RFR (S): 8243672: Short term pause time ratio calculation in G1 off

[Thomas Schatzl]

Hi all,

   can I have reviews for this change that improves our calculation of 
the short term pause time ratio calculation in G1?

Previously we calculated the short term pause time ratio by assuming 
that the current pause occurred X times in the long term pause time 
tracking interval spanning X garbage collections.

That, unfortunately, emphasizes the current gc pause, giving completely 
off results particularly for pause time increases e.g. during mixed gc 
(see the comment in the CR for a sample calculation), in turn tending to 
affect heap sizing negatively in that the heap expansion is far higher 
than expected.

(There is no real issue at the moment for too low results as we do not 
use these calculated time ratios to shrink the heap yet).

Arguably now the short term gc time ratio will be more spiky now with 
that change, but short term pause time ratio is not the only metric used 
for heap expansion, single outliers are not relevant anyway, and there 
are limits on the per-GC sizing too.
So overall it mostly works out as before with significant improvements 
in heap sizing behavior together with JDK-8244603 (which this change 
will be pushed with.)

CR:
https://bugs.openjdk.java.net/browse/JDK-8243672
Webrev:
http://cr.openjdk.java.net/~tschatzl/8243672/webrev/index.html
Testing:
hs-tier1-5, perf testing with JDK-8244603

Overall, with both changes on throughput benchmarks there is not much 
difference than before as startup typically expands the heap similarly 
to before and shrinking the heap at more places will be added only later.
There are some significant performance regressions with OOB settings, at 
significant reduced heap size though. If you set heap sizes manually, 
there is *no* change.