netdev - Re: [tbench regression fixes]: digging out smelly deadmen.

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Date:	Mon, 27 Oct 2008 10:30:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	zbr@...emap.net, akpm@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	efault@....de, jkosina@...e.cz, rjw@...k.pl,
	s0mbre@...rvice.net.ru, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [tbench regression fixes]: digging out smelly deadmen.

* David Miller <davem@...emloft.net> wrote:

> From: Evgeniy Polyakov <zbr@...emap.net>
> Date: Sun, 26 Oct 2008 13:05:55 +0300
> 
> > I'm not surprised there were no changes when I reported hrtimers to be
> > the main guilty factor in my setup for dbench tests, and only when David
> > showed that they also killed his sparks via wake_up(), something was
> > done. Now this regression even dissapeared from the list.
> > Good direction, we should always follow this.
> 
> Yes, this situation was in my opinion a complete fucking joke.  
> Someone like me shouldn't have to do all of the hard work for the 
> scheduler folks in order for a bug like this to get seriously looked 
> at.

yeah, that overhead was bad, and once it became clear that you had 
high-resolution timers enabled for your benchmaking runs (which is 
default-off and which is still rare for benchmarking runs - despite 
being a popular end-user feature) we immediately disabled the hrtick via 
this upstream commit:

  0c4b83d: sched: disable the hrtick for now

that commit is included in v2.6.28-rc1 so this particular issue should 
be resolved.

high-resolution timers are still default-disabled in the upstream 
kernel, so this never affected usual configs that folks keep 
benchmarking - it only affected those who decided they want higher 
resolution timers and more precise scheduling.

Anyway, the sched-hrtick is off now, and we wont turn it back on without 
making sure that it's really low cost in the hotpath.

Regarding tbench, a workload that context-switches in excess of 100,000 
per second is inevitably going to show scheduler overhead - so you'll 
get the best numbers if you eliminate all/most scheduler code from the 
hotpath. We are working on various patches to mitigate the cost some 
more - and your patches and feedback is welcome as well.

But it's a difficult call with no silver bullets. On one hand we have 
folks putting more and more stuff into the context-switching hotpath on 
the (mostly valid) point that the scheduler is a slowpath compared to 
most other things. On the other hand we've got folks doing 
high-context-switch ratio benchmarks and complaining about the overhead 
whenever something goes in that improves the quality of scheduling of a 
workload that does not context-switch as massively as tbench. It's a 
difficult balance and we cannot satisfy both camps.

Nevertheless, this is not a valid argument in favor of the hrtick 
overhead: that was clearly excessive overhead and we zapped it.

	Ingo
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