linux-kernel - Re: Network slowdown due to CFS

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Date:	Wed, 26 Sep 2007 08:40:35 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Schwartz <davids@...master.com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Martin Michlmayr <tbm@...ius.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Subject: Re: Network slowdown due to CFS

On Wed, 26 Sep 2007 15:31:38 +0200
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * David Schwartz <davids@...master.com> wrote:
> 
> > > > I think the real fix would be for iperf to use blocking network
> > > > IO though, or maybe to use a POSIX mutex or POSIX semaphores.
> > >
> > > So it's definitely not a bug in the kernel, only in iperf?
> > 
> > Martin:
> > 
> > Actually, in this case I think iperf is doing the right thing
> > (though not the best thing) and the kernel is doing the wrong
> > thing. [...]
> 
> it's not doing the right thing at all. I had a quick look at the
> source code, and the reason for that weird yield usage was that
> there's a locking bug in iperf's "Reporter thread" abstraction and
> apparently instead of fixing the bug it was worked around via a
> horrible yield() based user-space lock.
> 
> the (small) patch below fixes the iperf locking bug and removes the 
> yield() use. There are numerous immediate benefits of this patch:
> 
>  - iperf uses _much_ less CPU time. On my Core2Duo test system,
> before the patch it used up 100% CPU time to saturate 1 gigabit of
> network traffic to another box. With the patch applied it now uses 9%
> of CPU time.
> 
>  - sys_sched_yield() is removed altogether
> 
>  - i was able to measure much higher bandwidth over localhost for 
>    example. This is the case for over-the-network measurements as
> well.
> 
>  - the results are also more consistent and more deterministic, hence 
>    more reliable as a benchmarking tool. (the reason for that is that
>    more CPU time is spent on actually delivering packets, instead of
>    mindlessly polling on the user-space "lock", so we actually max out
>    the CPU, instead of relying on the random proportion the workload
> was able to make progress versus wasting CPU time on polling.)
> 
> sched_yield() is almost always the symptom of broken locking or other 
> bug. In that sense CFS does the right thing by exposing such bugs =B-)
>  
> 	Ingo

A similar patch has already been submitted, since BSD wouldn't work
without it.
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