Hi Alen,

On 2023-11-13 19:05, Alen Vrečko wrote:

Hello everyone,

o) young gen reference processor

A bit puzzled by reading in a thread on the list:

> mentioning that we decided to not ship a young generation reference processor for 21

Unless you made changes to ByteBuffer#allocateDirect it uses reference processor to free native memory. If I am not mistaking just using standard library API such as Files.readAllBytes will in some cases do BB#allocateDirect in the internals.

Or maybe I am misunderstanding something? I made a toy program and indeed I could easily get a situation where 20% of reference handlers are not called like ever.

This will cause issues for code that is using reference handlers.

The reference processing will happen when the GC performs a major collection, which collects both the young and old generation. If you add a System.gc() you should see that the reference processor is kicking in for your program. Could you share your toy program?

o) seeing weird byte[] corruption in production

On CentOS 7 Generational works fine. No issues observed. But on Alma Linux 9.2 either reading byte[] from file or sending byte[] over the network corrupts the byte[]. Didn't investigate at all. Just observed corruption in some cases for some byte[] arrays - not all - just some. On the same Alma Linux 9.2 without generational zgc no byte[] corruption is observed and everything works fine as before.

It's hard to say if this is a ZGC bug, compiler bug, OS bug, etc. Here are some suggestions for how to help pin-point the problem:
1) Could you provide the output from 'java -version'?
2) Is it possible to reproduce this with a small reproducer?
3) What CPU is this running on?
4) Does it happen with -XX:UseAVX=0
5) Do you know the sizes of the corrupted byte[]s? Do you know the offset to where it is corrupted?

StefanK

To me Generational ZGC looks more like an experimental feature for now. I am a bit surprised it doesn't require the extra flag to unlock experimental features.

Thanks

Alen