Messages in this thread | | | Date | Thu, 29 Oct 2009 11:36:58 +0100 | From | Andrea Arcangeli <> | Subject | Re: RFC: Transparent Hugepage support |
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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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