2016-04-22 21:07 GMT+08:00 Radim Krčmář <rkrcmar@xxxxxxxxxx>: > 2016-04-22 09:40+0800, Wanpeng Li: >> 2016-04-21 23:29 GMT+08:00 Radim Krčmář <rkrcmar@xxxxxxxxxx>: >>> x86 vcpu_id encodes APIC ID and APIC ID encodes CPU topology by >>> reserving blocks of bits for socket/core/thread, so if core or thread >>> count isn't a power of two, then the set of valid APIC IDs is sparse, >> >> ^^^^^^^^^^^^^^^^^^^ >> ^^^^^^^ >> Is this the root reason why recommand max vCPUs per vm is 160 and the >> KVM_MAX_VCPUS is 255 instead of due to perforamnce concern? > > No, the recommended amout of VCPUs is 160 because I didn't bump it after > PLE stopped killing big guests. :/ > > You can get full 255 VCPU guest with a proper configuration, e.g. > "-smp 255" or "-smp 255,cores=8" and the only problem is scalability, > but I don't know of anything that doesn't scale to that point. > > (Scaling up to 2^32 is harder, because you don't want O(N) search, nor > full allocation on smaller guests. Neither is a big problem now.) I see, thanks Radim. Regards, Wanpeng Li
