RFR (XS): 8232951: TestG1ParallelPhases.java fails with phase NonYoungFreeCSet not found

[Thomas Schatzl]

Hi all,

   can I have reviews for this small fix to the 
TestG1ParallelPhases.java test so that it is more robust?

As far as I can tell from the failure and program execution the test 
tries to force mixed gcs expecting particular JFR events.

In particular the failure is about the failure to get the 
"NonYoungFreeCSet" parallel phase.

This event is sent when freeing the remembered sets of an old region 
(during mixed gc).

However due to how the test is set up, while it successfully forces 
mixed gcs, it does not make sure that there ever is waste in old regions 
(even if the test sets the threshold to 0.0). In my unsuccessful 
reproduction tries I noticed that the actual waste at the start of mixed 
gc is very close to 0.0 (or even 0.0) in all mixed gcs (often something 
like 8 bytes to reclaim only in total), and that while the threshold 
forces a mixed gc, sometimes during region selection no old gen regions 
are selected, and so no freeing of an old gen region and the JFR event 
occurs.

This seems to be correct to me, so I changed the test a little to be 
sure to actually generate waste.

The alternative would be to send a fake "NonYoungFreeCSet" parallel 
phase jfr event in the collector, but I do not like sending fake events 
for the sake of a test.

I also added some useful logging options in case this occurs again in 
CI, and curbed the amount of (young only) GCs performed.

Note that I did not manage to reproduce the issue myselves - the only 
occurrence has been a month ago that has been linked to a wrong bug. 
Obviously the failure (that we do not get any reclaimable old gen 
region) depends on a lot of other factors.

CR:
https://bugs.openjdk.java.net/browse/JDK-8232951
Webrev:
http://cr.openjdk.java.net/~tschatzl/8232951/webrev/
Testing:
400 runs of the changed test without issues

Thanks,
   Thomas