On Mon, 1 Jul 2024, Daniel P. Berrangé wrote: > On Mon, Jul 01, 2024 at 02:48:07PM +0200, Michal Prívozník wrote: > > On 6/30/24 20:49, Mikulas Patocka wrote: > > > > > > > > > On Sun, 30 Jun 2024, Tejun Heo wrote: > > >> > > >> Do you happen to know why libvirt is doing that? There are many other > > >> implications to configuring the system that way and I don't think we want to > > >> design kernel behaviors to suit topology information fed to VMs which can be > > >> arbitrary. > > > > Firstly, libvirt's not doing anything. It very specifically avoids doing > > policy decisions. If something configures vCPUs so that they are in > > separate sockets, then we should look at that something. Alternatively, > > if "default" configuration does not work for your workflow well, > > document recommended configuration. > > Actually in this particular case, it is strictly speaking libvirt. > If the guest XML config does not mention any <topology> info, then > libvirt explicitly tells QEMU to set sockets=N,cores=1,threads=1. > That matches QEMU's own historical built-in default topology. > > None the less, my advice for mgmt applications using libvirt would > likely be to explicitly request sockets=1,cores=N,threads=1. This > is because it gives slightly better compatibility with unpleasant > software that applies licensing / subscription rules that penalize > use of many sockets, while being happy with any number of cores. > > > Either way though, the topology is a lie when the guest CPUs > are not pinned to host CPUs, so making performance decisions based > on this is unlikely to yield the desired results. Historically the > cores vs sockets distinction hasn't seemed to make much difference > to guest OS performance, as the OS' haven't made significant > decisions on this axis. Exposing threads != 1 though has always been > a big no though, unless strictly pinning 1:1 guest:host CPUs, as that > has had notable impacts on scheduling decisions. > > With regards, > Daniel I think there should be some way how to tell the guest kernel "the vCPUs are free-floating, the topology is a lie", so that it can stop making incorrect decisions based on the fake topology. Mikulas
