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

Christoph Lameter
2007-03-02T05:53:44Z

On Fri, 2 Mar 2007, Nick Piggin wrote:

> > You do not have to deal with TLB entries if you do buffered I/O.
>
> Where does the data come from?

>From the I/O controller and from the application.

> > We currently have problems with the kernel limits of 128 SG
> > entries but the fundamental issue is that we can only do 2 Meg of I/O in
> > one go given the default limits of the block layer. Typically the number
> > of hardware SG entrie is also limited. We never will be able to put a
>
> Seems like changing the default limits would be the easiest way to
> fix it then?

This would only be a temporary fix pushing the limits to the double or so?

> As far as hardware limits go, I don't think you need to scale that
> number linearly with the amount of memory you have, or even with the
> IO throughput. You should reach a point where your command overhead
> is amortised sufficiently, and the controller will be pipelining the
> commands.

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.