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Date: Thu, 10 Jan 2002 19:18:23 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: nigel@nrg.org
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>, Rob Landley <landley@trommello.org>,
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Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
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Nigel Gamble wrote:
> 
> On Thu, 10 Jan 2002, Alan Cox wrote:
> > The fun below 1mS comes from
> >
> >       1.      APM bios calls where the bios decides to take >1mS to have
> >               a chat with your batteries
> >       2.      Video cards pulling borderline legal PCI tricks to get
> >               better benchmarketing by stalling the entire bus
> 
> Don't forget the embedded space, where the hardware vendor can ensure
> that their hardware is well-behaved.  Even on a PC, it is possible for
> someone who cares about realtime to spec a reasonable system.
> 
> On good hardware, we can easily do much better than 1ms latency with a
> preemptible kernel and a spinlock cleanup.  I don't think the
> limitations of some PC hardware should limit our goals for Linux.
> 

On 700MHz x86 running Cerberus we can do 50 microseconds average
and 1300 microseconds worst-case today.  

Below 1000 uSec, the required changes get exponentially larger
and more complex.  I doubt that it's sane to try to go below
a millisecond on a desktop-class machine with desktop-class
workload, disk, memory and swap capacities.

On a more constrained system, which is what I expect you're
referring to, 250 microseconds should be achievable.  Whether
or not that is achieved via preemptability is pretty irrelevant.

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