RFR (S): 8067469: G1 ignores AlwaysPreTouch

Thomas Schatzl thomas.schatzl at oracle.com
Wed Dec 17 14:26:53 UTC 2014


Hi all,

On Wed, 2014-12-17 at 14:42 +0100, Thomas Schatzl wrote:
> 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.

After discussion with StefanK I changed the comment for the
pretouch_memory() method slightly. I reuploaded the webrevs to the same
locations.

Thanks,
  Thomas




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