linux-kernel - Re: iperf yield usage

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Date:	Mon, 1 Oct 2007 21:09:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Chris Friesen <cfriesen@...tel.com>
Cc:	Jarek Poplawski <jarkao2@...pl>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	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: iperf yield usage

* Chris Friesen <cfriesen@...tel.com> wrote:

> >See the background and answers to that in:
> >
> >   http://lkml.org/lkml/2007/9/19/357
> >   http://lkml.org/lkml/2007/9/19/328
> >
> >there's plenty of recourse possible to all possible kinds of apps. 
> >Tune the sysctl flag in one direction or another, depending on which 
> >behavior the app is expecting.
> 
> Yeah, I read those threads.
> 
> It seems like the fundamental source of the disconnect is that the 
> tasks used to be sorted by priority (thus making it easy to bump a 
> yielding task to the end of that priority level) while now they're 
> organized by time (making it harder to do anything priority-based).  
> Do I have that right?

not really - the old yield implementation in essence gave the task a 
time hit too, because we rotated through tasks based on timeslices. But 
the old one requeued yield-ing tasks to the 'active array', and the 
decision whether a task is in the active or in the expired array was a 
totally stohastic, load-dependent thing. As a result, certain tasks, 
under certain workloads saw a "stronger" yield, other tasks saw a 
"weaker" yield. (The reason for that implementation was simple: yield 
was (and is) unimportant and it was implemented in the most 
straightforward way that caused no overhead anywhere else in the 
scheduler.)

( and to keep perspective it's also important to correct the subject
  line here: it's not about "network slowdown" - nothing in networking
  slowed down in any way - it was that iperf used yield in a horrible
  way. I changed the subject line to reflect that. )

	Ingo
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