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SubjectRe: RFC: Transparent Hugepage support
Hello Ingo, Andi, everyone,

On Thu, Oct 29, 2009 at 10:43:44AM +0100, Ingo Molnar wrote:
>
> * Andi Kleen <andi@firstfloor.org> wrote:
>
> > > 1GB pages can't be handled by this code, and clearly it's not
> > > practical to hope 1G pages to materialize in the buddy (even if we
> >
> > That seems short sightened. You do this because 2MB pages give you x%
> > performance advantage, but then it's likely that 1GB pages will give
> > another y% improvement and why should people stop at the smaller
> > improvement?
> >
> > Ignoring the gigantic pages now would just mean that this would need
> > to be revised later again or that users still need to use hacks like
> > libhugetlbfs.
>
> I've read the patch and have read through this discussion and you are
> missing the big point that it's best to do such things gradually - one
> step at a time.
>
> Just like we went from 2 level pagetables to 3 level pagetables, then to
> 4 level pagetables - and we might go to 5 level pagetables in the
> future. We didnt go from 2 level pagetables to 5 level page tables in
> one go, despite predictions clearly pointing out the exponentially
> increasing need for RAM.

I totally agree with your assessment.

> So your obsession with 1GB pages is misguided. If indeed transparent
> largepages give us real benefits we can extend it to do transparent
> gbpages as well - should we ever want to. There's nothing 'shortsighted'
> about being gradual - the change is already ambitious enough as-is, and
> brings very clear benefits to a difficult, decade-old problem no other
> person was able to address.
>
> In fact introducing transparent 2MBpages makes 1GB pages support
> _easier_ to merge: as at that point we'll already have a (finally..)
> successful hugetlb facility happility used by an increasing range of
> applications.

Agreed.

> Hugetlbfs's big problem was always that it wasnt transparent and hence
> wasnt gradual for applications. It was an opt-in and constituted an
> interface/ABI change - that is always a big barrier to app adoption.
>
> So i give Andrea's patch a very big thumbs up - i hope it gets reviewed
> in fine detail and added to -mm ASAP. Our lack of decent, automatic
> hugepage support is sticking out like a sore thumb and is hurting us in
> high-performance setups. If largepage support within Linux has a chance,
> this might be the way to do it.

Thanks a lot for your review!

> A small comment regarding the patch itself: i think it could be
> simplified further by eliminating CONFIG_TRANSPARENT_HUGEPAGE and by
> making it a natural feature of hugepage support. If the code is correct
> i cannot see any scenario under which i wouldnt want a hugepage enabled
> kernel i'm booting to not have transparent hugepage support as well.

The two reasons why I added a config option are:

1) because it was easy enough, gcc is smart enough to eliminate the
external calls so I didn't need to add ifdefs with the exception of
returning 0 from pmd_trans_huge and pmd_trans_frozen. I only had to
make the exports of huge_memory.c visible unconditionally so it doesn't
warn, after that I don't need to build and link huge_memory.o.

2) to avoid breaking build of archs not implementing pmd_trans_huge
and that may never be able to take advantage of it

But we could move CONFIG_TRANSPARENT_HUGEPAGE to an arch define forced
to Y on x86-64 and N on power.


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