Re: [PATCH] powerpc: Introduce address space "slices"

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2007-02-19 at 09:33 -0600, Olof Johansson wrote:
> On Mon, Feb 19, 2007 at 05:43:38PM +1100, Benjamin Herrenschmidt wrote:
> > powerpc: Introduce address space "slices"
> > 
> > This patch provide some infrastructure that will allow proper creation
> > of special VMAs with different page sizes on powerpc.
> > 
> > The basic issue is to be able to do what hugetlbfs does but with
> > different page sizes for some other special filesystems, more
> > specifically, my need is:
> > 
> >  - hugetlbfs should still work of course :-)
> > 
> >  - SPE local store mappings using 64K pages on a 4K base page size
> > kernel on Cell
> 
> Why? What is the reason they can't use 4K pages?

Reduce TLB/ERAT trashing. SPE MMUs are fairly small, and in setups where
an SPE maps all the others, we take a performance hit due to trashing
with 4K pages. However, going to full 64K base page size has other
drawbacks, especially in setups with little main memory.

Ben.