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

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change to the policy that ends
adding regions to the collection set?

In some earlier review we noticed that G1 tries to fill up all remaining
space completely.

E.g. in

466 if ((2.0 * _sigma) * (double) bytes_to_copy > (double) free_bytes)
{ 
467 // end condition 3: out-of-space (conservatively!) 
468 } 

_sigma is .5 by default, and I have never seen anyone changing
G1ConfidencePercent.

This is a recipe for evacuation failure we want to avoid.

So I thought of a formula that better represents the assumptions in the
GC code, and came up with this:

 487   double safety_factor = (100.0 / G1ConfidencePercent) * (100 +
TargetPLABWastePct) / 100.0;
 488   size_t expected_bytes_to_copy = safety_factor * bytes_to_copy;
 489 
 490   if (expected_bytes_to_copy > free_bytes) {

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.

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.

Thanks,
  Thomas