From owner-freebsd-smp  Wed May  1 13:22:55 2002
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From: Andrew Gallatin <gallatin@cs.duke.edu>
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Date: Wed, 1 May 2002 16:22:14 -0400 (EDT)
To: John Baldwin <jhb@FreeBSD.org>
Cc: freebsd-smp@FreeBSD.org
Subject: RE: hlt when idle?
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John Baldwin writes:
 > 
 > On 01-May-2002 Andrew Gallatin wrote:
 > > 
 > > 
 > > Can somebody remind me why we do not hlt in the idle loop on MP x86s?
 > > Is this because a HLTed CPU is not going to notice a new runnable job
 > > (possibly migrating from another CPU) until it gets an interrupt to
 > > wake it up?
 > 
 > Yes.

This seems to be an acceptable "loss" in performance in environments
where cooling is a concern.  Is there a deadlock danger?  Or is it
just a performance tweak to not HLT SMPs?  Would you object to making
it a sysctl (machdep.smp_idle_hlt)?


 > > Do both CPUs get clock interrupts on x86?
 > 
 > No, the interrupts seem to be round-robin, but each clock intr is only
 > sent to one CPU unlike on alpha where they are broadcast.

So each CPU gets (1/num_cpu) * hz  clock interrupts/sec?

Drew

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