Re: [00/41] Large Blocksize Support V7 (adds memmap support)

[Christoph Lameter]

On Tue, 11 Sep 2007, Mel Gorman wrote:

> > Well Christoph seems to still be spinning them as a solution for VM
> > scalability and first class support for making contiguous IOs, large
> > filesystem block sizes etc.
> > 
> 
> Yeah, I can't argue with you there. I was under the impression that we
> would be dealing with this strictly as a second class solution to see
> what it bought to help steer the direction of fsblock.

I think we all have the same impression. But should second class not be 
okay for IO and FS in special situations?

> As you say, a difference is if we fail to allocate a hugepage, the world
> does not end. It's been a well known problem for years and grouping pages
> by mobility is aimed at relaxing some of the more painful points. It has
> other uses as well, but each of them is expected to deal with failures with
> contiguous range allocation.

Note that this patchset is only needing higher order pages up to 64k not 
2M.

> > And I would have kept quiet this time too, except for the worrying idea
> > to use higher order pages to fix the SLUB vs SLAB regression, and if
> > the rationale for this patchset was more realistic.
> > 
> 
> I don't agree with using higher order pages to fix SLUB vs SLAB performance
> issues either. SLUB has to be able to compete with SLAB on it's own terms. If
> SLUB gains x% over SLAB in specialised cases with high orders, then fair
> enough but minimally, SLUB has to perform the same as SLAB at order-0. Like
> you, I think if we depend on SLUB using high orders to match SLAB, we are
> going to get kicked further down the line.

That issue is discussed elsewhere and we have a patch in mm to address the 
issue.

> > In theory (and again for the filesystem guys who don't have to worry about
> > it). In practice after seeing the patch it's not a nice thing for the VM to
> > have to do.
> > 
> 
> That may be a good enough reason on it's own to delay this. It's a
> technical provable point.

It would be good to know what is wrong with the patch? I was surprised how 
easy it was to implement mmap.

> I might regret saying this, but it would be easier to craft an attack
> using pagetable pages. It's woefully difficult to do but it's probably
> doable. I say pagetables because while slub targetted reclaim is on the
> cards and memory compaction exists for page cache pages, pagetables are
> currently pinned with no prototype patch existing to deal with them.

Hmmm... I thought Peter had a patch to move page table pages?

> If we hit this problem at all, it'll be due to gradual natural degredation.
> It used to be a case that jumbo ethernets reported problems after running
> for weeks and we might encounter something similar with large blocks while it
> lacks a fallback. We no longer see jumbo ethernet reports but the fact is we
> don't know if it's because we fixed it or people gave up. Chances are people
> will be more persistent with large blocks than they were with jumbo ethernet.

I have seen a failure recently with jumbo frames and order 2 allocs on 
2.6.22. But then .22 has no lumpy reclaim.
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