[nSLUG] no networking

Peter Cordes peter at llama.nslug.ns.ca
Tue Jan 6 16:25:04 AST 2004

On Tue, Jan 06, 2004 at 03:06:57PM -0400, Dop Ganger wrote:
> On Tue, 6 Jan 2004, M Taylor wrote:
> > I am running the SMP kernel due it the system being a P4 with hyper
> > threading and my limited (mis)understanding is that later SMP in 2.4
> > kernels makes use of HT. Perhaps I'm wrong.

 Yup, turning on SMP in an x86 kernel includes support for hyperthreading,
and it's been in for probably over a year.  Unfortunately, the scheduler
doesn't know the difference between physical and logical CPUs, so it doesn't
realize that a nice=19 thread running on the same physical CPU as a nice=-19
thread will slow down the nice=-19 thread.  (HT doesn't support hardware
priorities.)  Some people are working on HT-aware scheduling:

 For anything other than a number-crunching machine, HT is probably a good

> You need to enable SMP for HT. I don't think it applies to you, but it
> seems like all the latest P4 chips have support for HT according to
> /proc/cpuinfo (eg, my laptop ("Intel(R) Pentium(R) 4 Mobile CPU 1.50GHz")
> has the ht flag set) but HT doesn't actually work. Not sure what Intel
> were thinking there.

 I've seen the same thing on my dad's P4M 1.7GHz laptop.  I think I actually
booted an SMP kernel with HT support, but there was only one logical CPU and
I think the kernel log said something about HT being disabled.  The ht flag
was set in the CPU features, though.  Maybe there's an HT interface that the
kernel can use to interrogate the hardware, but laptop CPUs support it but
only have one logical CPU?

#define X(x,y) x##y
Peter Cordes ;  e-mail: X(peter at cor , des.ca)

"The gods confound the man who first found out how to distinguish the hours!
 Confound him, too, who in this place set up a sundial, to cut and hack
 my day so wretchedly into small pieces!" -- Plautus, 200 BC

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