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

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Message-ID: <Pine.LNX.4.64.0703012150290.1768@schroedinger.engr.sgi.com>
Date:	Thu, 1 Mar 2007 21:53:42 -0800 (PST)
From:	Christoph Lameter <clameter@...r.sgi.com>
To:	Nick Piggin <npiggin@...e.de>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@...net.ie>, mingo@...e.hu,
	jschopp@...tin.ibm.com, arjan@...radead.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

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.

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