Tag Archives: cpu scheduling

Example Architectural Decision – Hyperthreading with Business Critical Applications (Exchange 2013)

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Problem Statement

When Virtualizing Exchange 2013 (which is considered by the customer as a Business Critical Application) in a vSphere cluster shared with other production workloads of varying sizes and performance requirements,  should Hyper Threading (HT) be used?

Assumptions

1. vSphere 5.0 or greater
2. Exchange Servers are correctly sized day one or are subsequently “Right Sized”
3. Cluster average CPU overcommitment of 3:1

Motivation

1. Ensure Optimal performance for BCAs (Exchange)
2. Ensure Optimal performance for other Virtual servers in the shared vSphere cluster

Architectural Decision

Enable Hyper Threading (HT)

Alternatives

1. Disable Hyper Threading (HT)
2. Enable Hyper Threading but configure Exchange Virtual machine/s with Advanced CPU, HT Sharing Mode of “None” to ensure Exchange is always scheduled onto physical CPU cores
3. Split off a limited number of ESXi hosts and form a dedicated BCA cluster w/ <= 2:1 overcommitment and disable HT
4. Disable HT on a number of nodes within the cluster but leave HT enabled on other nodes and use DRS rules to pin Exchange VMs to non HT hosts

Justification

1. Enabling Hyper Threading (HT) improves the efficiency of the CPU scheduler, which will minimize the possibility of CPU Ready for the Exchange server/s and other virtual machines on the host where a low level of overcommitment exists (<2:1)
2. For optimal performance, DRS “Should” rules will be used to keep Exchange (BCA) workloads on specific ESXi hosts within the cluster where <=2:1 CPU overcommitment is maintained
3. Configuring Advanced CPU, HT Sharing Mode to “None” (to guarantee only pCore’s are used) may result in increased CPU Ready as the CPU scheduler is forced to find (and wait) for available pCore’s which may result in degraded or inconsistent performance.
4. Sizing for the Exchange solution was completed taking into account only pCore’s (Not HT cores) to simplify sizing
5. As the cluster where Exchange is virtualized is shared with other server workloads with varying levels of importance and performance, HT benefits the vast majority of workloads and results in a higher consolidation ratio and better performance for the vSphere cluster as a whole.
6. In physical servers, enabling Hyper Threading on Exchange servers resulted in wasted or excessive RAM usage for .NET garbage collection due to memory for .NET being allocated based on logical cores. This does not impact “Right Sized” Virtual Machines as only the required number of vCPUs are assigned to the VM, and therefore available to the Guest OS. This avoids the issue of memory being wasted for HT cores.
7. The CPU scheduler in vSphere 5.0 or later is very efficient and can intelligently schedule workload on a hyper-thread or a physical core depending on the VMs CPU demand. While the Exchange server will at some point be scheduled onto a HT thread, this is not likely to be for any extended duration or have any significant impact on performance.
8. Splitting the cluster into BCA’s and server workloads would increase the HA overhead, and effective reduce the usable compute capacity of the infrastructure.
9. Having a cluster with varying configurations (eg: HT enabled on some hosts and not others) is not advisable as it may lead to inconsistent performance and adds unnecessary complexity to the environment

Implications

1. In the event the vCPU to pCore ratio is > 2:1 for any reason (including HA event & Virtual Server Sprawl) the number of users supported and/or the performance of the Exchange environment may be impacted
2. DRS “Should” rules will need to be created to keep Exchange workloads on hosts with <2:1 vCPU to pCore ratio

 

 

 

High CPU Ready with Low CPU Utilization?

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I have noticed an increasing amount of search engine terms which results in people accessing my blog similar to

* High CPU Ready Low CPU usage
* CPU ready and Low utilization
* CPU ready relationship to utilization

So I wanted to try and clear this issue up.

First lets define CPU Ready & CPU Utilization.

CPU ready (percentage) is the percentage of time a virtual machine is waiting to be scheduled onto a physical (or HT) core by the CPU scheduler.

CPU utilization measures the amount of Mhz or Ghz that is being used.

Next to find out how much CPU ready is ok, check out my post How Much CPU ready is OK?

CPU Ready and CPU utilization have very little to do with each other, high CPU utilization does not mean you will have high CPU ready, and vice versa.

So it is entirely possible to have either of the below scenarios

Scenario 1 : An ESXi host has 20% CPU utilization and VMs to suffer high CPU ready (>10%).
Scenario 2: An ESXi host has 95% CPU utilization and VMs to have little or no CPU ready (<2.5%)

How are the above two scenarios possible?

