Re: [Qemu-devel] QEMU -smp paramater: Will multiplue threads and cores improve performance?

[Daniel P. Berrange]

On Wed, Dec 21, 2016 at 11:23:42AM +0000, Stefan Hajnoczi wrote:
> On Wed, Dec 21, 2016 at 04:44:08PM +0800, Bob Chen wrote:
> > -smp 16
> > 
> > -smp cores=4,threads=4,sockets=1
> > 
> > 
> > Which one has better performance? The scenario is guest VMs running on
> > cloud server.
> 
> It's more complicated than that.
> 
> The CPU topology (cores, threads, sockets) can be used by guest kernels
> to make scheduling decisions.

cores vs sockets will have little difference, but cores or sockets
vs threads will definitely affect guest schedular.

> If you pin vcpu threads to host cpus and configure the guest with
> an accurate CPU topology then you can expect the guest kernel to make
> the best decisions.
> 
> If you do not pin vcpu threads then you cannot expect any consistent
> performance advantage by specifying the CPU topology.

In fact if you do not pin threads and you specify a CPU topology that
has > 1 hyperthread, you may well make performance of the guest worse.

Essentially you never want to use threads>1, unless pinning guest CPUs
to host CPUs 1:1

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://entangle-photo.org       -o-    http://search.cpan.org/~danberr/ :|