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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 15:59, Daniel Phillips wrote:

> And while I'm enumerating differences, the preemptable kernel (in this 
> incarnation) has a slight per-spinlock cost, while the non-preemptable kernel 
> has the fixed cost of checking for rescheduling, at intervals throughout all 
> 'interesting' kernel code, essentially all long-running loops.  But by clever 
> coding it's possible to finesse away almost all the overhead of those loop 
> checks, so in the end, the non-preemptible low-latency patch has a slight 
> efficiency advantage here, with emphasis on 'slight'.

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?

	Robert Love

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