jcmd: separate metadata statistics from NMT?

[Thomas Stüfe]

Hi all,

JDK-8189688 introduced metadata printing into NMT. This is a very useful
feature. However, it is tied to NMT, which is unnecessary.

Currently, we have to trigger metadata statistics with jcmd with the
NMT-printing command "VM.native_memory metadata". In order for that to
work, NMT must have been enabled first in the monitored VM - so, the VM
must have been started with -XX:NativeMemoryTracking=... . If not, "jcmd
VM.native_memory" will give us an error. This diminishes its usefulness. We
have many customer scenarios where we cannot restart the VM to enable NMT
but still would like to see metadata statistics.

The metadata statistics do not need the NMT infrastructure, they work
perfectly fine without it. I propose to remove metadata statistics
from "VM.native_memory"
and make it an own first-class diagnostic command. I have made a quick
patch to check if that would work and it works just fine.

What do you think?

Thanks and Kind Regards, Thomas