[PATCh v3] powerpc: add hugepagesz boot-time parameter

[Mel Gorman]

On (04/01/08 00:34), Arnd Bergmann didst pronounce:
> On Thursday 03 January 2008, Jon Tollefson wrote:
> > Paul, please include this in 2.6.25 if there are no objections.
> 
> No objections to this version from me, just questions.
> 
> > This patch adds the hugepagesz boot-time parameter for ppc64.  It lets
> > one pick the size for huge pages. The choices available are 64K and 16M
> > when the base page size is 4k. It defaults to 16M (previously the only
> > only choice) if nothing or an invalid choice is specified.
> 
> We started discussing this in v1, but the discussion got sidetracked:
> Is there a technical reason why you don't also allow 1M pages, which
> may be useful in certain scenarios?
> 

I cannot see why not although the ideal would be that the necessary
information to support the pagesize would be provided by the firmware
instead of hard-coded values.

> On the Cell/B.E. platforms (IBM/Mercury blades, Toshiba Celleb, PS3), the
> second large page size is an option that can be set in a HID SPR
> to either 64KB or 1MB. Unfortunately, we can't do these two simultaneously,
> but the firmware can change the default and put it into the device tree,
> or you could have the kernel override the firmware settings.
> 
> Going a lot further, do you have plans for a fully dynamic hugepage size,
> e.g. using a mount option for hugetlbfs?

A mount option for hugetlbfs to select a hugepage size is a long-term
goal. However, a lot of changes are required to support such a thing. Selecting
the hugepage size at boot-time is a reasonable starting point and one that
might be usable with some additional work by x86-64 on AMD processors
supporting 1GiB pages.

> I can see that as rather useful,
> but at the same time it's probably much more complicated than the boot time
> option.
> 

Not probably at all. It is *certainly* much more complicated than the
boot-time option :)

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab