The Hyper-V Processor Virtual Machine Limit

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Introduction

Hyper-V offers 3 ways of managing or tweaking the CPU scheduler to provide the best possible configuration for certain scenarios and use cases. The defaults normally work fine but of certain conditions you might want to tweak them for the best possible outcome.  The CPU resource controls at your disposal are:

  • Virtual machine reserve  – Think of this as the minimum CPU “QoS”
  • Virtual machine limit – Think of this as the maximum CPU “QoS”
  • Relative Weight – Think of this as the scale defining what VM is more important.

Note that you should understand what these setting are and can do. Threat them like spices. Select the ones you need and don’t overdo it. They’re there to help you, if needed you can leverage all three. But it’s highly unlikely you’ll need to do so. Using one or two will server you best if and when you need them.

In this blog post we’ll look at the virtual machine limit.

Virtual Machine Limit

The virtual machine limit is a setting on the vCPU configuration of a virtual machine that you can set to limit the % of CPU resources a virtual machine can grab from the host. This setting limits the vCPU, preventing it to use more than the defined maximum percentage. The default percentage is 100.

Let’s look at some examples below based on a simple 4 core host with a VM.

On a 1 vCPU virtual machine with a virtual machine limit of 100% this means it can grab the equivalent of compute time slices of maximum 1 CPU on the host. Which is 25% of the total system CPU resources

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So in case of a home lab PC that has 4 cores you can see that setting the virtual machine limit to 100% means it’s limited to 100% of the total system CPU resources.

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When I set the number of vCPUs to 2 this drops to 50% of the total system CPU resources.

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Now by playing with the virtual machine limit you can configure the desired maximum of the total CPU system resources a VM can grab. A 2 vCPU VM with 60% as a virtual machine limit get 30% of the total system CPU resources at the most on 4 CPU host.

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Use cases

When you’re worried about run away virtual machines due to either misbehaving applications or developers who tend to run CPU stress tools in the guest and you really want to cap them to a certain limit without reducing the number of vCPUs (which they might need for testing parallelism, NUMA awareness) this might be a tool to use.

Limitations

This setting is always enforced. Even if a host has 32 cores and is only using 40% of them, a virtual machine could consume more CPU cycles without causing issues it won’t be able to. It’s a hard cap that’s always enforced.

The setting is enforced per vCPU. This means that when a single threaded app in a 4 vCPU virtual machine consumes the virtual machine limit it is capped even when the 3 other vCPU are totally idle.  So it’s not 30% of all vCPUs is 30% per vCPU maximum.

Conclusion

It’s a tool you have at your disposal but it’s probably the least used one. It has limited uses case due to it’s limitations. For most scenarios you’re better of leveraging virtual machine reserve or the relative weight. These are more flexible and are only enforced when needed, providing a smarter use of resource.

Discrete Device Assignment with Storage Controllers

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Discrete Device Assignment with Storage Controllers is the second type of DDA we’ll look at. I have written before on Discrete Device Assignment in Windows 2016. A the time of writing officially the 2 supported use cases are GPU and NVMe disk pass-through. I have demonstrated the configuration of DDA with a NVIDIA GRID K1 GPU here.

Meanwhile I have also successfully configured DDA with a NVMe disk. I’ll demonstrate how to do this later but in this blog post I want to address a consequence of this experiment. So let’s take a preliminary at Discrete Device Assignment with storage controllers

With an Intel NMVe disk you do not assign the actual disk to the virtual machine. It’s the controller. By disabling and dismounting the standard NVM Express Controller from the host and assigning it to the guest you make the NVME disk available in the guest.

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This is supported. This makes me wonder if MSFT would consider officially supporting other storage controllers. What if you need 8TB of high performance storage dedicated to a single VM? You could assign an extra controller in the host with a RAID 10 SSD virtual disk to the virtual machine. How different would that be from NVMe? No too much I guess. Smile

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By the way this assigning & un-assigning keeps data intact. This means that is also a roundabout sort of way to get data in and out of a virtual machine. On of the funky, crazy ideas I have already is to use this to export & import data. Maybe.

