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Hyper-V and Processor utilisation

Posted on 2013-01-17
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Last Modified: 2014-11-12
Hi guys,

Maybe a simple question but...
Got a client who has 2 x Xeon processors. They have Hyper-V on server 2008 R2 and 3 virtual guests (1 x SBS 2011, 2 x Server 2008 R2).

Windows detects 2 x CPU. When I check the properties of The VM, is it right to only see 4 logical processors? If this is correct, does that just mean that it will use 4 of the 8 cores that the server has installed? Or have I got that wrong.

Any help would be great - thanks.
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Question by:Talds_Alouds
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Lee W, MVP earned 500 total points
ID: 38790498
Could help if you posted screen shots so I know exactly what you're referring to, but in general, the HOST OS (Hyper-V Server 2008 R2 or Windows Server 2008 R2 with Hyper-V role will use ALL CPUs*.  When you assign CPUs to the VMs, it does not distinguish between CORES and Hyperthreading virtual CPUs - a CPU is a CPU to it.  So if you assign 4 CPUs to a VM on a server that recognizes 2 Quad Core processors with HyperThreading enabled (which means 16 total CPUs if you look at Task Manager on the host), then the VM will only use 4 CPUs WORTH of resources.  It will be able to execute UP TO 4 threads at once.

In some respects you can consider each CPU to be a thread that can be executed.  A dual Quad Core with Hyper-Threading supports up to 16 simultaneous threads.  Hyper-V in 2008 R2 only supports 4 virtual CPUs per VM.  This means with 3 VMs, if all 3 have been allocated 4 virtual CPUs, then 4 CPUs are left potentially "unassigned" and are otherwise free to the host server.

NOTE: CPU "assignment" is not actually "assigning" the CPU.  It's more like granting the VM the ability to run a thread. It's NOT EXCLUSIVE to that VM. When you look at most systems, the CPUs are VERY idle most of the time.  Hence a single Quad core CPU could be assigned to 10 VMs with each having 4 CPUs alloted to them and still perform just fine.
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by:Talds_Alouds
ID: 38791830
Perfect!

Thanks mate. Sorry I meant to refer to the VM's instead of VM. So essentially, I can set all 3 VMs to 4 logical processors and I'll still have some logical CPUs (or cores) left over.

Thanks
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