RFR (S): 8202842: G1 footprint regressions in jdk11+10

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes (and actually improves
upon the baseline) some startup footprint regression introduced by
changes in jdk10+10?

Between jdk11+9 and jdk11+10 several G1 related changes added to the
startup footprint:

- https://bugs.openjdk.java.net/browse/JDK-8196485 ("FromCardCache
default card index can cause crashes") doubled the size of the
FromCardCache internal data structure due to some memory overflow bug
(which is #threads * #regions * 8 now instead of * 4) 

- https://bugs.openjdk.java.net/browse/JDK-8180415 ("Rebuild remembered
sets during the concurrent cycle") added at least one per-region * per-
thread table (mark stats cache, result array), and also increased
FromCardCache size due to more threads concurrently adding remembered
set entries.

This change modifies the allocation of the FromCardCache to be
allocated using mmap, which means that until it is touched, no actual
memory is used.
This works combined with a change to make the "invalid" FromCardCache
entry have a value of zero - this is possible because the Java heap can
never start at address 0x0.
This is the main change and yields almost all footprint gains.

There is also some change to not initialize the MarkStatsCaches at
initialization either, and although these are malloc'ed, also
apparently not causing the respective page faults.
The impact of this change is much smaller, as we don't control
alignment of these arrays - and the size of that data structure is also
much smaller.

However it would be nice to implement a more controlled allocation of
these kinds of arrays in the future to make sure they are not
initialized/touched at startup and only on use. That seemed to be out
of scope here, and the other change already improved startup footprint
beyond the original anyway. It was a very low-hanging fruit to pick
though :)

CR:
https://bugs.openjdk.java.net/browse/JDK-8202842
Webrev:
http://cr.openjdk.java.net/~tschatzl/8202842/webrev/
Testing:
hs-tier1 to hs-tier3

Thanks,
  Thomas