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[PATCH, PPC] Use 64k for COMMONPAGESIZE

Alan Modra amodra@gmail.com
Fri Dec 19 08:16:00 GMT 2014 
On Thu, Dec 18, 2014 at 04:41:19PM +1030, Alan Modra wrote:
> On Thu, Dec 18, 2014 at 10:26:03AM +1100, Chris Johns wrote:
> > On 17/12/2014 12:34 pm, Richard Henderson wrote:
> > >It seems to me that most powerpc hardware these days is server based,  and very
> > >little remains at the desktop class.
> > 
> > What about embedded devices with as Freescale's QorIQ T2080 and T4240 ?
> > 
> > >And in the server environment, IBM has
> > >been recommending a 64k page size.
> > 
> > Would this change effect RTEMS and it devices ?
> 
> Yes, it would.  However, the effect isn't huge one way or another.
> 
> Richard quoting IBM's recommendation of a 64k page size really hasn't
> anything to do with COMMONPAGESIZE, or at least not as much as you
> might think..  You can quite happily run a binary linked with
> COMMONPAGESIZE set to 4k on a system using 64k pages.  COMMONPAGESIZE
> or -z common-page-size is really about where the linker starts the
> data segment, following on from the text segment.  It boils down to
> a trade-off between memory pages and disk pages, and the net result of
> increasing COMMONPAGESIZE to 64k for a system running with 4k pages 
> is that you'll tend to have bigger on-disk binaries but won't use any
> more memory than with the "proper" 4k COMMONPAGESIZE.  On the other
> hand if you really are running with 64k pages, there will be binaries
> where you could save a 64k page of memory if you'd specified the
> proper COMMONPAGESIZE at link time.
> 
> Overall, I think the increased COMMONPAGESIZE is beneficial, so I'm
> happy with the patch.

One other thing I'd forgotten about is that COMMONPAGESIZE is used to
decide where the relro segment ends (which after reading some bug
reports, I guess is the driving force behind this patch).  This
particular usage *can* increase memory footprint so my "won't use more
memory" remark above is incorrect.

You could argue that using COMMONPAGESIZE for this purpose is a bug,
and that MAXPAGESIZE should determine the relro segment end instead..

-- 
Alan Modra
Australia Development Lab, IBM
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