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From: Daniel Phillips <phillips@bonn-fries.net>
To: Roger Larsson <roger.larsson@skelleftea.mail.telia.com>,
        Luigi Genoni <kernel@Expansa.sns.it>
Subject: Re: Preemtive kernel (Was: Re: [2.4.17/18pre] VM and swap - it's really unusable)
Date: Wed, 9 Jan 2002 08:48:41 +0100
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Cc: Andrea Arcangeli <andrea@suse.de>, Anton Blanchard <anton@samba.org>,
        Dieter N?tzel <Dieter.Nuetzel@hamburg.de>,
        Marcelo Tosatti <marcelo@conectiva.com.br>,
        Rik van Riel <riel@conectiva.com.br>,
        Linux Kernel List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@zip.com.au>, Robert Love <rml@tech9.net>
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On January 9, 2002 08:25 am, Roger Larsson wrote:
> (the subject has been wrong for some time now...)
> 
> On Wednesday den 9 January 2002 07.26, Daniel Phillips wrote:
> > On January 9, 2002 12:02 am, Luigi Genoni wrote:
> > > On Tue, 8 Jan 2002, Daniel Phillips wrote:
> > > > On January 8, 2002 04:29 pm, Andrea Arcangeli wrote:
> > > > > but I just wanted to make clear that the
> > > > > idea that is floating around that preemptive kernel is all goodness
> > > > > is very far from reality, you get very low mean latency but at a
> > > > > price.
> > > >
> > > > A price lots of people are willing to pay
> > >
> > > Probably sometimes they are not making a good business.
> >
> > Perhaps.  But they are happy customers and their music sounds better.
> >
> > Note: the dominating cost of -preempt is not Robert's patch, but the fact
> > that you need to have CONFIG_SMP enabled, even for uniprocessor, turning
> > all those stub macros into real spinlocks.  For a dual processor you have
> > to have this anyway and it just isn't an issue.
> 
> Well you don't - the first versions used the SMP spinlocks macros but
> replaced them with own code. (basically an INC on entry and a DEC and test
> when leaving)
> 
> Think about what happens on a UP
> There are two cases
>  - the processor is in the critical section, it can not be preempted = no
>    other process can take the CPU away from it.
>  - the processor is not in a critical section, no process can be executing
>    inside it = can never be busy.
> => no real spinlocks needed on a UP

Right, thanks, it was immediately obvious when you pointed out that the 
macros are just used to find the bounds of the critical regions.  So the cost 
of -preempt is somewhat less than I had imagined.

--
Daniel
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