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

[Christoph Lameter <clameter@engr.sgi.com>]

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.

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