Dual core CPU's, but only 2 CPU's in-use?

[Tigger]

On Mon, 25 Feb 2008 11:30:13 +0100
Kris Kennaway <kris at FreeBSD.org> wrote:

> Tigger wrote:
> > On Sun, 24 Feb 2008 15:59:49 +0100
> > Kris Kennaway <kris at FreeBSD.org> wrote:
> > 
> >> Tigger wrote:
> >>> On Sun, 24 Feb 2008 12:46:23 +0100
> >>> Dominic Fandrey <kamikaze at bsdforen.de> wrote:
> >>>
> >>>> Tigger wrote:
> >>>>> Hello. I have a Xeon system with 2 CPU's installed (dual-cores).
> >>>>> Under FreeBSD 6.2, a systat reported 4 CPUs in use and so did
> >>>>> dmesg. 
> >>>>>
> >>>>> Under FreeBSD 6.3, dmesg is reporting the 4 CPUs, but systat is
> >>>>> only reporting 2 CPUs (CPU 0 and CPU 2). 
> >>>>>
> >>>>> How do I enabled the second cores on the CPU's or are they
> >>>>> really running but systat is not reporting the CPU load across
> >>>>> the cores?
> >>>> You can run 'top -S' to check how many idle processes exist. If
> >>>> there are four, everything is fine.
> >>>>
> >>>
> >>> Sadly, no luck. Only cpu0 and cpu2 are reported:
> >>>
> >>>   PID USERNAME  THR PRI NICE   SIZE    RES STATE  C   TIME   WCPU
> >>> COMMAND 11 root        1 171   52     0K     8K CPU2   2 143:43
> >>> 92.38% idle: cpu2 13 root        1 171   52     0K     8K RUN    0
> >>> 142:33 92.04% idle: cpu0
> >> Are your CPUs really dual-core, or single core + hyperthreaded?
> >>
> >> Kris
> >>
> > 
> > Thank you! Your email sent me off to do what I should have done
> > first - some research. I assumed (wrongly) that 'Logical CPUs per
> > core: 2' meant 'Dual Core', and when I saw the following:
> > 
> > FreeBSD/SMP: Multiprocessor System Detected: 4 CPUs
> >  cpu0 (BSP): APIC ID:  0
> >  cpu1 (AP): APIC ID:  1
> >  cpu2 (AP): APIC ID:  6
> >  cpu3 (AP): APIC ID:  7
> > 
> > I also thought 'Dual Core', however this is not the case. The CPU's
> > are single core with Hyper Threading Tech.
> > 
> > But this research has also lead to some confusion. Its clear there
> > are 2 CPU with 2 Logical cores each, but only 2 cores are in use -
> > why?
> > 
> > Also, the 'man (4) smp' talks about (possible) performance issues
> > with Hyper Threading enabled - does this mean I should set
> > 'machdep.hlt_logical_cpus 1' (it is currently set to 0) - or
> > because the Logical Cores are not being used I can forget about
> > this?
> 
> hyperthreading is not enabled by default on 6.x.  See the security 
> advisory for discussion and how to enable it.
> 
> Kris
> 

The only security advisory I could find was for 5.x and contained info
on how to disable, not enable.

http://security.freebsd.org/advisories/FreeBSD-SA-05:09.htt.asc

Also, 'machdep.hlt_logical_cpus' is already set to 0 (I'm assuming the
'hlt' means 'halt' in the sysctl switch).

-Tig