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SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
On January 8, 2002 10:17 pm, Robert Love wrote:
> 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?

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.

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