netdev - Re: Low performance Intel 10GE NIC (3.2.10) on 2.6.38 Kernel

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Message-ID: <1302803357.2744.1.camel@edumazet-laptop>
Date:	Thu, 14 Apr 2011 19:49:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	Wei Gu <wei.gu@...csson.com>, netdev <netdev@...r.kernel.org>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	Mike Galbraith <efault@....de>
Subject: Re: Low performance Intel 10GE NIC (3.2.10) on 2.6.38 Kernel

Le jeudi 14 avril 2011 à 18:57 +0200, Eric Dumazet a écrit :
> Le jeudi 14 avril 2011 à 18:56 +0200, Peter Zijlstra a écrit :
> > On Thu, 2011-04-14 at 09:42 -0700, Alexander Duyck wrote:
> > 
> > > I'm doing some more digging into this now.  One thought that occurred to 
> > > me is that if the patch you mention is having some sort of effect this 
> > > could be a sign of perhaps a kernel timer or scheduling problem.
> > 
> > Right, so the removal of the NO_HZ throttle will allow the CPU to go
> > into C states more often, this could result in longer wake-up times for
> > IRQs.
> > 
> > We reverted because:
> >   - it caused significant battery drain due to not going into C states
> >     often enough, and
> >   - its a much better idea to implement these things in the idle
> >     governor since it already has the job of guestimating the idle
> >     duration.
> > 
> > I really can't remember back far enough to even come up with a theory of
> > why kernels prior to merging the NO_HZ throttle would not exhibit this
> > problem.
> > 
> > 
> > 
> 
> Normally, Wei Gu already asked to not use C states.
> 
> http://h20000.www2.hp.com/bc/docs/support/SupportManual/c01804533/c01804533.pdf
> 
> How can we/he check this ?
> 
> 

Anyway, this could explain a latency problem, not packet drops.

With NAPI, we should get few hardware irqs under load.

Once softirq started, scheduler is out of the equation.



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