Re: [00/17] Large Blocksize Support V3

[Nick Piggin]

Christoph Lameter wrote:
> On Thu, 26 Apr 2007, Nick Piggin wrote:
>
>
> > > > I am working now and again on some code to do this, it is a big job but
> > > > I think it is the right way to do it. But it would take a long time to
> > > > get stable and supported by filesystems...
> > >
> > > Ummm... We already have a radix tree for this???? What more is needed? You
> > > just need to go through all filesystems and make them use extends.
> >
> > I'm talking about block size > page size in the buffer layer.
>
>
> I fail to see the point of adding another layer when you already have a 

It isn't another layer. We already have this layer.

> mapping through the radix tree. You just need to change the way the 
> filesystem looks up pages.

You didn't think any of the criticisms of higher order page cache size
were valid?


> What are the exact requirement you are trying to address?

Block size > page cache size.


> You fundamentally cannot address the large blocksize requirements with 4k 
> pages since you simply must have larger contiguous memory.
>
> Large blocksize means that the device can do I/O on blocks of that size.
>
> What can be done is to create some kind of fake linearity. At one level 
> the radix tree and the address space already provide that. The radix tree 
> allows you to find the next page etc. Another approach would be to create 
> a virtual address space that fakes linearity even for the processor.
>
> Then there are ways with I/O mmus to avoid the issues again.
>
> However, you still have not addressed the underlying problem of the device 
> not being able to do I/O to a larger block of memory.

With iommus and sg lists?

You guys have a couple of problems, firstly you need to have ia64
filesystems accessable to x86_64. And secondly you have these controllers
without enough sg entries for nice sized IOs.

I sympathise, and higher order pagecache might solve these in a way, but
I don't think it is the right way to go, mainly because of the fragmentation
issues.

Increasing PAGE_SIZE, support for block size > page cache size, and getting
io controllers matched to a 4K page size IMO would be some good ways to
solve these problems. I know they are probably harder...

--
SUSE Labs, Novell Inc.
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