Re: [Qemu-devel] Enabling Hyperthreading for Guest

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2011-09-22 at 18:43 +0200, Erik Rull wrote:
> Alex Williamson wrote:
> > See the extended -smp options:
> >
> > -smp n[,maxcpus=cpus][,cores=cores][,threads=threads][,sockets=sockets]
> >                  set the number of CPUs to 'n' [default=1]
> >                  maxcpus= maximum number of total cpus, including
> >                  offline CPUs for hotplug, etc
> >                  cores= number of CPU cores on one socket
> >                  threads= number of threads on one CPU core
> >                  sockets= number of discrete sockets in the system
> >
> > Try something like:
> >
> > -smp 4,cores=2,threads=2,sockets=1
> >
> > Alex
> >
> 
> Great - the correct combination made it :-)
> 
> But the SMP-Performance-Benchmark is horrible :-(
> "Only" between 0.35 and 1.05 for the above combination.

I'm not sure what that means...

> I have the same architecture on the host (2 cores w/ ht enabled) so there 
> are enough real cores available for computation.
> 
> Any idea what could slow down the performance here?

Note that threads aren't real "full" cores, so you're likely going to
see some scheduling mismatches between physical threads and virtual
threads.  One thing that often helps smp guests is to pin vCPUs to
pCPUs.  You can get the vCPU thread IDs from 'info cpu' in the monitor
and pin each to a physical CPU with taskset.  If you're using libvirt,
virt-manager can configure it to do this too (as well as the cpu
topology).

Alex