[Libguestfs] [p2v PATCH 2/6] restrict vCPU topology to (a) fully populated physical, or (b) 1 * N * 1

[Richard W.M. Jones]

(Adding Dan for input)

On Thu, Sep 08, 2022 at 03:23:41PM +0200, Laszlo Ersek wrote:
> On 09/08/22 10:03, Richard W.M. Jones wrote:
> > On Mon, Sep 05, 2022 at 01:25:27PM +0200, Laszlo Ersek wrote:
> >> +  "p2v.vcpu.dense_topo" => manual_entry->new(
> >> +    shortopt => "", # ignored for booleans
> >> +    description => "
> >> +Copy the physical machine's CPU topology, densely populated, to the
> >> +guest.  Disabled by default.  If disabled, the C<p2v.vcpu.cores> setting
> >> +takes effect.",
> > 
> > 
> > I just realised I'm not completely sure what "densely populated"
> > means here.  I think we should have a bit more explanation.
> > 
> > How about something like:
> > 
> >   "p2v.vcpu.dense_topo" => manual_entry->new(
> >     shortopt => "", # ignored for booleans
> >     description => "
> > Copy the physical machine's complete CPU topology (sockets, cores and
> > threads) to the guest.  Disabled by default.  If disabled, the
> > C<p2v.vcpu.cores> setting takes effect.",
> > 
> > (Which might also imply that we rename this something like
> > "complete_topo" or "full_topo" but I'll leave that to you.)
> 
> By "dense", I meant to express that there are no gaps in the onlining of
> the CPU topology.
> 
> Assume we have 2 sockets, 2 cores/socket, 2 theads/core. Assume CPU#1
> (in socket#1) is hot-pluggable, but isn't currently plugged, only CPU#0
> (in socket#0) is present -- making for 1*2*2 = 4 logical processors in
> total. A physical machine may well boot like this. Then our topology is
> 2*2*2, but we only have 4 logical processors, so the topology is not
> densely populated. The language is supposed to express that in any such
> case, we'll ignore the online / plugged / etc count, and we'll just grab
> the static topology, and fully / densely populate it with logical
> processors.
> 
> "Complete topology" does not express this. Sticking with the above
> example, the topology is already complete on the physical machine (we
> have full information about the levels of the hierarchy), but it's not
> densely populated.
> 
> Another example would be 1 * 4 * 2 physical (a normal low-end machine by
> today's standards), where the sysadmin disables (say) cores #1 and #2
> using /sys/devices/system/cpu/cpu{1,2}/online. (I think this may even be
> possible on the kernel command line, for whatever reason necessary.) In
> this case, during conversion, if "dense_topo" is set, we carry over not
> just the topology (= the 1 * 4 * 2 hierarchy), but we also densely
> populate it (producing 8 logical processors in the conversion output,
> disregarding the "gaps" on the source; i.e. that only 4 logical
> processors were available on the physical machine originally.)
> 
> I considered "complete", and thought it didn't express my intent. "Full"
> is so-so -- my problem is it seems to have two meanings; one is in fact
> what I'm trying to say with "dense", but the other meaning is just
> "complete", which I don't find good.
> 
> The choices p2v should offer are:
> 
> - Just carry over a flat VCPU count N --> this maps to a 1 socket * N
> cores/socket * 1 thread / core topology, fully populated.
> 
> - Otherwise (i.e., when the dense_topo knob is enabled), convert the
> original topology (S sockets * C/S cores/socket * T threads/core), *AND*
> fully populate that topology (disregarding the original "online count"
> on the physical machine, which may easily be less than the (S * C * T)
> product.)

I think the "mot juste" has to express that we're trying to model as
closely as possible the real physical topology.  (The denseness
doesn't seem to be so important - are there many machines where CPUs
are not online?  Can that even happen when virt-p2v is running?)

How about:

authentic_topo
physical_topo
accurate_topo

...?

The patch is totally fine, we're just quibbling about the
word "dense" :-)

Rich.

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