Hyper-V and Multiprocessor VMs

re: Hyper-V and Multiprocessor VMs

Phil — Tue, 07 Dec 2010 22:52:11 GMT

If I may ask a supplementary question, if I have a single threaded CPU bound application (i.e. a scientific workload) and that is the only work I wish to run on that VM then, as I understand the previous answer, creating a VM with 4VPs will result in 4 logical processors being scheduled to the VM each and every time it is dispatched by Hyper-V. So even though my application can only use one VP at a time (dispatched by Windows across all 4VPs) most of the time (Windows overheads and interrupts for this type of workload are small), Hyper-V will still schedule 4VP.

Does this mean that approximately 3 logical processors are effectively wasted, because no other VM can get access to these LPs while the application is running on this VM and that I would be better off configuring the VM with 1 or 2 LPs?

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If you are running a single threaded application, then you are better off configuring the VM with 1 or 2 virtual processors (VPs). More information about configuring the VM in terms of VP count and monitoring performance can be found on http://www.microsoft.com/whdc/system/sysperf/Perf_tun_srv-R2.mspx.

The Tuning Guide covers things like how VPs are scheduled on the system's logical processors (LPs) and other topics you might find useful for your use case.

re: Hyper-V and Multiprocessor VMs

Phil — Wed, 01 Dec 2010 06:40:18 GMT

I was wondering what happens where I have a single threaded application that can only take advantage of a single processor but I choose to run it on a VM that supports up to 4 virtual processors (i.e. Windows 2008 R2). Will my VM be scheduled with all four virtual CPUs even though it only needs one to execute ?

I can't see anything on any of Microsoft's sites that explicitly clarifies this, however, V2.0 of the Hypervisor Functional Specification in section 17 "Parition Save and Restore" seems to imply that this is the case in 17.1.8 "... Create new virtual processors. The count and IDs of the virtual processors should match those of the partition that was previously saved.". That said, in other places individual processors can be relinquished when events such as spinlocks occur.

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The behavior in a virtualized OS is similar to running natively: If an application is single threaded, then it can only run on one processor at any given point in time with the possibility of migrating to other processors based on other activities in the VM. Other activities include interrupts, DPCs, processes, and system threads.

With that in mind, if you are creating a VM to primarily run a single threaded application, creating a VM with 2VPs makes sense because another VP can be available for other system work. If this is a VM that has been saved and started again as a result of Live Migration or something similar then the VM will come up with 4VPs and the application will only use one.

re: Hyper-V and Multiprocessor VMs

Christos Pournaras — Tue, 21 Sep 2010 14:29:33 GMT

If the guest OS is the Windows 2003 R2 ENT edition, up to how many CPUs can the HyperV "provide" to the vm ?

Hi Christos, Hyper-V supports up to 2 virtual processors for a Windows 2003 R2 ENT guest OS. For a complete listing of the supported number of virtual processors for other guest OS, please see http://technet.microsoft.com/en-us/library/cc794868(WS.10).aspx. Thanks.

re: Hyper-V and Multiprocessor VMs

John — Sun, 24 Jan 2010 12:50:01 GMT

I've noticed that the Hyper-V settings will allow you to assign 4 processors to a Windows 2003 Server virtual machine. I understand that only 2 are supported. We have tried 4 in a test enviroment and the virtual machine seems to work fine. What are the potential consequences of this configuration?

It's an unsupported scenario, but not explicitly blocked. Windows Server 03 will only utilize 2 processors in this case. There's potential for performance degradation using more processors.

Tom Basham, Virtualization Performance PM, Windows Fundamentals Team

re: Hyper-V and Multiprocessor VMs

Saeed Alsaeed — Mon, 07 Dec 2009 14:07:09 GMT

I ran a test to compare physical Windows 2008 with 4 processors vs. one VM running Windows 2008 on Hyper-V with 4 virtual processors.

I found the overhead to be a little bit more than 20%. I tried to optimize the Hyper-V according to Microsoft documentation available on TechNet. In TechNet, it is mentioned that Hyper-V CPU overhead is between 9-12%.

Does a multiprocessor VM suffers more overhead?

