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

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

[Bob Chen]

Hi,

-smp 16

-smp cores=4,threads=4,sockets=1


Which one has better performance? The scenario is guest VMs running on
cloud server.


-Bob

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

[Stefan Hajnoczi]

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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.

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.

Stefan

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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
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