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

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Date:	Wed, 14 Nov 2007 05:14:27 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Miller <davem@...emloft.net>
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

On Wednesday 14 November 2007 17:12, David Miller wrote:
> 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.

No, it's an x86-64, so no highmem.

What's also interesting is that SLAB apparently doesn't have this
condition. The first thing that sprung to mind is that SLAB caches
order > 0 allocations, while SLUB does not. However if anything,
that should actually favour the SLUB numbers if network is avoiding
order > 0 allocations.

I'm doing some oprofile runs now to see if I can get any more info.
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