On Tue, Mar 14, 2000 at 09:54:14PM +0100, Ingo Molnar wrote:
>
> On Tue, 14 Mar 2000 yodaiken@fsmlabs.com wrote:
>
> > > people will have the nicely preemptable UP kernel. The latency-quality of
> > > the SMP kernel will be much worse than the UP kernel's latency.
> >
> > I don't see why. Can you explain?
>
> yep: the SMP kernel will not be preemptible like the UP kernel.
>
> > Why ? The length of time between scheduler calls under SMP should be
> > smaller than the length of time between scheduler calls on UP - no?
>
> no, they would be bigger because with Linus' suggestion the UP kernel uses
> spinlocks to make the kernel fully preemptible - we will be able to
> reschedule the kernel in a preemptive way at any point that does not hold
> any spinlock (or lock). The SMP kernel would still reschedules in a
> 'cooperative' way, because spinlocks would not have this 'collect total
> current spinlock count' property (and thus preempting kernel space would
> not be allowed).
But the SMP system should do n-times as many user<->kernel transitions
and so should still run into preemption points at a high speed: even
if each processor schedules less often.
> IMPORTANT: to avoid any misunderstanding, this does not make the SMP
> kernel worse in any way than it is right now. But this would be a
> (dramatic wrt. latencies!) 'UP-only' improvement, which _feels_ wrong, to
> me at least.
If the latencies in question are process schedule latencies, then
improving runq operation is the obvious strategy.
> of course it has more footprint than 0 instructions [ == the current SMP
> kernel].
I thought that we had had this conversation in another setting and you
had argued that cache bloat was absolutely critical path.
-- --------------------------------------------------------- Victor Yodaiken FSMLabs: www.fsmlabs.com www.rtlinux.com FSMLabs is a servicemark and a service of VJY Associates L.L.C, New Mexico.- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu Please read the FAQ at http://www.tux.org/lkml/
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