linux-kernel - Re: Network slowdown due to CFS

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Date:	Thu, 27 Sep 2007 11:30:02 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	David Schwartz <davids@...master.com>,
	"Linux-Kernel\@Vger\. 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>,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: Re: Network slowdown due to CFS

On 26-09-2007 15:31, Ingo Molnar 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:
...
> 
> 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-)

...Only if it were under some DEBUG option. Even if iperf is doing
the wrong thing there is no explanation for such big difference in
the behavior between sched_compat_yield 1 vs. 0. It seems common
interfaces should work similarly and predictably on various
systems, and here, if I didn't miss something, linux looks like a
different kind?

Regards,
Jarek P.
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