[openj9-dev] Performance questions for low-latency app

[akrus <akrus@xxxxxxxxxxx>]

Hello everyone!

I was able to switch one of our app instances to OpenJ9 finally and it
works good so far. It was running on Azul Zing before, so I can compare
what we have now and what we had before.

There are couple of questions:
1) I have the following flags set so far: -Xmx200g
-Xshareclasses:name=myapp -Xgcpolicy:metronome -Xgc:targetPauseTime=1
Are there any other flags to consider for low-latency app?

2) I've tried enabling hugepages support for Java heap, but it was
complaining that such amount of memory is unavailable (which is not
true). I suppose that this can happen because hugepages were enabled on
a working system so it's advised in docs somewhere to reboot the
machine to fix this. Is anyone using hugepages for heap? Is there any
performance comparison for OpenJ9 with hugepages and without?

3) there is CUDA support available. Although we have no nVidia cards on
the servers, is it worth adding one? Is there any performance
comparison for this as well?

4) I've tried switching JIT optLevel to scorching and this is consuming
too much CPU. Is it worth changing default JIT level to any other?

5) I can see that Java memory usage never exceeds 50% of Xmx value, is
it supposed to be like that? Is there any flag to change this?

6) OpenJ9 supports RDMA, but there is no -Dcom.ibm.net.rdma.conf
configuration example I found. As I can see this was tested with
Mellanox cards, did anyone test Mellanox VMA?

7) comparing with Zing again, I can see that some app threads are
sometimes 'hanging' for ~20-40 msec and this happens quite frequently.
While the server is quite loaded and this can be normal behaviour, with
Zing it's happening much less frequently, but pauses are more
noticeable.

8) are there support options available (IBM?)?

Sorry for such a bunch of questions, but this mailing list is the only
place to find out the answers :)

Regards,
akrus.