OperationType in SafepointBegin event

[Stefan Karlsson]

Hi,

I'll let others answer your proposal, but I want to give another 
perspective on this:

Whenever I use JFR to debug latencies, I almost always change the 
threshold of ExecuteVMOperation to 0ms. (I do the same for other events 
as well, but I don't know them by heart) Maybe it would make sense to 
make it the default for this event? I don't think the system would be 
overwhelmed by getting a few more events, especially now that we are 
replacing more and more VM operations with thread-local handshakes.

StefanK

On 2020-11-24 16:57, Jean-Philippe Bempel wrote:
> Hello All,
> 
> With default JFR settings (default.jfc) you may end up with no 1:1
> mapping between SafepointBegin event and ExecuteVMOperation event.
> While having ExecuteVMOperation without SafepointBegin could be normal
> (Some VMOperation does not require Safepoint like Handshake for
> example), Having a SafepointBegin should normally be associated with a
> VM operation.
> 
> By default JFR settings record SafepointBegin & ExecutionVMOperation
> events greater than 10ms (threshold). So your VM Operation could be
> quick (less than 10ms, so not recorded along with VMOperation type)
> but the SafepointBegin event can have a long duration because bringing
> all Java threads at safepoint (so called Time-To-SafePoint) can take
> several ms in itself (ex System.arraycopy call on large buffers).
> 
> So you’re ending up with a long SafepointBegin event, but without
> ExecuteVMOperation event associated, and could not figure out what
> triggers this safepoint.
> Currently, SafepointBegin event does not record the VMOperation
> type[1] unlike ExecuteVMOperation event[2] which does not facilitate
> the troubleshooting.
> 
> Note: Xlog tracing (-Xlog:safepoint) give us the VMOperation type[3]
> 
> My proposal is the following:
> 
> Add the same operation field in SafepointBegin than in
> ExecuteVMOperation event in metadata.xml and modify
> post_safepoint_begin_event to add VMThread::vm_op_type() and set it
> into the event.
> If you are ok with that I can open a ticket and push a PR.
> 
> Regards,
> Jean-Philippe
> 
> [1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/jfr/metadata/metadata.xml#L560
> [2] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/jfr/metadata/metadata.xml#L589
> [3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/runtime/safepoint.cpp#L1114-L1125
>