RFR: 8272807: Permit use of memory concurrent with pretouch [v3]

[Kim Barrett]

> [Trying again, after a couple previous proposals.]
> 
> Please review this change to os::pretouch_memory to permit use of the memory
> being pretouched concurrently with the pretouch operation.  This will allow
> future changes that can make use of such overlap.
> 
> Testing:
> mach5 tier1-3 with -XX:+AlwaysPreTouch.
> 
> Verified no performance impact by comparing startup times with Epsilon GC and
> -XX:+AlwaysPretouch on a large enough heap that the pretouch takes a
> significant amount of time.  Tested on both linux-x64 and linux-aarch64.
> 
> None of these test concurrent use; that will wait for future changes that use
> the new feature.

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  change loop termination test

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7343/files
  - new: https://git.openjdk.java.net/jdk/pull/7343/files/a80ac44f..2e4e66a0

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7343&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7343&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7343/head:pull/7343

PR: https://git.openjdk.java.net/jdk/pull/7343