Shenandoah Events

[zgu at redhat.com]

On Tue, 2019-01-29 at 18:25 +0100, Aleksey Shipilev wrote:
> On 1/29/19 6:03 PM, Ken Dobson wrote:
> > Just following up on the possibility of running the benchmarks to
> > measure the performance overhead.
> > Please let me know if this would be possible and what I would have
> > to do to get this done.
I was initially worry about the amount of region state transition
events generated. After adding should_commit() guard, I am now less
concerned.

Some overheads during recoding time, I think, are expected. So the
overhead, that we are talking about, is down to the additional guard
test when recording is off, I doubt it is measurable.

Thanks,

-Zhengyu


> 
> It is possible, and should be as simple as running the benchmarks
> with/without -XX:+FlightRecorder?
> You are working with Zhengyu on JFR support, right? Zhengyu knows how
> to run benchmarks.
> 
> > On Thu, Jan 24, 2019 at 11:28 AM Ken Dobson <kdobson at redhat.com
> > <mailto:kdobson at redhat.com>> wrote:
> >     The G1 version currently intercepts individual transitions so
> > I'd hope they've measured the
> >     overhead and found it was acceptable but can't be certain of
> > that. Yes I agree that's definitely
> >     the first step. Generally the default JFR profiling
> > configuration is ~2% overhead but detailed
> >     events such as these are not enabled in that configuration.
> > When using these events I think it
> >     would be best to disable all the default events and only enable
> > the two Shenandoah Events to
> >     reduce the overhead. If you think measuring the benchmarks is
> > the best way to get this data I'd
> >     be happy to do this if you can point me in the right direction.
> 
> The first rule of benchmarking is not assuming anything, including
> that someone else did them,
> especially for a different implementation.
> 
> There is also a bigger question: how much additional latency this
> brings to Shenandoah (tiny)
> pauses, when there are lots of transitions happen? Shenandoah logs
> times with -Xlog:gc, and summary
> times with -Xlog:gc+stats.
> 
> -Aleksey