Hyper-V and Multiprocessor VMs

re: Hyper-V and Multiprocessor VMs

Michael — Fri, 04 Apr 2008 20:05:45 GMT

Are there any advantages/disadvantages to configuring a Guest with more Virtual Processors then are physically present on the Host? (for example, a 4P Guest running on a 2P Host?)

John: Each virtual processor needs to be scheduled on a physical processor. This means that the number of concurrently running virtual processors cannot exceed the number of physical processors in Hyper-V. Most operating systems are designed to expect that all of their processors are running at the same time, so having more virtual processors than physical processors may result in poor performance.

re: Hyper-V and Multiprocessor VMs

Rekha — Thu, 08 May 2008 07:45:16 GMT

How many VMs does hyper-v support?

John: The number of active VMs that Hyper-V supports is only limited by the resources available on the host machine. For example, Hyper-V V1 supports upto 4 virtual processors per logical processor, so a host with 4 logical processors on it can support up to 16 virutal processors. These 16 virtual processors can be spread across the active VMs in a number of different configurations (such as 16 1VP VMs, or 8 2VP VMs, or 4 VP VMs, or some other mix).

You should plan the capacity of the virtualization server based on its CPU, RAM, and I/O resources and the expected and peak loads on the VMs.

re: Hyper-V and Multiprocessor VMs

James — Wed, 25 Jun 2008 14:42:03 GMT

What is the maximum number of virtual processors that the root partition can have?

or does the root partition Windows 2008 OS have acess to all available CPUs in the box (via the hypervisor layer)

Is this configurable ?

Thanks

Thanks for your question James.

The root partition always has access to the same number of virtual processors as logical processors present on the machine. Each root virtual processor is affinitized to each logical processor.

Currently Windows Server 2008 Hyper-V officially supports up to 24 logical processors, while Windows Server 2008 R2 Hyper-V supports up to 64 logicial processors.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

hwow — Thu, 03 Jul 2008 18:00:53 GMT

What about memory sharing / overcommitted memory in hyper-v.

How can I prevent from every VM to lock the MAX memory was assigned to it, while at the same time the VM does not using all the memory?

---

At the moment Hyper-V doesn't support memory sharing or overcommit.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

Andy — Thu, 18 Sep 2008 19:12:40 GMT

Are there any setting that can throttle CPU for the VMs so that overall host hyper-v cpu doesn't go to 100%?

---

Andy, yes it is possible to throttle the maximum CPU resources that an individual virtual machine gets.

Under the settings for a virtual machine, select the “Processor” category and within the “Resource Control” section it is possible to set the limit on the amount of virtual CPU that can be allocated to this machine, as a percentage of the CPU resources available to this virtual machine. This limit setting applies regardless of whether or not other virtual machines running.

If you want to ensure that the host CPU resources aren’t completely consumed, then you will need to ensure that sum of the limit settings of all the running virtual machines does not exceed 100%.

However remember that server virtualization is about consolidating workloads of underutilized machines onto a smaller number of better utilized machines, so over committing on the virtual CPU is something that Hyper-V was designed to do.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

Doug White — Thu, 02 Oct 2008 04:20:57 GMT

Any tweaks for going over 4 cores? I'd like to virtualize a Ubuntu-based app that requires as many cores and as much memory as possible.

At the moment Hyper-V only supports 4 virtual processors (VPs) per virtual machine.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

Vybez — Mon, 09 Mar 2009 13:28:37 GMT

I've heard that Microsoft permits only 4 virtual machines per physical server - is this true?

If not what's the maximum number of virtual machines permitted per physical machine

Thanks for your question Vybez.

The number of active virtual machines that Hyper-V can support is certainly higher than 4, and is dependent on the physical characteristics of the machine.

Currently Windows Server 2008 Hyper-V supports up to 24 logical processors, with upto 8 virtual processors per logical processor. This means that a machine with 24 logical processors can run up to 192 virtual processors (24 * 8 = 192).

How the 192 virtual processors (VP) are used is quite flexible, it could be 192 VMs - each with 1 VP, or 96 VMs - each with 2 VPs, or 48 VMs - each with 4 VPs, or some combination in between.

Tim Litton, Program Manager, Windows Server Performance.

re: Hyper-V and Multiprocessor VMs

Brady Houser — Thu, 16 Apr 2009 16:57:25 GMT

Is there a difference between Windows Server 2008 Enterprise and Standard with the amount of processors that the virtual machines can access?

Thanks for your question Brad.

When Windows Server 2008 is used as the guest operating system of a virtual machine, it can access up to 4 virtual processors. Standard Edition supports up to 4 processors, so both Enterprise and Standard editions will let the virtual machine use all 4 virtual processors.

If your machine has more than 4 physical processors, you need to run Enterprise Edition of Windows Server 2008 for the root operating system to use all the processors to run the virtual processors.

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?

---

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.

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

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.

---

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

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.