Windows EC2 CPU hog

Stress a configurable number of CPU cores at a configurable percentage on one or more Windows EC2 instances (selected by ID or tag) for a configurable duration so you can test how Windows-hosted workl

Windows EC2 CPU hog is an AWS chaos fault that stresses CPU_CORES cores at CPU_PERCENTAGE percent on one or more Windows EC2 instances for a configurable duration. Targets are selected by EC2_INSTANCE_ID (explicit) or by EC2_INSTANCE_TAG (any instance carrying the tag). The fault dispatches a PowerShell stress command via AWS Systems Manager Run Command and removes the stress at the end of the fault.

Use this fault to test how a Windows-hosted workload behaves under sustained CPU pressure: whether request latency degrades predictably, whether autoscaling alarms fire, and whether application-level circuit breakers shed load.

RUN YOUR FIRST EXPERIMENT

If you have not configured the chaos infrastructure yet, go to Quickstart to install the chaos infrastructure and run an experiment end to end.

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Use cases

Run this fault when you want to answer concrete questions like:

• CPU pressure on the workload: When a fraction of replicas is CPU-saturated, do remaining replicas absorb the traffic without breaching latency SLOs?

• Autoscaling: Does the CPU-based scaling policy fire and add capacity within the expected window?

• IIS / Windows-service behaviour: Do IIS or Windows services running under high CPU pressure recover when the stress ends, or do they need manual intervention?

• Noisy-neighbour isolation: Does CPU pressure on one EC2 instance affect other tenants on the same Auto Scaling group?

• Recovery time: When the stress ends, how quickly does latency return to baseline?

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Prerequisites

• Kubernetes version: 1.21 or later for the chaos infrastructure cluster. Go to What's supported to confirm distribution support.

• Target Windows EC2 instances: Each instance is in the running state and registered with AWS Systems Manager.

• Selection criteria: Either EC2_INSTANCE_ID is set (explicit list) or EC2_INSTANCE_TAG is set (Key:Value matching any tag attached to candidate instances).

• AWS credentials available: Either an AWS credentials file uploaded as a File Secret in Harness Secret Manager (see Authentication below) or an IAM role for service accounts (IRSA) bound to the chaos infrastructure service account.

• IAM permissions granted: The credentials or role include the permissions listed below.

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Supported environments

Platform	Support status
Windows Server 2016 / 2019 / 2022 EC2	Supported
EKS or ECS Windows worker nodes	Supported (the fault stresses the host; workload behaviour depends on the orchestrator)
Linux EC2 instances	Not supported (use EC2 CPU hog)
AWS regions	Supported in every commercial region; pass the region in REGION

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Permissions required

The IAM principal that the chaos pod uses (the credentials mounted from the Harness Secret Manager file secret, the IRSA role on the chaos service account, or the role assumed via ASSUME_ROLE_ARN) needs the following AWS actions.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:DescribeInstances",
        "ec2:DescribeInstanceStatus"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ssm:SendCommand",
        "ssm:CancelCommand",
        "ssm:GetCommandInvocation",
        "ssm:DescribeInstanceInformation"
      ],
      "Resource": "*"
    },
    {
      "Effect": "Allow",
      "Action": [
        "ec2messages:AcknowledgeMessage",
        "ec2messages:DeleteMessage",
        "ec2messages:FailMessage",
        "ec2messages:GetEndpoint",
        "ec2messages:GetMessages",
        "ec2messages:SendReply"
      ],
      "Resource": "*"
    }
  ]
}

Go to common policy for all AWS faults to use a single superset IAM policy across every AWS fault.

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Authentication

The fault supports three credential delivery models. Pick one based on how your chaos infrastructure is deployed.

Method	When to use it	How to configure
Harness Secret Manager file secret	Chaos infrastructure runs outside EKS, or you want explicit static credentials	Upload the AWS credentials file as a File Secret in Harness Secret Manager and reference its identifier via AWS_AUTHENTICATION_SECRET
IAM Roles for Service Accounts (IRSA)	Chaos infrastructure runs in EKS and uses an OIDC-bound service account	No tunable changes; the chaos pod inherits the role automatically. Go to AWS IAM integration to set it up
Assume role	The fault needs to act in a different account or with elevated permissions	Set ASSUME_ROLE_ARN to the role ARN; the chaos pod assumes the role on top of its base credentials

When using the Harness Secret Manager method, the contents of the File Secret should be the AWS credentials file in the standard ~/.aws/credentials format:

[default]
aws_access_key_id = REPLACE_WITH_ACCESS_KEY_ID
aws_secret_access_key = REPLACE_WITH_SECRET_ACCESS_KEY

Upload this file as a File Secret in Harness Secret Manager (Project Setup → Secrets → New File Secret), and pass the secret identifier in AWS_AUTHENTICATION_SECRET when configuring the fault.

Go to AWS named profile for chaos to switch between profiles inside a single credentials file.

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Fault tunables

Configure the following fault parameters when you add Windows EC2 CPU hog to an experiment in Chaos Studio. Defaults are shown for reference.

