mary Sunitha Joseph mary.joseph at
Tue Feb 2 19:30:40 UTC 2021

Hi team,

Our Production application runs on a 320G heap and uses ZGC with large
pages enabled. We have not done any tuning and are using ZGC with defaults.
Since upgrading to JDK 15.0.1 we've started to notice that once a day the
app experiences allocation stalls (during peak hours) and this happens when
there is a huge drop in the number of classes loaded. We have a bi-monthly
release cycle and can see that the allocation stalls start small a business
day after a  release and slowly increase as the week progresses.

At the moment the app seems to be doing fine but it could escalate anytime
by the looks of it. There is an increase in the app's response time as well
at the same time and a small spike in heap which seem like side effects.
Any pointers in terms of tuning would be much appreciated.

The app currently always makes use of at least 200G of heap space which
leaves a 37% head space for ZGC.


Mary Sunitha Joseph (She/her)

Lead Developer

Fiix Software

p: 1 (855) 884-5619

e: mary.joseph at



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