Subject: Re: scheduler woes on MPACPI kernel
To: Johnny Billquist <bqt@Update.UU.SE>
From: Manuel Bouyer <>
List: current-users
Date: 01/18/2005 22:59:03
On Tue, Jan 18, 2005 at 10:53:26PM +0100, Johnny Billquist wrote:
> On Tue, 18 Jan 2005, Manuel Bouyer wrote:
> >On Tue, Jan 18, 2005 at 05:09:48PM +0200, Teemu Rinta-aho wrote:
> >>Does anyone know if there has been any analysis on the
> >>effect of P4 hyperthreading and MP support of NetBSD,
> >>compared to a UP kernel?
> >
> >I tested with a make -j4 kernel build. A few % improvements with a MP 
> >kernel.
> >But this is because we have a big kernel lock I think. On linux 2.6, which
> >has finer-grained locking, a MP kernel increased the build time instead of
> >decreasing it.
> Huh? I didn't understand this...
> Are you claiming that a finer grained lock will decrease performance? Or 
> is there a serious bug in the Linux kernel?

It decrease performances for this type of workload, on hyperthreaded
single-CPU systems. This isn't surprising: finer grained lock cost more
CPU time, without giving much more concurency as a hyperthreaded CPU
can't do that much things in parallel.
On real multi-CPU systems things are probably different.

Manuel Bouyer <>
     NetBSD: 26 ans d'experience feront toujours la difference