implications of SMP kernel on UP

Robert Watson rwatson at
Fri Apr 2 14:35:38 PST 2004

On Thu, 1 Apr 2004, Thierry Herbelot wrote:

> Le Thursday 01 April 2004 09:10, Bjoern A. Zeeb a écrit :
> > Hi,
> >
> > what are the implications on running an SMP enabled kernel on a UP
> > machine ?
> >
> > I first thought of things like:
> > - performence (most likely not worth the discussion ?)
> I got an improvement with a factor of ten between an SMP and a UP kernel
> on a HTT-enabled P4/2,6GHz/800MHz FSB on network transfers (with gigabit
> Ethernet boards : SMP gives about 6MB/s for FTP transfer rate, and UP
> gives up to 75MB/s) 
> So : as long as the network stack is not fully locked (this is coming -
> perhaps for 5.3), a server should definitely run a UP kernel. 

I would instead phrase this as "A kernel-bound network server may benefit
from running a UP server".  For compute-bound tasks, running SMP has
pretty dramatic effects :-).  It's also worth pointing out that in many
existing configurations, even with Giant over the network stack, we
already see performance benefits running 5.x with SMP over 4.x with SMP.
BTW, look for network locking patches coming to the arch@ mailing list in
the next couple of days to try out (subject to limitations).

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert at      Senior Research Scientist, McAfee Research

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