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Yes, adding multiple virtual processors will increase the overhead (the exact amount will vary based on both the underlying hardware and the workload being executed). The Performance Tuning Guidelines for Windows Server 2008 R2 recommends: VMs that have loads that are not CPU intensive should be configured to use one virtual processor. This is because of the additional overhead that is associated with multiple virtual processors, such as additional synchronization costs in the guest operating system. More CPU-intensive loads should be placed in 2-VP or 4-VP VMs if the VM requires more than one CPU of processing under peak load.

Tom Basham, Program Manager, WIndows Performance

re: Hyper-V and Multiprocessor VMs

Gata — Thu, 12 Nov 2009 11:12:34 GMT

My condition is the same as Naveed's and Jen's [see previous posts] -- (I have a dual quad core machine and I run windows 2008 server VM and configure 4 virtual processors to this VM. )

When you consider the contents of those posts, I understood that, if only one VM is running, its VM can not use full power of all physical CPU because, when only one VM is running, if affinity("virtual machine limit") is 100 then percentage of all CPU resource is 50%.

Is this true?

That’s correct. The system has a total of 8 logical processors. A single VM can have up to 4 virtual processors. Therefore, in this example, a single VM can utilize a maximum of 50% of that physical host’s CPUs.

Tom Basham – Program Manager, Windows Performance

re: Hyper-V and Multiprocessor VMs

Jens — Tue, 15 Sep 2009 12:40:48 GMT

In the settings for a VM, is it possibile to change the "Percent of total system resources" in the processor resource control? I have a VM that is running too slow, because there is a process running that dosn't support multicore properly, so I need more power on only one core (like in Virtual Server 2005).

Thanks for your question Jens.

You've managed to stumble across one of the major differences between Virtual Server and Hyper-V.

Virtual Server will only ever give one CPU to a virtual PC, so the maximum that you can allocate to any given virtual PC is 100 percent of one CPU..

With Hyper-V's support for multiprocessors it gets a little bit more complicated, as the amount of CPU resources that you are configuring will depend on the number of logical processors that are assigned to the virtual machine (VM).

Here's an example, based on VM configured with 1 virtual processor on a quad-core machine (so it has 4 logical processors).

I've assigned this VM just one virtual processor, so when I set the "Virtual machine reserve" to 100%, the "Percentage of total system resources" that this VM can use is 25% (because I'm dedicating 100% of one of my four logical processors to this VM)

On Virtual Server, this is the same as setting Maximum Capacity to 100% for the virtual PC, because as noted above the maximum that you can allocate to any given virtual PC is 100 percent of one CPU. For information about Virtual Server and configuring CPU resources can be found at TechNet here: http://technet.microsoft.com/en-us/library/cc720296(WS.10).aspx.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

John R — Tue, 11 Aug 2009 20:56:07 GMT

Is there an improvement in performance in setting the Virtual Machine Reserve for each of my VM's rather than leaving that value at the default of 0?

I have plenty of resources on the host but was wondering if performance is changed by specifically reserving processors for my VM's.

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Specifying a Virtual Machine Reserve will ensure that a machine gets the specified CPU allocation - this can be very useful if you want to make sure that a particular VM is not CPU starved due to other VMs workloads.

If you have plenty of CPU resource on the host to handle what you expect will be the peak load from the VMs (especially if you haven't overcommitted your logical processors), then setting a reserve is not likely change the performance for an individual VM.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

David Rails — Mon, 13 Jul 2009 13:40:13 GMT

Is there currently or will there be an option in Hyper V to use more than 4 virual processors per VM as we have an application which is cpu hungry we have 4x quad core processors but can only assign 4 logical processors per vm.

At present, only 4 VPs are supported per VM in Hyper-V.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

Naveed — Tue, 07 Jul 2009 10:30:54 GMT

I have a dual quad core machine and I run windows 2008 server VM and configure 4 virtual processors to this VM.

I have only one VM running on this hyper-v host, can this VM utilize the complete power of the dual quad core CPU?

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Naveed - if the host is a dual quad core machine, it has 8 logical processors. If you only have one VM on the host, configured for 4 virtual processors, then Hyper-V will use up to 4 of the logical processors to run the workload of the VM. In this configuration, It is unlikely that all the logical processors on the host will be busy all the time.

Tim Litton, Program Manager, Windows Server Performance.