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

(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" :-)