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

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Date:	Fri, 2 Mar 2007 06:49:44 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Christoph Lameter <clameter@...r.sgi.com>
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 Thu, Mar 01, 2007 at 09:40:45PM -0800, Christoph Lameter wrote:
> On Fri, 2 Mar 2007, Nick Piggin wrote:
> 
> > So what do you mean by efficient? I guess you aren't talking about CPU
> > efficiency, because even if you make the IO subsystem submit larger
> > physical IOs, you still have to deal with 256 billion TLB entries, the
> > pagecache has to deal with 256 billion struct pages, so does the
> > filesystem code to build the bios.
> 
> You do not have to deal with TLB entries if you do buffered I/O.

Where does the data come from?

> For mmapped I/O you would want to transparently use 2M TLBs if the 
> page size is large.
> 
> > So you are having problems with your IO controller's handling of sg
> > lists?
> 
> 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?

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.

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