RFR (XS): 8200723: Suppress rs_length and predicted_cards sampling during mixed gcs

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that fixes a throughput
problem G1 may show after mixed gc phase?

The problem is that after mixed gc young gen sizing is heavily
influenced by mixed gc due to in many cases highly different RS lengths
and number of cards processed. Almost always in a negative way (if not
neutral), i.e. during mixed gc these values are almost always much
higher than during young gc, meaning that young gen of the first few
young-only gcs is much smaller than it should be.

There is another related problem with adaptive IHOP, where due to these
changes to the young gen, the survival rate increases (smaller young
gen means less time to die), which decreases IHOP, which in turn
increases the number of mixed phases (e.g. mixed gc's), which in turn
make young gen stay small.

JDK-8035557 has a graph showing how eden size over time in relation to
rs_length could look like without this change.

Contrary to JDK-8035557 this change kind of arbitrarily suppresses the
sampling for rs_length and predicted_cards during mixed gc, so these
"bad" values never enter the prediction logic. Mixed gc is not
affected, since we do not use any prediction for sizing young during
that time.

The proper fix would be JDK-8035557, completely separating predictions
for young-only and mixed gc.

CR:
https://bugs.openjdk.java.net/browse/JDK-8200723
Webrev:
http://cr.openjdk.java.net/~tschatzl/8200723/webrev
Testing:
hs-tier 1-5 with many other changes over time; manual inspection of
eden sizing behavior of several known to be affected benchmarks

Thanks,
  Thomas