Subject: Re: CONFIG_PREEMPT and server workloads
From: Tom Sightler <ttsig@tuxyturvy.com>
To: Andrea Arcangeli <andrea@suse.de>
Cc: Andrew Morton <akpm@osdl.org>, mjy@geizhals.at,
       Linux-Kernel <linux-kernel@vger.kernel.org>
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Date: Thu, 18 Mar 2004 18:54:30 -0500
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On Thu, 2004-03-18 at 10:37, Andrea Arcangeli wrote:
> On Thu, Mar 18, 2004 at 10:20:23AM -0500, Tom Sightler wrote:
> > Well, I reported an issue on my laptop several weeks ago where network
> > activity via my aironet wireless adapter would use 60-70% of the CPU but
> > only when PREEMPT was enabled.  Looking back over the list I see other
> 
> sounds like a preempt bug triggering in a softirq, or maybe an SMP bug
> triggering in UP.
> 
> I certainly agree with Andrew that preempt cannot cause a 60/70%
> slowdown with a network card at least (if you do nothing but spinlocking
> in a loop then it's possible a 60/70% slowdown instead but the system
> time that a wifi card should take is nothing compared to the idle/user
> time).

Actually, I managed to get two bugs mixed together, my system had a
problem with PREEMPT and ACPI but this certainly seems to be fixed in
the current kernels, at least in 2.6.3-mm2 (my current everyday kernel)
and 2.6.5-rc1-mm2 (just a quick test tonight).  Somehow the combination
of PREEMPT and any little applet that used ACPI (battery monitor) would
use a lot of CPU, then using the aironet adapter caused the CPU to
spike.  Turning off either PREEMPT or ACPI would get rid of the
problem.  I forgot about the ACPI part being in the mix.

I still think the aironet driver acts strange, but I'm not really sure
how to describe it.  In 2.6.3-mm2 which I transfer a file top will show
60% of the cpu time in 'irq' whatever that means.  It seems vmstat still
calls this idle.  Perhaps I'm interpreting those numbers wrong.

When I enable preempt on the same system my network performance with the
aironet driver drops tremendously, probably by 75%, and the rest of the
systems seems to crawl.  Somehow without preempt this isn't a problem,
even though it shows 60% of the time in 'irq' the rest of the system
feels responsive.  What would be the proper way to track down this type
of problem?

I'm having more problems with 2.6.5-rc1-mm2 and the aironet card.  It
seems to preform poorly no matter if preempt is on or off.  I'm still
looking into that.  I don't think the drivers has changed significantly
between the two version so I'm hoping I just made a mistake.

Later,
Tom


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