Stability of SMP guests

Hello,

I'm just wondering what the current best practices are in regard to SMP guests. FYI, I'm running RedHat kvm-kmod/qemu-kvm which is ~85 I think. I have 8 physical cores (+ 8 HT) and I'd like to virtualize multiple SMP guests. The RHEL manual talks about single vCPU guests as being preferred and as such a 1:5 physical:virtual core ratio can be achieved with low/balanced loads. However I'm wondering how this will be affected by running SMP guests?

Ideally I plan on running 2 and 4 vCPU guests with fairly low loads in general, almost all will be running Linux/virtio/LVM. Does any have any experience with this type of usage? Is performance/stability severely affected as the number of guests rises? Can we apply any tweaks on the host to improve scheduling for SMP guest workloads?

Thanks