RFR (XS): 8138684: G1 decision about taking regions into the collection set is too aggressive

Thomas Schatzl thomas.schatzl at oracle.com
Thu Nov 5 12:40:22 UTC 2015


Hi Mikael,

On Thu, 2015-11-05 at 13:10 +0100, Mikael Gerdin wrote:
> Hi Thomas,
> 
> On 2015-11-05 11:49, Thomas Schatzl wrote:
> > Hi all,
[...]
> > I.e. the safety factor is first dependent on our target PLAB waste, and
> > on the confidence of our predictions in general, scaling inversely (i.e.
> > with high G1ConfidencePercent we assume that our assumption about
> > keeping TargetPLABWastePct is more correct, and with lower
> > G1ConfidencePercent we allow more slack).
> >
> > A (really) conservative buffer here is particularly interesting for
> > dynamic IHOP, as it tends to try to push out marking rounds as long as
> > possible.
> 
> Just so I'm reading this code correctly, we are essentialy changing from 
> a "safety factor" of 1 to a safety factor of 2,x where x depends on the 
> PLAB waste percent?
> 

Yes. A value of "1" is no safety at all. I plan to look at the default
value of G1ConfidenceFactor, because 0.5 seems "pretty uncertain" to me
already as that value is located halfway the possible range.
I would be fine to drop the G1ConfidenceFactor term though if somebody
objects.

I am pretty certain that the TargetPLABWastePct is a good idea - that is
after all our goal in how much we can waste for PLAB sizing. And GC
threads typically allocate in PLABs from the heap, not in actual
objects.

Also consider that this check is applied for every single region, so
e.g. it's not as if we want to evacuate 100 half-full (1M) regions (=50M
data), this value of 2.2 does not means that we sort of keep >50M as
reserve, only around 1M (with these settings).

> > Since we are also in the process of investigating the policy surrounding
> > mixed gcs and the predictions, this seems a lot better starting point
> > too.
> >
> > CR:
> > https://bugs.openjdk.java.net/browse/JDK-8138684
> > Webrev:
> > http://cr.openjdk.java.net/~tschatzl/8138684/webrev
> > Testing:
> > jprt, benchmark runs with and without dIHOP without any real
> > differences.
> 
> Given that you've performance tested this without any issues I think 
> this is good to go.
> 
> /Mikael

Thanks,
  Thomas




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