RFR: 8272807: Permit use of memory concurrent with pretouch

[Kim Barrett]

On Mon, 23 Aug 2021 11:35:18 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change to os::pretouch_memory to permit use of the memory
> concurrently with the pretouch operation.  This is accomplished by using an
> atomic add of zero as the operation for touching the memory, ensuring the
> virtual location is backed by physical memory while not changing any values
> being read or written by the application.
> 
> While I was there, fixed some other lurking issues in os::pretouch_memory.
> There was a potential overflow in the iteration that has been fixed.  And if
> the range arguments weren't page aligned then the last page might not get
> touched.  The latter was even mentioned in the function's description.  Both
> of those have been fixed by careful alignment and some extra checks.  The
> resulting code is a little more complicated, but more robust and complete.
> 
> This change doesn't make use of the new capability; I have some other
> changes in development to do that.
> 
> Testing:
> mach5 tier1-3.
> 
> I've been using this change while developing uses of the new capability.
> Performance testing hasn't found any regressions related to this change.

I'm withdrawing this PR, with the intent of taking a different tack, adding a new function for the concurrent use-case.

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

PR: https://git.openjdk.java.net/jdk/pull/5215