RFR (M): 8157952: Parallelize Memory Pretouch

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that parallelizes memory pretouch?
Particularly on large systems (like hundreds of GB) this often improves
startup time by an order of magnitude.

Like in some systems with 1TB+ heap I have running from 12mins to
~1min.

It does so by having multiple threads doing the pretouching, using more
of the available memory bandwidth of the computer.

This change only has an FC extension approval by the group lead for
now, but I expect to get the release teams' approval soon.

CR:
https://bugs.openjdk.java.net/browse/JDK-8157952
Webrev:
http://cr.openjdk.java.net/~tschatzl/8157952/webrev
Testing:
jprt, vm.gc run with -XX:+AlwaysPreTouch, nightly test suite run

Thanks,
  Thomas