netdev - Re: 2.6.24-rc2: Network commit causes SLUB performance regression with tbench

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Date:	Tue, 13 Nov 2007 22:12:42 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	nickpiggin@...oo.com.au
Cc:	clameter@....com, netdev@...r.kernel.org,
	herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org
Subject: Re: 2.6.24-rc2: Network commit causes SLUB performance regression
 with tbench

From: Nick Piggin <nickpiggin@...oo.com.au>
Date: Wed, 14 Nov 2007 04:36:24 +1100

> On Wednesday 14 November 2007 12:58, David Miller wrote:
> > I suspect the issue is about having a huge skb->data linear area for
> > TCP sends over loopback.  We're likely getting a much smaller
> > skb->data linear data area after the patch in question, the rest using
> > the sk_buff scatterlist pages which are a little bit more expensive to
> > process.
> 
> It didn't seem to be noticeable at 1 client. Unless scatterlist
> processing is going to cause cacheline bouncing, I don't see why this
> hurts more as you add CPUs?

Is your test system using HIGHMEM?

That's one thing the page vector in the sk_buff can do a lot,
kmaps.
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