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Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable
From: Robert Love <rml@tech9.net>
To: Daniel Phillips <phillips@bonn-fries.net>
Cc: Andrew Morton <akpm@zip.com.au>, Anton Blanchard <anton@samba.org>,
        Andrea Arcangeli <andrea@suse.de>,
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On Tue, 2002-01-08 at 16:57, Daniel Phillips wrote:

> > True (re spinlock weight in preemptible kernel) but how is that not
> > comparable to explicit scheduling points?  Worse, the preempt-kernel
> > typically does its preemption on a branch on return to interrupt
> > (similar to user space's preemption).  What better time to check and
> > reschedule if needed?
> 
> The per-spinlock cost I was refering to is the cost of the inc/dec per 
> spinlock.  I guess this cost is small enough as to be hard to measure, but
> I have not tried so I don't know.  Curiously, none of the people I've heard
> making pronouncements on the overhead of your preempt patch seem to have 
> measured it either.

;-)
 
If they did I suspect it would be minimal.  Andrew's point on complexity
and overhead in this manner is exact -- such thinks are just not an
issue.

I see two valid arguments against kernel preemption, and I'll be the
first to admit them:

- we introduce new problems with kernel programming.  specifically, the
issue with implicitly locked per-CPU data.  honestly, this isn't a huge
deal.  I've been working on preempt-kernel for awhile now and the
problems we have found and fixed are minimal.  admittedly, however,
especially wrt the future, preempt-kernel may introduce new concerns.  I
say let's rise to meet them.

- we don't do enough for the worst-case latency.  this is where future
work is useful and where preempt-kernel provides the framework for a
better kernel.

I want a better kernel.  Hell, I want the best kernel.  In my opinion,
one factor of that is having a preemptible kernel.

	Robert Love

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