Re: The performance and behaviour of the anti-fragmentation related patches

[Nick Piggin]

On Thu, Mar 01, 2007 at 10:51:00PM -0800, Christoph Lameter wrote:
> On Fri, 2 Mar 2007, Nick Piggin wrote:
> 
> > > There was no talk about slightly. 1G page size would actually be quite 
> > > convenient for some applications.
> > 
> > But it is far from convenient for the kernel. So we have hugepages, so
> > we can stay out of the hair of those applications and they can stay out
> > of hours.
> 
> Huge pages cannot do I/O so we would get back to the gazillions of pages 
> to be handled for I/O. I'd love to have I/O support for huge pages. This 
> would address some of the issues.

Can't direct IO from a hugepage?

> > > Writing a terabyte of memory to disk with handling 256 billion page 
> > > structs? In case of a system with 1 petabyte of memory this may be rather 
> > > typical and necessary for the application to be able to save its state
> > > on disk.
> > 
> > But you will have newer IO controllers, faster CPUs...
> 
> Sure we will. And you believe that the the newer controllers will be able 
> to magically shrink the the SG lists somehow? We will offload the 
> coalescing of the page structs into bios in hardware or some such thing? 
> And the vmscans etc too?

As far as pagecache page management goes, is that an issue for you?
I don't want to know about how many billions of pages for some operation,
just some profiles.

> > Is it a problem or isn't it? Waving around the 256 billion number isn't
> > impressive because it doesn't really say anything.
> 
> It is the number of items that needs to be handled by the I/O layer and 
> likely by the SG engine.

The number is irrelevant, it is the rate that is important.

> > I understand you have controllers (or maybe it is a block layer limit)
> > that doesn't work well with 4K pages, but works OK with 16K pages.
> 
> Really? This is the first that I have heard about it.
>

Maybe that's the issue you're running into.

> > This is not something that we would introduce variable sized pagecache
> > for, surely.
> 
> I am not sure where you get the idea that this is the sole reason why we 
> need to be able to handle larger contiguous chunks of memory.

I'm not saying that. You brought up this subject of variable sized pagecache.

> How about coming up with a response to the issue at hand? How do I write 
> back 1 Terabyte effectively? Ok this may be an exotic configuration today 
> but in one year this may be much more common. Memory sizes keep on 
> increasing and so is the number of page structs to be handled for I/O. At 
> some point we need a solution here.

Considering you're just handwaving about the actual problems, I
don't know. I assume you're sitting in front of some workload that has
gone wrong, so can't you elaborate?

Eventually, increasing x86 page size a bit might be an idea. We could even
do it in software if CPU manufacturers don't for us.

That doesn't buy us a great deal if you think there is this huge looming
problem with struct page management though.

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