linux-kernel - Re: eth_type_trans(): Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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Date:	Tue, 18 Nov 2008 09:30:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	torvalds@...ux-foundation.org, dada1@...mosbay.com, rjw@...k.pl,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl,
	shemminger@...tta.com
Subject: Re: eth_type_trans(): Re: [Bug #11308] tbench regression on each
	kernel release from 2.6.22 -&gt; 2.6.28


* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Mon, 17 Nov 2008 22:26:57 +0100
> 
> > eth->h_proto access.
> 
> Yes, this is the first time a packet is touched on receive.
> 
> > Given that this workload does localhost networking, my guess would be 
> > that eth->h_proto is bouncing around between 16 CPUs? At minimum this 
> > read-mostly field should be separated from the bouncing bits.
> 
> It's the packet contents, there is no way to "seperate it".
> 
> And it should be unlikely bouncing on your system under tbench, the 
> senders and receivers should hang out on the same cpu unless the 
> something completely stupid is happening.
> 
> That's why I like running tbench with a num_threads command line 
> argument equal to the number of cpus, every cpu gets the two thread 
> talking to eachother over the TCP socket.

yeah - and i posted the numbers for that too - it's the same 
throughput, within ~1% of noise.

	Ingo
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