On Thu, Jun 16, 2022, David Matlack wrote: > On Tue, Jun 14, 2022 at 1:28 AM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote: > > > > David Matlack <dmatlack@xxxxxxxxxx> writes: > > > > > On Mon, Jun 13, 2022 at 11:35 AM Kyle Meyer <kyle.meyer@xxxxxxx> wrote: > > >> > > >> Increase KVM_MAX_VCPUS to 2048 so we can run larger virtual machines. > > > > > > Does the host machine have 2048 CPUs (or more) as well in your usecase? > > > > > > I'm wondering if it makes sense to start configuring KVM_MAX_VCPUS > > > based on NR_CPUS. That way KVM can scale up on large machines without > > > using more memory on small machines. > > > > > > e.g. > > > > > > /* Provide backwards compatibility. */ > > > #if NR_CPUS < 1024 > > > #define KVM_MAX_VCPUS 1024 > > > #else > > > #define KVM_MAX_VCPUS NR_CPUS > > > #endif > > > > > > The only downside I can see for this approach is if you are trying to > > > kick the tires a new large VM on a smaller host because the new "large > > > host" hardware hasn't landed yet. > > Heh. My point here doesn't make sense. The actual number of CPUs in > the host machine wouldn't matter, just the host kernel's NR_CPUS. > > > > > FWIW, while I don't think there's anything wrong with such approach, it > > won't help much distro kernels which are not recompiled to meet the > > needs of a particular host. > > But is there a use-case for running a VM with more vCPUs than the > kernel's NR_CPUS? With custom configs yes, e.g. my host kernel config for my anemic old system has CONFIG_NR_CPUS=12 and I occasionally run VMs with >12 vCPUs for testing purposes. But that obviously doesn't apply if KVM_MAX_VCPUS has a lower bound of 1024, so I agree that making KVM_MAX_VCPUS=max(1024, NR_CPUS) should do just fine.
