linux-kernel - Re: Network slowdown due to CFS

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Date:	Fri, 28 Sep 2007 16:10:00 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jarek Poplawski <jarkao2@...pl>
Cc:	Ingo Molnar <mingo@...e.hu>, David Schwartz <davids@...master.com>,
	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 Friday 28 September 2007 00:42, Jarek Poplawski wrote:
> On Thu, Sep 27, 2007 at 03:31:23PM +0200, Ingo Molnar wrote:
> > * Jarek Poplawski <jarkao2@...pl> wrote:
>
> ...
>
> > > OK, but let's forget about fixing iperf. Probably I got this wrong,
> > > but I've thought this "bad" iperf patch was tested on a few nixes and
> > > linux was the most different one. The main point is: even if there is
> > > no standard here, it should be a common interest to try to not differ
> > > too much at least. So, it's not about exactness, but 50% (63 -> 95)
> > > change in linux own 'definition' after upgrading seems to be a lot.
> > > So, IMHO, maybe some 'compatibility' test could be prepared to compare
> > > a few different ideas on this yield and some average value could be a
> > > kind of at least linux' own standard, which should be emulated within
> > > some limits by next kernels?
> >
> > you repeat your point of "emulating yield", and i can only repeat my
> > point that you should please read this:
> >
> >     http://lkml.org/lkml/2007/9/19/357
> >
> > because, once you read that, i think you'll agree with me that what you
> > say is simply not possible in a sane way at this stage. We went through
> > a number of yield implementations already and each will change behavior
> > for _some_ category of apps. So right now we offer two implementations,
> > and the default was chosen empirically to minimize the amount of
> > complaints. (but it's not possible to eliminate them altogether, for the
> > reasons outlined above - hence the switch.)
>
> Sorry, but I think you got me wrong: I didn't mean emulation of any
> implementation, but probably the some thing you write above: emulation
> of time/performance. In my opinion this should be done experimentally
> too, but with something more objective and constant than current
> "complaints counter". And the first thing could be a try to set some
> kind of linux internal "standard of yeld" for the future by averaging
> a few most popular systems in a test doing things like this iperf or
> preferably more.

By definition, yield is essentially undefined as to the behaviour between
SCHED_OTHER tasks at the same priority level (ie. all of them), because
SCHED_OTHER scheduling behaviour itself is undefined.

It's never going to do exactly what everybody wants, except those using
it for legitimate reasons in realtime applications.
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