The following diagram shows how a VM gets its resource from ESXi. Unlike a physical server, a VM has dynamic resources given to it. It is not static. Contention, Demand and Entitlement are concepts that do not exist in the physical world. It is a pretty complex diagram, so let me walk you through it.
The tall rectangular area represents a VM. Say this VM is given 8 GB of virtual RAM. The bottom line represents 0 GB and the top line represents 8 GB. The VM is configured with 8 GB RAM. We call this Provisioned. This is what the Guest OS sees, so if it is running Windows, you will see 8 GB RAM when you log into Windows.
Unlike a physical server, you can configure a Limit and a Reservation. This is done outside the Guest OS, so Windows or Linux does not know. You should minimize the use of Limit and Reservation as it makes the operation more complex.
Entitlement means what the VM is entitled to. In this example, the hypervisor entitles the VM to a certain amount of memory. I did not show a solid line and used an italic font style to mark that Entitlement is not a fixed value, but a dynamic value determined by the hypervisor. It varies every minute, determined by Limit, Entitlement, and Reservation of the VM itself and any shared allocation with other VMs running on the same host.
Obviously, a VM can only use what it is entitled to at any given point of time, so the Usage counter does not go higher than the Entitlement counter. The green line shows that Usage ranges from 0 to the Entitlement value.
In a healthy environment, the ESXi host has enough resources to meet the demands of all the VMs on it with sufficient overhead. In this case, you will see that the Entitlement, Usage, and Demand counters will be similar to one another when the VM is highly utilized. This is shown by the green line where Demand stops at Usage, and Usage stops at Entitlement. The numerical value may not be identical because vCenter reports Usage in percentage, and it is an average value of the sample period. vCenter reports Entitlement in MHz and it takes the latest value in the sample period. It reports Demand in MHz and it is an average value of the sample period. This also explains why you may see Usage a bit higher than Entitlement in highly-utilized vCPU. If the VM has low utilization, you will see the Entitlement counter is much higher than Usage.
An environment in which the ESXi host is resource constrained is unhealthy. It cannot give every VM the resources they ask for. The VMs demand more than they are entitled to use, so the Usage and Entitlement counters will be lower than the Demand counter. The Demand counter can go higher than Limit naturally. For example, if a VM is limited to 2 GB of RAM and it wants to use 14 GB, then Demand will exceed Limit. Obviously, Demand cannot exceed Provisioned. This is why the red line stops at Provisioned because that is as high as it can go.
The difference between what the VM demands and what it gets to use is the Contention counter. Contention is a special counter that tracks all these competition for resources. It’s a counter that only exists in the virtual world.
So Contention, simplistically speaking, is Demand – Usage. I said simplistically as that’s not the actual formula. The actual formula does not really matter for all practical purpose as it’s all relative to the expectation that you’ve set to your customers (VM Owner).
Contention happens when what the VM demands is more than it gets to use. So if the Contention is 0, the VM can use everything it demands. This is the ultimate goal, as performance will match the physical world. This Contention value is useful to demonstrate that the infrastructure provides a good service to the application team. If a VM owner comes to see you and says that your shared infrastructure is unable to serve his or her VM well, both of you can check the Contention counter.
The Contention counter should become a part of your Performance SLA or Key Performance Indicator (KPI). It is not sufficient to track utilization alone. When there is contention, it is possible that both your VM and ESXi host have low utilization, and yet your customers (VMs running on that host) perform poorly. This typically happens when the VMs are relatively large compared to the ESXi host. Let me give you a simple example to illustrate this. The ESXi host has two sockets and 20 cores. Hyper-threading is not enabled to keep this example simple. You run just 2 VMs, but each VM has 11 vCPUs. As a result, they will not be able to run concurrently. ESXi VMkernel will schedule them sequentially as there are only 20 physical cores to serve 22 vCPUs. Here, both VMs will experience high contention.
Hold on! You might say, “There is no Contention counter in vSphere and no memory Demand counter either.”
This is where vR Ops comes in. It does not just regurgitate the values in vCenter. It has implicit knowledge of vSphere and a set of derived counters with formulae that leverage that knowledge.
You need to have an understanding of how the vSphere CPU scheduler works.
The following diagram shows the various states that a VM can be in:
The preceding diagram is taken from The CPU Scheduler in VMware vSphere®
5.1: Performance Study. This is a whitepaper that documents the CPU scheduler with
a good amount of depth for VMware administrators. I highly recommend you read this
paper as it will help you explain to your customers (the application team) how your
shared infrastructure juggles all those VMs at the same time. It will also help you pick
the right counters when you create your custom dashboards in vRealize Operations.