RFR (S): 8067469: G1 ignores AlwaysPreTouch

[Thomas Schatzl]

Hi David,

On Wed, 2014-12-17 at 12:18 +1000, David Holmes wrote:
> Hi Thomas,
> 
> On 16/12/2014 10:23 PM, Thomas Schatzl wrote:
> > Hi all,
> >
> >    can I have reviews for the following small change?
> >
> > It adds pre-touching of allocated memory in G1, which has been forgotten
> > to be implemented when changing this code in JDK-8038423.
> >
> > This change adds this and refactors the pre-touch code to a single
> > method in the os class.
> >
> > CC'ed runtime since it changes code in os.hpp/cpp.
> 
> Just looking at that, we seem to be assuming/requiring that start and 
> end are suitably page aligned and/or an integral number of pages apart - 
> otherwise for an arbitrary pair of addresses it seems we could fail to 
> touch the page containing end.

  this is indeed a problem. And we cannot assert this because code doing
the pre-touching and now calling this method, actually pass unaligned
(end) addresses.

The code to commit memory also may pass unaligned memory sizes (although
I guess the OS will round up anyway), so I think we should not align
ourselves here either.

I filed JDK-8067785 for that.

I added a comment about this for now, new webrev at
http://cr.openjdk.java.net/~tschatzl/8067469/webrev.1/
diff at
http://cr.openjdk.java.net/~tschatzl/8067469/webrev.0_to_1/

I also changed the actual access to what Erik suggested to be easier
understandable.

Thanks,
  Thomas