I really do wonder how things will evolve here. Perhaps these are too much “niche” use case scenarios but it’s interesting none the less. But perhaps the advances in NVMe Fabrics and the added performance available via VHDX outpace the need for DDA storage solutions.

Anuway enough musing as we’ll be taking a more hands on look at assigning a NVMe disk to a VM and Discrete Device Assignment with storage controllers via PowerShell in a later blog and video.

The Hyper-V Processor Virtual Machine Reserve

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Introduction

Hyper-V offers 3 ways of managing or tweaking the CPU scheduler to provide the best possible configuration for certain scenarios and use cases. The defaults normally work fine but of certain conditions you might want to tweak them for the best possible outcome.  The CPU resource controls at your disposal are:

  • Virtual machine reserve  – Think of this as the minimum CPU “QoS”
  • Virtual machine limit – Think of this as the maximum CPU “QoS”
  • Relative Weight – Think of this as the scale defining what VM is more important.

Note that you should understand what these setting are and can do. Threat them like spices. Select the ones you need and don’t overdo it. They’re there to help you, if needed you can leverage all three. But it’s highly unlikely you’ll need to do so. Using one or two will server you best if and when you need them.

In this blog post we’ll look at the virtual machine reserve.

Virtual Machine Reserve

The virtual machine reserve is a minimum CPU guarantee. This is configured at the virtual machine level for all vCPUs of that virtual machine. This means a couple of things. The reserve is guaranteed for the virtual machine that has it configured. This means that when there is contention for CPU resources the reserve is enforced. When there is no reserve configured are left to compete for the available resources, they don’t have any “SLA” minimum guaranteed CPU cycles however

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The virtual machine reserve is configured as the minimal % of CPU usage we’ll guarantee a virtual machine to get when there is contention. 0% reserve means that the virtual machine reserve is disabled. This is the default.

So how is this reserve accomplished? When a virtual machine is started there is a check if the virtual machine reserve can be met. If not, the virtual machine will not be started. This is also the case when you want to quick or live migrate a virtual machine. You cannot migrate a virtual machines to a host that cannot deliver the reserve.

The reserve is guaranteed. When you have started enough virtual machines with a reserve to consume 100% of the host CPU resources you cannot start another virtual machines that has a reserve configured. This is even the case if all the running virtual machines who together have reserved 100% of the host CPU resources are completely idle. That’s the cost of reserving CPU resources.

Also note you can mix virtual machines with and without a reserve configured. That’s perfectly fine.

Below is an extreme example. This single virtual machines has 56 vCPU configured with a 100% reserve. This means that this virtual machines will only start on a host that has no other virtual machines with a reserve running. Virtual machines without a reserve will run but when this virtual machines requires the minimum to be met they’ll lose their CPU cycles.

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So how does this work

When the virtual machines are running and contention for CPU resources materializes on the host than the reserve will kick in and guarantee the virtual machines get there guaranteed minimum. If there is no contention this isn’t enforced and all virtual machines can grab CPU resources as long as they are available. Only when a reserve can’t be met due to lack of resources the CPU scheduler intervenes. This means there is no waste by not allowing other virtual machines to use what is not needed at any given moment by others. Yes virtual machines can use more than their reserve if it’s available and other virtual machines don’t have a need for their reserved CPU cycles. But the moment there is contention, the reserve is guaranteed. That’s why you cannot reserve more that 100% in total. This however makes things a bit more complex in real life. A reserve is not enforced when not needed but it does have to be there. This means that if you reserve a 30% for a virtual machine with 56vCPU you can’t even reserve 80% for a virtual machine with 8 vCPU and run those together.

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Even when you have 56 cores left with 70% CPU unreserved you cannot find one single CPU with 80% free, let alone 8 of them. This means you cannot start such a virtual machine when the first one is running and vice versa. So that’s another caveat you need to consider.

