On 14.07.21 15:21, Vitaly Kuznetsov wrote:
Juergen Gross <jgross@xxxxxxxx> writes:On 14.07.21 13:45, Vitaly Kuznetsov wrote:Personally, I'd vote for introducing a 'ratio' parameter then so generally users will only have to set 'kvm.max_vcpus'.Okay. Default '4' then? Or '2 ^ (topology_levels - 2)' (assuming a topology_level of 3 on Intel: thread/core/socket and 4 on EPYC: thread/core/package/socket).I'd suggest we default to '4' for both Intel and AMD as we haven't given up completely on cross-vendor VMs (running AMD VMs on Intel CPUs and vice versa). It would be great to leave a comment where the number comes from of course.
Thinking more about it I believe it would be better to make the parameter something like "additional vcpu-id bits" with a default of topology_levels - 2 (cross-vendor VMs are so special that I think the need to specify another value explicitly in this case is acceptable). Reasons are: - the ability to specify factor values not being a power of 2 is weird - just specifying the additional number of bits would lead to compatible behavior (e.g. a max vcpu-id of 1023 with max_vcpus being 288 and the default value of 1) - the max vcpu-id should (normally) be 2^n - 1 Juergen
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