3056_hyper-v2-large-msg_thumb_0D8890ECIn my webcast on Hyper-V, this past Wednesday, I had a question about Hyper-V and how Hyper-threading effects that.

The current version of Hyper-V in Server 2008R2 we recommend an 8 to 1 consolidation ratio for virtual processors to logical processors and a maximum of 384 virtual machines on a single host (whichever is lower).  Intel, SMT (aka Intel Hyper Threading) is available on many current CPUs, so how does this effect performance in a VM and how is VM consolidation ratios affected? 

The Hyper-V team has reviewed performance data showing that enabling SMT may have a positive impact on overall performance when running under Hyper-V.

For the purposes of capacity planning with Hyper-V, you should use the number of physical processor cores on the machine, rather than the number of execution threads. The Windows Task Manager (and many other tools) show the number of execution threads, so care should be taken when determining the number of processor cores in a system.  For example, if I have a quad core CPU with hyper threading the taskmgr will show 8 CPUs. So for planning use the actual number of cores and then you can increase consolidation ratios after testing.

While the addition of SMT threads may enhance overall performance under Hyper-V, the additional performance is not a constant factor. The additional performance is heavily dependent on the target workload, overall machine utilization, and other factors.

Generally speaking, you can utilize the additional performance of SMT threads in two ways:

For computationally intensive workloads, you may benefit from increasing the number of Virtual Processors (VPs) assigned to a VM. For example, a 1 VP VM may be able to do more work if you assign it two VPs. Alternatively, or in combination with the above, if keeping the VPs per VM constant, you may be able to increase your consolidation ratio.

For more information on Hyper-V limits see this TechNet article:
http://technet.microsoft.com/en-us/library/ee405267(v=WS.10).aspx