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

[Richard W.M. Jones]

On Fri, Sep 18, 2020 at 02:03:00PM +0200, Pino Toscano wrote:
> On Friday, 18 September 2020 11:44:04 CEST Richard W.M. Jones wrote:
> >    let g = open_guestfs ~identifier:"v2v" () in
> >    g#set_memsize (g#get_memsize () * 14 / 5);
> > +  (* Setting the number of vCPUs allows parallel mkinitrd. *)
> > +  g#set_smp (Sysconf.nr_processors_online ());
> 
> IMHO this is not a good idea, for few reasons:
> 
> a) it unconditionally uses all the available CPUs with no way to change
> it to any other value

My second suggestion would be to use:

+  g#set_smp (min 8 (Sysconf.nr_processors_online ()));

(limit to be discussed, but here I arbitrarily chose 8).  I agree that
it's not a good idea to use all CPUs.  However:

> b) virt-v2v is run also on systems that are not specifically dedicated
> as conversion servers, for example the oVirt hosts (that run the oVirt
> VMs); in this context, suddently using more CPUs than requested may
> saturate the system resources

It's fairly unlikely this would happen.  All the libguestfs steps are
still serialized.  Only a few steps would use all CPUs, including
dracut/mkinitrd, and possibly some RPM commands.

> IMHO the right approach is to add a --smp N option like available in
> other libguestfs tools; this way, the user can decide how many
> resources virt-v2v can use.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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