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

Christoph Lameter
2007-03-02T06:19:50Z

On Fri, 2 Mar 2007, Nick Piggin wrote:

> > >From the I/O controller and from the application.
>
> Why doesn't the application need to deal with TLB entries?

Because it may only operate on a small section of the file and hopefully
splice the rest through? But yes support for mmapped I/O would be
necessary.

> > This would only be a temporary fix pushing the limits to the double or so?
>
> And using slightly larger page sizes isn't?

There was no talk about slightly. 1G page size would actually be quite
convenient for some applications.

> > Amortized? The controller still would have to hunt down the 4kb page
> > pieces that we have to feed him right now. Result: Huge scatter gather
> > lists that may themselves create issues with higher page order.
>
> What sort of numbers do you have for these controllers that aren't
> very good at doing sg?

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.

> Isn't the issue was something like your IO controllers have only a
> limited number of sg entries, which is fine with 16K pages, but with
> 4K pages that doesn't give enough data to cover your RAID stripe?
>
> We're never going to do a variable sized pagecache just because of that.

No, we need support for larger page sizes than 16k. 16k has not been fine
for a couple of years. We only agreed to 16k because that was the common
consensus. Best performance was always at 64k 4 years ago (but then we
have no numbers for higher page sizes yet). Now we would prefer much
larger sizes.