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EC2 CPU hog

Stress a configurable number of CPU cores inside a target EC2 instance via AWS Systems Manager so you can test how the workload behaves when the host is starved of CPU.

EC2 CPU hog is an AWS chaos fault that drives a configurable number of CPU cores to a configurable load percentage inside a target EC2 instance for a configurable duration. The fault dispatches the stress workload via AWS Systems Manager Run Command, so the target instance must have the SSM Agent installed and an IAM role that permits SSM messages.

Use this fault to test how a workload reacts when its host is CPU-starved: does autoscaling react, do latency-sensitive paths break SLO, does the load balancer detach the instance because of failing health checks?

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.

Use cases

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

  • Latency under CPU pressure: When all cores are saturated, does p99 latency stay within SLO?

  • Autoscaling triggers: Do CloudWatch CPU alarms fire and does the ASG add capacity in the expected window?

  • Health-check correctness: Does the target group detach the saturated instance, and does it reattach when the fault ends?

  • Co-tenant workloads: When one workload monopolizes CPU, do neighbouring workloads still meet their SLOs?

  • Burst-credit exhaustion (T-family): For burstable instance types (t3, t4g), does sustained 100% CPU exhaust burst credits and degrade further?

Prerequisites

  • Kubernetes version: 1.21 or later for the chaos infrastructure cluster.

  • Target instance is reachable via SSM: The instance has the SSM Agent running and an instance profile with AmazonSSMManagedInstanceCore (or equivalent). Confirm with aws ssm describe-instance-information --filters "Key=InstanceIds,Values=<id>".

  • Selector provided: Either EC2_INSTANCE_ID or EC2_INSTANCE_TAG is set.

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

  • IAM permissions granted: The credentials or role include the SSM and EC2 permissions listed below.

Supported environments

Platform
Support status

Amazon EC2 (Linux instances with SSM Agent)

Supported

Amazon EKS managed worker nodes

Supported (if SSM Agent is installed)

Amazon EKS self-managed worker nodes

Supported (if SSM Agent is installed)

Targeting by tag

Supported via EC2_INSTANCE_TAG

Targeting by ID

Supported via EC2_INSTANCE_ID

Burstable instance types (T family)

Supported (be aware of credit exhaustion; see Limitations)

Windows instances

Not supported (Linux-only stress payload)

Permissions required

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

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 File Secret should contain an AWS credentials file in the standard ~/.aws/credentials format:

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.

Fault tunables

Required parameters

Tunable
Description
Default

REGION

AWS region that hosts the target instance.

(required)

EC2_INSTANCE_ID or EC2_INSTANCE_TAG

One of these must be set to select the target instance(s).

""

Chaos parameters

Tunable
Description
Default

CPU_CORES

Number of CPU cores to hog. 0 hogs all available cores.

0

CPU_LOAD

Percentage of CPU load to generate per core (0-100).

100

TOTAL_CHAOS_DURATION

Duration of the fault in seconds.

60

INSTANCE_AFFECTED_PERC

Percentage of matching instances to target (only with EC2_INSTANCE_TAG). 0 targets one instance.

0

INSTALL_DEPENDENCIES

Install the in-instance stress tool if missing. Set to False to skip.

True

PROXY

HTTP/HTTPS proxy used by the in-instance installer (for example https_proxy=http://proxy.server:3128).

""

SEQUENCE

Order in which multiple instances are processed: parallel or serial.

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.

""

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.

TUNE CPU_CORES TO YOUR BLAST RADIUS

Setting CPU_CORES=0 (all cores) is the most severe option. For a softer test, set CPU_CORES to half of the instance's vCPU count and observe whether the workload starts to degrade before saturation.

Fault execution in brief

Sends an SSM Run Command to the selected instance(s) in REGION that spawns a CPU-stress workload pinned to CPU_CORES cores at CPU_LOAD percent each for TOTAL_CHAOS_DURATION seconds.

Expected behavior during fault execution

  • Per-core CPU utilization on the target instance climbs to CPU_LOAD percent across CPU_CORES cores.

  • CloudWatch CPUUtilization rises to the corresponding fraction of overall capacity.

  • Latency-sensitive workloads typically see p99 latency spike; CPU-bound throughput drops.

  • For burstable instance types, CPU credits drain rapidly; once exhausted, baseline performance degrades further.

  • Load balancer health checks may begin to fail if they share CPU with the workload.

WHEN THE FAULT ENDS

The chaos pod terminates the stress workload. CPU utilization returns to baseline within seconds; burst credits replenish according to the instance's credit accrual rate.

Signals to watch

  • CPU utilization: Use a Prometheus probe on aws_ec2_cpuutilization_average (CloudWatch exporter) to confirm the spike.

  • Application latency: Use an HTTP probe to detect latency regression.

  • Burst credits: For T-family instances, watch aws_ec2_cpucredit_balance_minimum to confirm credit exhaustion.

Verify the fault execution effect

While the experiment is running:

  1. Check CPU on the instance via SSM.

  2. Confirm CloudWatch metric spike.

Recovery and cleanup

  • End of duration: The chaos pod terminates the stress workload.

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

  • Manual recovery: If the stress workload outlives the experiment (rare), kill it via SSM: pkill -f stress or pkill -f <stress-binary>.

Limitations

  • Linux-only payload: This fault stresses Linux instances. For Windows hosts, use windows-ec2-cpu-hog.

  • SSM Agent required: Instances without the SSM Agent online cannot be targeted.

  • Burst-credit consumption: On T-family instances, credits are consumed by the fault and may not be fully replenished by the time the next experiment runs. Allow recovery time between runs.

  • Workload eviction: On EKS nodes, severe CPU pressure may cause kubelet to evict pods. Use INSTANCE_AFFECTED_PERC and CPU_CORES carefully on production-like clusters.

Troubleshooting

EC2 CPU hog experiment fails with InvalidInstanceId in Harness Chaos Engineering

The SSM Agent is not online for the target instance. Confirm with aws ssm describe-instance-information --filters 'Key=InstanceIds,Values='. If the instance is missing, install the SSM Agent and attach an instance profile that includes AmazonSSMManagedInstanceCore.

EC2 CPU hog runs but CPUUtilization metric stays flat

The most common causes are: the in-instance stress tool failed to install (set INSTALL_DEPENDENCIES=True and verify network egress from the instance, or use PROXY); CPU_CORES is greater than the instance vCPU count; or CloudWatch metric resolution is coarse and is averaging out the spike. Run 'top' via SSM during the fault to confirm load locally before relying on CloudWatch.

EC2 CPU hog fails with AccessDeniedException calling ssm:SendCommand

The chaos pod's IAM principal lacks ssm:SendCommand or related SSM permissions. Add ssm:SendCommand, ssm:GetCommandInvocation, ssm:DescribeInstanceInformation, ssm:CancelCommand, ssm:GetDocument, and ssm:DescribeDocument to the policy. If using ASSUME_ROLE_ARN, confirm the trust policy allows the source identity.

Related faults

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