Amazon Elastic Compute Cloud
User Guide for Linux Instances

Unlimited Mode for Burstable Performance Instances

A burstable performance instance configured as unlimited can sustain high CPU performance for any period of time whenever required. The hourly instance price automatically covers all interim spikes in usage if the average CPU utilization of the instance is at or below the baseline over a rolling 24-hour period or the instance lifetime, whichever is shorter. If the instance runs at higher CPU utilization for a prolonged period, it can do so for a flat additional rate per vCPU-hour. For information about instance pricing, see Amazon EC2 Pricing and the section for Unlimited pricing in Amazon EC2 On-Demand Pricing.

Important

If you use a t2.micro instance under the AWS Free Tier offer and configure it as unlimited, charges may apply if your average utilization over a rolling 24-hour period exceeds the baseline of the instance.

Unlimited Mode Concepts

The unlimited mode is a credit configuration option for burstable performance instances. It can be enabled or disabled at any time for a running or stopped instance.

  • T3 instances are launched as unlimited by default.

  • T2 instances are launched as standard by default.

A burstable performance instance configured as unlimited can burst above the baseline for as long as required. This enables you to enjoy the low instance hourly price for a wide variety of general-purpose applications, and ensures that your instances are never held to the baseline performance.

The basic hourly instance price automatically covers all CPU usage spikes if the average CPU utilization of an instance configured as unlimited over a rolling 24-hour period is at or below the baseline. For the vast majority of general-purpose workloads, instances configured as unlimited provide ample performance without any additional charges. If the average CPU utilization exceeds the baseline over a 24-hour period, there is a flat additional rate per vCPU-hour.

How Unlimited Burstable Performance Instances Work

If a burstable performance instance configured as unlimited depletes its CPU credit balance, it can spend surplus credits to burst beyond the baseline. When its CPU utilization falls below the baseline, it uses the CPU credits that it earns to pay down the surplus credits that it spent earlier. The ability to earn CPU credits to pay down surplus credits enables Amazon EC2 to average the CPU utilization of an instance over a 24-hour period.

Surplus Credits Can Incur Charges

If the average CPU utilization of an instance is at or below the baseline, the instance incurs no additional charges. Because an instance earns a maximum number of credits in a 24-hour period (for example, a t3.micro instance can earn a maximum of 288 credits in a 24-hour period), it can spend surplus credits up to that maximum without being charged.

However, if CPU utilization stays above the baseline, the instance cannot earn enough credits to pay down the surplus credits that it has spent. The surplus credits that are not paid down are charged at a flat additional rate per vCPU-hour.

Surplus credits that were spent earlier are charged when any of the following occurs:

  • The spent surplus credits exceed the maximum number of credits the instance can earn in a 24-hour period. Spent surplus credits above the maximum are charged at the end of the hour.

  • The instance is stopped or terminated.

  • The instance is switched from unlimited to standard.

Spent surplus credits are tracked by the CloudWatch metric CPUSurplusCreditBalance. Surplus credits that are charged are tracked by the CloudWatch metric CPUSurplusCreditsCharged. For more information, see Additional CloudWatch Metrics for Burstable Performance Instances.

No Launch Credits for T2 Unlimited

T2 Standard instances receive launch credits, but T2 Unlimited instances do not. A T2 Unlimited instance can burst beyond the baseline at any time with no additional charge, as long as its average CPU utilization is at or below the baseline over a rolling 24-hour window or its lifetime, whichever is shorter. As such, T2 Unlimited instances do not require launch credits to achieve high performance immediately after launch.

If a T2 instance is switched from standard to unlimited, any accrued launch credits are removed from the CPUCreditBalance before the remaining CPUCreditBalance is carried over.

Note

T3 instances never receive launch credits.

Enabling Unlimited Mode

T3 instances launch as unlimited by default. T2 instances launch as standard by default, but you can enable unlimited at launch.

You can switch from unlimited to standard, and from standard to unlimited, at any time on a running or stopped instance. For more information, see Launching a Burstable Performance Instance as Unlimited or Standard and Modifying the Credit Specification of a Burstable Performance Instance.

You can check whether your burstable performance instance is configured as unlimited or standard using the Amazon EC2 console or the AWS CLI. For more information, see Viewing the Credit Specification of a Burstable Performance Instance.

What Happens to Credits when Switching between Unlimited and Standard

CPUCreditBalance is a CloudWatch metric that tracks the number of credits accrued by an instance. CPUSurplusCreditBalance is a CloudWatch metric that tracks the number of surplus credits spent by an instance.

When you change an instance configured as unlimited to standard, the following occurs:

  • The CPUCreditBalance value remains unchanged and is carried over.

  • The CPUSurplusCreditBalance value is immediately charged.

When a standard instance is switched to unlimited, the following occurs:

  • The CPUCreditBalance value containing accrued earned credits is carried over.

  • For T2 Standard instances, any launch credits are removed from the CPUCreditBalance value, and the remaining CPUCreditBalance value containing accrued earned credits is carried over.

Monitoring Credit Usage

To see if your instance is spending more credits than the baseline provides, you can use CloudWatch metrics to track and set up hourly alarms to be notified of credit usage. For more information, see Monitoring Your CPU Credits.

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