Scenario 1 may occur when

* One or more VMs are oversized (ie: not utilizing the resources they are assigned)
* The host (or cluster) is highly overcommited (either with or without right sized VMs)
* Where power management settings are set to Balanced / Low Power or custom

Scenario 2 may occur when

* VMs are correctly sized
* The ESXi hosts are well sized for the virtual machine workloads
* The VM to host ratio has been well architected

So the question on everyone lips, How can high CPU ready with Low CPU utilization be addressed/avoided?

If you have a situation where you are experiencing high CPU ready and low ESXi host utilization the following steps should be taken

* Right size your VMs

This is by far the most important thing to do. I Recommend using a tool such as vCenter Operations to assist with determining the correct size for VMs.

* Ensure your hosts/clusters are not excessively overcommited

I generally find 4:1 vCPU overcommitment is achievable with right sized VMs where the avg VM size is <4 vCPUs. The higher the vCPU per VM average, the lower CPU overcommitment you will achieve.)
If you have an average VM size of 8 vCPUs then you may only see <1.5:1 overcommitment before suffering contention (CPU ready).

* Use DRS affinity rules to keep complimentary workloads together
VMs with high CPU utilization and VMs with very low CPU utilization can work well together. You  also may have an environment where some servers are busy overnight and others are only busy during business hours, these are examples of workload to keep together.

* Use DRS anti-affinity rules to keep non-complimentary workloads apart

VMs with very high CPU utilization (assuming the high utilization is at the same time) can be spread over a number of hosts to avoid stress on the CPU scheduler.

* Ensure your ESXi hosts are chosen with the virtual machine workloads in mind
If your VMs are >=8vCPUs choose a CPU with >=8 cores per socket and more sockets per host, like 4 socket hosts as opposed to 2 socket hosts. If the bulk of your VMs are 1 or 2 vCPUs, then even older 2 socket 4 core processors should generally work well.

* Use Hyperthreading
Assuming you have a mix of workloads and not all VMs require large amounts of cores and Ghz, using hyper threading increases the efficiency of the CPU schedulure by effectively doubling the scheduling opportunities. Note: A HT core will generally give much less than half the performance of a pCore.

* Use “High Performance” for your Power Management Policy

The above seven (7) steps should resolve the vast majority of issues with CPU ready.

For an example of the benefits of right sizing your VMs, check out my earlier post – VM Right Sizing , An example of the benefits.

Also please note, using CPU reservations does not solve CPU ready, I have also written an article on this topic – Common Mistake – Using CPU reservations to solve CPU ready

I hope this helps clear up this issue.

How much CPU ready is OK?

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I have noticed a lot of search results hitting my blog asking

Question: How much CPU ready is OK?

so I thought I would address this question with a quick post.

Of course the answer is it depends, for example Server workloads have a lower tolerance to CPU ready than desktop workloads but as a rule of thumb, here is my thoughts.

For Production server workloads

<2.5% CPU Ready
Generally No Problem!

2.5%-5% CPU Ready
Minimal contention that should be monitored during peak times

5%-10% CPU Ready
Significant Contention that should be investigated & addressed

>10% CPU Ready
Serious Contention to be investigated & addressed ASAP!

In my experience, the above have been good for a rule of thumb.

However, applications which are latency sensitive may be severely impacted even with low levels of CPU ready, these types of VMs should be on clusters with lower CPU overcommitment, leverage DRS rules to separate the contending workloads or in extreme cases, dedicated clusters.

On the flip side, Some servers are much more tolerant to CPU ready, and 5%-10% CPU ready or higher may not noticeably impact performance.

Keep in mind that setting CPU Reservations does not solve CPU Ready, see my post on the topic for more details.

VMware vCenter Operations is a tool which can help easily identify contention (including CPU) within your vSphere environment.

For Virtual Desktop workloads, what level of CPU ready is acceptable will largely depend on the individual user (ie: Power User verses Task Worker). Keep in mind virtual desktop deployments generally have high CPU consolidation ratios of  around 6:1 all the way to >12:1.

I would suggest the following , again as a rule of thumb

<5% CPU Ready
Generally No Problem!

5%-10% CPU Ready
Minimal contention that should be monitored during peak times

>10% CPU Ready
Contention to be investigated & addressed where the end user experience is being impacted.

Any Higher CPU ready will likely be impacting your users, and should be investigated.

VMware have recently released vCenter Operations for View, which you could use to monitor your VMware View environment.