On Thu, Sep 08, 2022 at 02:36:15PM +0100, Richard W.M. Jones wrote:
> (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" :-)
Why not 'host_topo' since it is mirroring the host ?
With regards,
Daniel
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