Use Cases

The primary use case of virtual machine reserve is to enforce an SLA for a virtual machines compute needs. When a DBA demands 100% of the 8 vCPUs his SQL Server virtual machine has configured you can set the reserve to 100%, which is 14*% of all CPU resources on the host. On our 56 core host that means at least 8 CPU have to have 100% of resources free or that SQL Server virtual machine won’t even start as the reserve can’t be guaranteed. imageWhen it’s running and there is no contention for resources other virtual machines can grab whatever they need. The CPU scheduler will only take resources away from the other virtual machines if it’s needed to guarantee the reserve configured for the SQL Server virtual machine.

Some people get creative and use the virtual machine reserve to allocate 100% of all host CPU resources to all virtual machines running on a host. This means all resources are taken and they’ll need to rebalance the reservation if you want to add more virtual machines with a certain reserve. This only works if you can enforce all the virtual machines are created with a reserve. If not some of the new virtual machines might not get any CPU cycles when here is a lot of contention.

Limitations

It can become high maintenance. When people set up CPU reserves for all virtual machines (let’s say 14 of them each with 2 vPCU)  on a host with 2*14 cores. That means we can give each virtual machine a reserve of 10%. They all get 2*10% of 28 cores. 

When you now add a virtual machine with a reserve to the host you’ll need to reset the reserves to accommodate 11 virtual machines. As said this can be used to make sure no VM get’s deployed and started without you noticing (some one will complain when a VM doesn’t start) but this is tedious to manage.

Also note that this game of percentages plays per host but those can be part of a cluster. So when you do this on all nodes on a cluster things will go wrong if you have reserved 100% of the CPU resources on all nodes, live migration and failover will run into issues as extra virtual machines can’t start on nodes that haven’t got sufficient CPU resources left. You need to account and plan for for this

Conclusion

Virtual machine reserve is a powerful tool to have when needed. You do need to understand its behavior very well and take that into behavior for startup, quick and live migrations.

If you do that and use it wisely it can help you achieve SLA’s and the best possible guaranteed performance when needed.

Live Export a Running Virtual Machine or a Checkpoint

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A remarkably little known feature in Windows Sever 2012 R2 (and Windows 8.1)  is the ability to export one or multiple running virtual machines.

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You just select right click in the Hyper-V manager and select Export from the context menu and follow the wizard to select an export location. Easy. This is also possible via PowerShell so you can automate this. The result is a VM you can import which gives you a copy of the original virtual machine in a saved state, at the point in time that you exported it.

More people seem to know about the capability to export a checkpoint of a running virtual machine, not so many of the capability to export a running VM itself. I noticed this because some people figured the latter was a new feature in Windows 2016. No it’s not. We’ve had this option since Windows 8.1 and Windows Server 2012 R2.

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So why even have the option of exporting a checkpoint of a running VM? Because this enables you to have exports from various points in time, which is pretty cool and handy during test and development and trouble shooting or lab work. As a standard checkpoint has state in Windows Server 2012 R2 I prefer to shut down the VM, create a checkpoint and start the VM again. When I then export that checkpoint I don’t have to worry about the state in the VM at that point in time as it was shut down.

For some workloads this isn’t a big deal bit for some this is not a great experience, hence the fact that checkpoints are “”not supported in production but for test and dev.

In Windows Server 2016 we now have production checkpoints. That means that when we apply such checkpoints we have a consistent state just like when we restore VM from a backup. You’ll have to boot it up after applying the checkpoint, they do not appear running with the state at the time the snapshot was taken. Well, not unless you opt to create standard checkpoints. The reduces the need for me to shut down a VM before I create a checkpoint to export in many cases.

When you export a running VM in Windows Server 2016 you’ll have a copy of it in saved state. Just like you did in Windows Server 2012 R2, no change there. When you import that you’ll have a VM in saved state that you need to start up. If you want an application consistent copy, create a production checkpoint first and export that one.

So there you go. The feature to live export a running virtual machine was here before and it’s still here. The real extra capability with live exports comes from leveraging the live export of a checkpoint of a running virtual machine and the fact that we now have production checkpoints.