Required parameters

Tunable	Description	Default
REGION	AWS region that hosts the target instances (for example us-east-1).	(required)

Targeting parameters

Tunable	Description	Default
EC2_INSTANCE_ID	Comma-separated list of EC2 instance IDs to stress.	""
EC2_INSTANCE_TAG	Tag in Key:Value form matching candidate instances when EC2_INSTANCE_ID is empty.	""
INSTANCE_AFFECTED_PERC	Percentage of matching instances (by tag) to affect. 0 corresponds to one instance.	0

Chaos parameters

Tunable	Description	Default
CPU_CORES	Number of CPU cores to stress on each target. 0 stresses every available core.	0
CPU_PERCENTAGE	Load percentage applied per stressed core (0-100).	50
TOTAL_CHAOS_DURATION	Duration of the fault in seconds.	60
INSTALL_DEPENDENCIES	Install the stress tooling on the Windows host if missing.	true
SEQUENCE	Order in which multiple instances are stressed: parallel stresses all selected instances at once; serial stresses one at a time.	parallel
RAMP_TIME	Wait period in seconds before and after the fault. Go to ramp time to read how it is applied.	0

Authentication

Tunable	Description	Default
ASSUME_ROLE_ARN	ARN of an IAM role to assume on top of the base credentials. Leave empty to use the base credentials directly.	""
AWS_AUTHENTICATION_SECRET	Identifier of the File Secret in Harness Secret Manager that contains the AWS credentials file. Not required when using IRSA.	""

Tunables that apply to every fault are documented in common tunables for all faults. AWS-specific shared tunables are documented in common AWS fault tunables.

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Fault execution in brief

Resolves the target instance list from EC2_INSTANCE_ID (or by EC2_INSTANCE_TAG), dispatches a PowerShell CPU-stress command via AWS Systems Manager Run Command to each selected Windows instance, holds the load for TOTAL_CHAOS_DURATION seconds, then stops the stress process.

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Expected behavior during fault execution

• CPU utilization on the targeted Windows instances rises to approximately CPU_PERCENTAGE across CPU_CORES cores.

• Workloads running on the host (IIS sites, Windows services, ECS/EKS Windows tasks) experience CPU contention; latency and throughput degrade.

• CloudWatch CPUUtilization for the affected instances rises.

• CPU-based autoscaling alarms may fire.

WHEN THE FAULT ENDS

The chaos pod stops the stress process on each host. CPU returns to baseline within seconds.

Signals to watch

Attach resilience probes to assert each layer:

• Application latency: Use an HTTP probe and assert percentile latency SLOs.

• CPU utilization: Use a Prometheus probe on aws_ec2_cpuutilization (CloudWatch exporter).

• Autoscaling alarms: Confirm CPU-based alarms fire.

• Service availability: For Windows services or IIS sites, monitor whether the service stays in the Running state.

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Verify the fault execution effect

While the experiment is running, confirm CPU pressure is applied:

1. Check CloudWatch CPU metrics.

   In the AWS console (CloudWatch → EC2 → Per-Instance Metrics), the target instance's CPUUtilization should rise during the chaos window and fall after recovery.

2. Inspect SSM command status.

   aws ssm list-command-invocations --region <region> --details --filters "key=Status,value=InProgress"

   During the fault, you should see in-progress commands on the affected instances.

3. Inspect from the Windows host (via RDP or SSM Session Manager).

   Get-Counter '\Processor(_Total)\% Processor Time' -SampleInterval 1 -MaxSamples 5

   Total processor utilization should be near the configured CPU_PERCENTAGE.

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Recovery and cleanup

• End of duration: The chaos pod stops the stress process on each host.

• Abort the experiment: Stopping the experiment from Chaos Studio cancels the SSM command.

• Manual recovery: If the fault exits before cleanup, kill the stress process on the host via SSM Run Command (PowerShell Stop-Process).

• Workload recovery: Most CPU-bound workloads recover automatically; long-running batch jobs may have produced incorrect or incomplete results during the stress.

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Limitations

• Windows-only: This fault uses PowerShell and Windows process tooling. For Linux, use EC2 CPU hog.

• SSM-managed hosts only: Instances not registered with SSM cannot be reached.

• CPU governor / power plans: Some Windows power plans cap CPU below 100%; the stress may not reach the configured load.

• Cross-region targeting: A single experiment targets one region (the value of REGION).

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Troubleshooting

Windows EC2 CPU hog fails with AccessDeniedException in Harness Chaos Engineering

The credentials supplied to the chaos pod do not have the required EC2 or SSM permissions. Confirm the IAM policy attached to the user, role, or IRSA service account includes ec2:DescribeInstances, ssm:SendCommand, ssm:GetCommandInvocation, and the ec2messages:* actions used by SSM.

Windows EC2 CPU hog reports instance is not SSM-managed

Confirm the SSM Agent is installed and running on the Windows host (the SSMAgent service should be in Running state), the host has an instance profile with AmazonSSMManagedInstanceCore (or equivalent), and the host appears in 'aws ssm describe-instance-information' with PlatformType=Windows. Newly bootstrapped instances may take a few minutes to register.

CPU utilization does not reach the configured percentage

The most common causes are: the Windows power plan caps the CPU (set to High Performance for the test); CPU_CORES is smaller than the available cores so total utilization is lower than expected; or another workload is throttling the chaos process. Set CPU_CORES=0 to stress every core and verify the High Performance power plan is active.

Tags match more instances than expected

EC2_INSTANCE_TAG matches any instance carrying that tag in the target REGION; if the tag is broadly used, the fault may affect more than the intended set. Use a more specific tag (or use INSTANCE_AFFECTED_PERC to bound the blast radius), and verify the candidate list with 'aws ec2 describe-instances --filters Name=tag:Key,Values=Value'.

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Related faults

• Windows EC2 memory hog: Stress memory on Windows instances instead of CPU.

• EC2 CPU hog: CPU stress on Linux EC2 instances.

• Windows EC2 process kill: Kill specific Windows processes on the instance.

• Common AWS fault tunables: Shared environment variables for AWS faults.