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SubjectRe: [PATCH] Yielding processor resources during lock contention
On Sun, 12 Sep 2004, Nick Piggin wrote:

> > Well currently it just enables preempt and spins like a mad man until the
> > lock is free. The idea is to allow preempt to get some scheduling done
> > during the spin.. But! if you accept this patch today, you get the
> > i386 version which will allow your processor to halt until a write to the
> > lock occurs whilst allowing interrupts to also trigger the preempt
> > scheduling, much easier on the caches.
> >
> That's the idea though isn't it? If your locks are significantly more
> expensive than a context switch and associated cache trashing, use a
> semaphore, hypervisor or no.
> I presume the hypervisor switch much incur the same sorts of costs as
> a context switch?

In the PPC64 and P4/HT case the spinning on a lock is a bad utilisation of
the execution resources and that's what we're really trying to avoid, not
necessarily cache thrashing from a context switch.


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