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What are the advantages and disadvantages of overcommitting CPU resources for VMs?

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Overcommitting CPU resources for VMs means allocating more virtual CPUs (vCPUs) to the virtual machines (VMs) than the physical cores available on the host. This can help optimize the utilization of the host CPU and increase the density of VMs per host. However, it also comes with some trade-offs and challenges that you should be aware of before deciding to overcommit CPU resources for your VMs. In this article, we will explore the advantages and disadvantages of overcommitting CPU resources for VMs and give you some tips on how to do it effectively.

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Paul Delory

Gartner VP, Analyst, and Research Portfolio Manager
Ehsan Kosari

Virtualization and Storage Solutions Architect and & Data Backup Engineer| Delivering Scalable, High-Availability…

1 Advantages of overcommitting CPU resources

One of the main advantages of overcommitting CPU resources for VMs is that it can improve the performance and efficiency of the host CPU. By allocating more vCPUs to the VMs, you can take advantage of the idle cycles and unused capacity of the host CPU and distribute the workload more evenly across the cores. This can reduce the CPU contention and increase the throughput of the VMs. Another advantage of overcommitting CPU resources for VMs is that it can reduce the cost and complexity of your virtual infrastructure. By overcommitting CPU resources, you can run more VMs on fewer hosts, which can lower your hardware, power, cooling, and licensing expenses. It can also simplify your management and maintenance tasks by reducing the number of hosts and clusters you need to monitor and update.

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Paul Delory

Gartner VP, Analyst, and Research Portfolio Manager
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Oversubscribing hardware resources is the only way to maximize utilization of the physical hardware, which is the most important benefit of virtualization. The whole idea behind virtualization is to trade a small amount of performance for a vast gain in efficiency. Oversubscribing CPUs is normal. It is the expected behavior in well-functioning virtualization environments, and an obvious best practice.

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Ehsan Kosari

Virtualization and Storage Solutions Architect and & Data Backup Engineer| Delivering Scalable, High-Availability Infrastructure and Disaster Recovery Solutions
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while overcommitting CPU resources can provide advantages such as improved resource utilization, cost savings, and flexibility, it can also result in disadvantages such as resource contention, VM migration difficulties, VM isolation compromises, lower VM performance, and VM compatibility issues. As such, organizations should carefully consider the potential advantages and disadvantages of overcommitting CPU resources for their specific workloads and environments.

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2 Disadvantages of overcommitting CPU resources

However, overcommitting CPU resources for VMs also has some drawbacks and risks that you should consider before applying it to your environment. One of the main disadvantages of overcommitting CPU resources for VMs is that it can degrade the performance and reliability of the VMs and the host. If the demand for CPU resources exceeds the supply, the host will have to schedule and prioritize the vCPUs and use techniques such as time slicing, co-scheduling, and hyperthreading to cope with the overload. This can introduce latency, jitter, and contention issues that can affect the responsiveness and quality of service of the VMs. Another disadvantage of overcommitting CPU resources for VMs is that it can increase the complexity and difficulty of your capacity planning and troubleshooting tasks. By overcommitting CPU resources, you will have to monitor and analyze more metrics and variables to ensure that your VMs are getting enough CPU resources and that your host is not oversubscribed. You will also have to deal with more potential causes and symptoms of performance problems and fine-tune your configuration and settings to optimize your CPU allocation.

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Paul Delory

Gartner VP, Analyst, and Research Portfolio Manager
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An overcommitted virtual CPU will have lower performance than the underlying physical core. (Some overhead is inevitable.) However, the performance degradation should be minimal--no more than 5%. Applications that are especially sensitive to latency (such as virtual desktops) may experience some adverse effects (such as screen jitter), but the vast majority of applications will not notice. In practice, there are very few downsides to CPU oversubscription, because RAM is almost always the constraint in virtualization environments. Usually, you'll run out of RAM long before you're taxing CPUs to the point where VMs are starved of processor cycles.

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3 Tips for overcommitting CPU resources effectively

To make the most of overcommitting CPU resources for VMs, you should follow some best practices and tips. Firstly, assess the workload and resource requirements of your VMs and applications to understand how much CPU resources they need, as well as their usage patterns and sensitivity to latency and contention. This will help you determine how much you can overcommit without compromising performance. Additionally, select a host with enough physical cores, memory, and I/O capacity to support your VMs, and configure them with the right number and size of vCPUs. Furthermore, monitor and optimize CPU utilization and performance metrics such as CPU Ready, CPU Co-stop, CPU Run Queue, CPU Usage, and CPU Balloon. Tools like vSphere Performance Charts or vRealize Operations Manager can collect and analyze these metrics to provide recommendations to optimize your CPU utilization.

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Paul Delory

Gartner VP, Analyst, and Research Portfolio Manager
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Oversubscribing hardware resources requires careful planning and close monitoring by a subject-matter expert to make sure VMs are never starved of resources. The performance overhead of CPU commitment should not exceed 5%. Measure this continuously and carefully. The metrics vary per hypervisor. For VMware ESXi: CPU Ready and CPU Wait should be under 5%; and Co-Stop should be zero. For Nutanix AHV: CPU Steal and I/O Wait should both be under 5%; and STATS_HYP_AVG_IO_LATENCY should be under 10 milliseconds. For Microsoft Hyper-V: CPU Wait Time per Dispatch should be under 50 milliseconds; Hyper-V does not measure time spent waiting in the host kernel. Overcommitment rates vary, but a good rule of thumb is 4 vCPUs per (hyperthreaded) core.

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4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the advantages and disadvantages of overcommitting CPU resources for VMs?

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1 Advantages of overcommitting CPU resources

2 Disadvantages of overcommitting CPU resources

3 Tips for overcommitting CPU resources effectively

4 Here’s what else to consider

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