Re: device apic on a single processor machine

On Wednesday 27 October 2004 02:47 pm, Mike Tancsa wrote:
> At 02:00 PM 27/10/2004, John Baldwin wrote:
> >On Friday 22 October 2004 11:40 am, Mike Tancsa wrote:
> > > When moving from RELENG_4 to RELENG_5, I noticed that in GENERIC, the
> > > options
> > >
> > > options         SMP             # Symmetric MultiProcessor Kernel
> > > device          apic            # I/O APIC
> > >
> > > are enabled by default.  Going forward, is this the best thing to leave
> > > in my  default kernel on a uniprocessor machine ? I am not using the
> > > ULE scheduler either and have hyperthreading disabled in the BIOS.
> > >
> > > I did a search on google, and in 2003 it was said not to having either
> > > on a single processor machine but its not clear if this is no longer
> > > the case.
> >
> >You do want to drop SMP.  As far as 'apic', that is less clear.  If you
> > have lots of PCI devices that share interrupts for the !apic case and you
> > do lots of interrupt intensive tasks, then 'device apic' might help. 
> > There may also be cases where it hurts.  There have been reports that
> > access to the apic registers for things like masking sources takes longer
> > than on the 8259As.
>
> Thanks for the feedback. I guess my question is, what constitutes "lots" ?
> Typically, I strip down boxes to their bare min hardware wise so in most
> cases, I dont have anything sharing interrupts (I usually turn off USB
> which is the most gratuitous).  But I do have a POS app that needs USB as
> well as 2  PCI serial cards.  In this case, I do have a lot of shared
> interrupts.  However, it almost never is CPU bound or has an interrupt rate
> higher than 10-20%.  In this case, stability is more important to me.  I
> have run into a number of cases where there are interrupt storms (e.g
> http://lists.freebsd.org/pipermail/freebsd-current/2004-September/036967.ht
>ml)
>
> ... So if it provides a cleaner / more stable way to talk to the devices, I
> will certainly run with it.

I would try it both ways and see if one works better than the other.

-- 
John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org