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SubjectRe: Processor affinity planned for 2.5?
Wasnt my idea ... I´ll forward this to the guy who asked ... On 128 CPU
machines affinity to processor groups may be a thing to consider but not
on 2 or 4 way SMPs ...

Anyway, I appreciate that he asked on linux-SMP and not here ... less
traffic in here ...


Jakob Østergaard wrote:
> On Wed, Oct 18, 2000 at 01:13:48PM +0200, Markus Pfeiffer wrote:
> > HI all
> >
> > I received and discussed Processor-affinity on the SMP-Kernel-list and I
> > only wanted to ask if there is something in preparation or if someone
> > (especially Linus :)) is against this ... I was asked this question
> > beacause someone wanted to do some Matrix-Operations on his SMP machine
> > and wnated to speed them up by trying affinity ...
> I don't know the exact answer to your question, but I can tell you that there
> has been patches floating around for a long time (somehwere) to provide
> processor affinity for processes in the Linux kernel.
> However, the patches has never been merged because:
> *) No matter how self-contained and simple and elegant they may be,
> they complicate the scheduler (maybe not much, but more than not)
> *) There is no performance gain what so ever, expcept for perhaps some
> very specially fabricated benchmark in some very specially fabricated
> test environment. In the best case, you will not get a slowdown from
> the explicit affinity.
> Think about it: It's the scheduler's job to distribute the threads of
> execution reasonably among the available processors. The scheduler knows the
> state of the system and *you* don't when you put in your static affinity
> decisions. You are bound to lose.
> Yes, I know that big-iron IRIX boxes have explicit processor affinity available
> to user-space. They also have gang-scheduling, which is more complicated and
> maybe more likely to actually give you a speedup than explicit affinity. IRIX
> has a lot of things, and let's not get into *that* one again ;)
> If you want to achieve anything in life, keep it simple. Be happy you have a
> scheduler that on any day can do a better job than you at scheduling the
> threads, and get on with your work :)
> Cheers,
> --
> ................................................................
> : : And I see the elder races, :
> :.........................: putrid forms of man :
> : Jakob Østergaard : See him rise and claim the earth, :
> : OZ9ABN : his downfall is at hand. :
> :.........................:............{Konkhra}...............:
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