This post is written by: Matthew Brunton, Senior Solutions Architect – WWPS

To ensure our customers have the appropriate machines available for their workloads, AWS offers a wide range of hardware options that include hundreds of types of instances that help customers achieve the best price performance for their workloads.  In some specialized circumstances, our customers need an even wider range of options, or more flexibility. This could be driven by a desire to optimize licensing costs, or the customer wanting more hardware configuration options. Some high performance workloads can improve by turning off simultaneous multithreading. In our AWS Well Architected Framework – High Performance Computing Lens we have the following recommendation “Unless an application has been tested with hyperthreading enabled, it is recommended that hyperthreading be disabled”. With these factors in mind, AWS offers the ability to configure some options regarding the CPU configuration in launched instances.

Our larger instance types that have a higher number of cores, and offer multithreaded cores will translate to a larger combination of potential options. The valid combinations of cores and threads per core can be found here. To consider utilizing the CPU options for both cores and threads per core, you will need to consider instance types that have multiple CPU’s and/or cores.

You can specify numerous CPU options for some of our larger instances via the console, command line interface, or the API. Moreover, you can remove CPUs from the launch configuration, or deactivate threading within CPUs that have multiple threads per core. Amazon Elastic Compute Cloud (Amazon EC2) FAQ’s for the Optimize CPU’s feature can be found here. You should be aware that this feature is only available during instance launch and cannot be modified after launch. The launch options persist after you reboot, stop, or start an instance.

You can easily determine how many CPUs and threads a machine has. There are numerous ways to see this via the AWS Management Console and the AWS Command Line Interface (CLI).

Within the AWS Management Console, under the ‘Instance Details’ section, opening up the ‘Host and placement group’ item reveals the number of vCPUs that your machine has.

Instance Details showing the number of vCPU's a particular EC2 machine has

Figure 1: Console details showing number of vCPU’s

This information is also available using the AWS CLI as follows:

aws ec2 describe-instances --region us-east-1 --filters "Name=instance-type,Values='c6i.*large'" ... "Instances": [ { "Monitoring": { "State": "disabled" }, "PrivateDnsName": "ip-172-31-44-121.ec2.internal", "PrivateIpAddress": "", "State": { "Code": 16, "Name": "running" }, "EbsOptimized": false, "LaunchTime": "2021-11-22T01:46:58+00:00", "ProductCodes": [], "VpcId": " vpc-7f7f1502", "CpuOptions": { "CoreCount": 32, "ThreadsPerCore": 1 }, "StateTransitionReason": "", ... } ]

Figure 2: ec2 describe-instances cli output

A handy AWS CLI command ‘describe-instance-types’ will show the list of valid cores, the possible threads-per-core, and the default values for the vCPUs and cores. These are listed in the ‘DefaultVCpus’ and ‘DefaultCores’ items in the returned JSON listed as follows.

aws ec2 describe-instance-types --region us-east-1 --filters "Name=instance-type,Values='c6i.*'"
{ "InstanceTypes": [ { "InstanceType": "c6i.4xlarge", "CurrentGeneration": true, "FreeTierEligible": false, "SupportedUsageClasses": [ "on-demand", "spot" ], "SupportedRootDeviceTypes": [ "ebs" ], "SupportedVirtualizationTypes": [ "hvm" ], "BareMetal": false, "Hypervisor": "nitro", "ProcessorInfo": { "SupportedArchitectures": [ "x86_64" ], "SustainedClockSpeedInGhz": 3.5 }, "VCpuInfo": { "DefaultVCpus": 16, "DefaultCores": 8, "DefaultThreadsPerCore": 2, "ValidCores": [ 2, 4, 6, 8 ], "ValidThreadsPerCore": [ 1, 2 ] }, "MemoryInfo": { "SizeInMiB": 32768 },


Figure 3: ec2 describe-instance-types cli output

To utilize the AWS CLI to launch one or multiple instances, you should use the run instances CLI command.

The following is the shorthand syntax:

aws ec2 run-instances --image-id xxx --instance-type xxx --cpu-options “CoreCount=xx,ThreadsPerCore=xx” --key-name xxx --region xxx

For example, the following command launches a 32-core machine with only 1 thread per core instead of the standard 2 threads per core:

aws ec2 run-instances --image-id ami-0c2b8ca1dad447f8a --instance-type c6i.16xlarge
--cpu-options " CoreCount =32, ThreadsPerCore =1" --key-name MyKeyPair --region xxx

If you are using the CPU options parameters in CloudFormation templates, then the following applies:

The following is an example of the YAML syntax for specifying the CPU configuration.

Resources: CustomEC2Instance: Type: "AWS::EC2::Instance" Properties: InstanceType: xxx ImageId: xxx CpuOptions:
CoreCount: xx
ThreadsPerCore: x

As can be seen in the following table, there are a number of valid CPU core options, as well as the option to set one or two threads per core for each CPU for certain instance types. This significantly opens up the number of combinations and permutations to meet your specific workload need. In the case of the c6i instance listed in the following table, there are an additional 32 CPU core and threading combinations available to customers.

Instance type Default vCPUs Default CPU cores Default threads per core Valid CPU cores Valid threads per core
c6i.16xlarge 64 32 2 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32 1, 2

Figure 4: Valid Core and Thread Launch Options

Note that you still pay the same amount for the EC2 instance, even if you deactivate some of the cores or threads.


This approach can let customers customize the EC2 hardware options even further to ensure a wider range of CPU/memory combinations over and above the already extensive AWS instance options. This lets customers finely tune hardware for their exact application requirements, whether that is running High Performance workloads or trying to save money where license restrictions mean you can benefit from a specific CPU configuration.

We have run through various scenarios in this post, which detailed how to launch instances with alternate CPU configurations, easily check the current configuration of your running instances via our API and console, and how to configure the options in CloudFormation templates.

We love to see our customers maximizing the flexibility of the AWS platform to deliver outstanding results. Have a look at some of your High Performance workloads and give the threading options a try, or take a deep dive into any of your more expensive licenses to see if you could benefit from an alternate CPU configuration. In order to get started, check out our detailed developer documentation for the optimize CPU options.