[Libguestfs] [PATCH v2v] v2v: Set the number of vCPUs to same as host number of pCPUs.

[Daniel P. Berrangé]

On Fri, Sep 18, 2020 at 11:22:15AM +0100, Richard W.M. Jones wrote:
> On Fri, Sep 18, 2020 at 10:52:58AM +0100, Daniel P. Berrangé wrote:
> > On Fri, Sep 18, 2020 at 10:44:03AM +0100, Richard W.M. Jones wrote:
> > > So it didn't make any noticable difference in my test.  I wonder if
> > > the test guest I'm using (Fedora 32 using dracut) doesn't use parallel
> > > compression?
> > 
> > Do you do anything special to optimize storage ? If the thing using
> > parallel CPUs in the guest is doing I/O you'd likely want to tune
> > storage at same time. In current QEMU git master there's a change[1]
> > that sets  viort-blk/virtio-scsi  num_queues to match SMP count,
> > so the guest OS get fully parallel I/O from meach guest CPU.
> > 
> > [1] a4eef0711b2cf7a7476c3e2c202a414b68a1baa0
> 
> I'm not using a new enough qemu to have that and it doesn't appear in
> a released version, but I'll test again once we have a new enough
> version.

You could set  num_queues explicitly in v2v if desired, as the actual
config knob has been around a while, only the default is changing in
QEMU.

> > > However I don't think it can cause a problem and it seems obvious that
> > > it could benefit some cases.
> > 
> > What's the default RAM size ?  On hosts with very large numbers of CPUs
> > (100's) you might need to scale up RAM size too so you keep a sane
> > RAM-per-CPU ratio.  Or default to host CPUs, but cap it at say 32.
> 
> I'll cap the number of CPUs in this patch.
> 
> Thanks,
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
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Regards,
Daniel
-- 
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