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Amazon Elastic Compute Cloud
User Guide for Windows Instances

CPU Credits and Baseline Performance

Traditional Amazon EC2 instance types provide fixed performance, while T2 instances provide a baseline level of CPU performance with the ability to burst above that baseline level. The baseline performance and ability to burst are governed by CPU credits. A CPU credit provides the performance of a full CPU core for one minute.

CPU Credits

One CPU credit is equal to one vCPU running at 100% utilization for one minute. Other combinations of number of vCPUs, utilization, and time can also equate to one CPU credit. For example, one CPU credit is equal to one vCPU running at 50% utilization for two minutes, or two vCPUs running at 25% utilization for two minutes.

Earning CPU Credits

Each T2 instance continuously earns (at a millisecond-level resolution) a set rate of CPU credits per hour, depending on the instance size. The accounting process for whether credits are accrued or spent also happens at a millisecond-level resolution, so you don't have to worry about overspending CPU credits; a short burst of CPU uses a small fraction of a CPU credit.

If a T2 instance uses fewer CPU resources than is required for baseline performance (such as when it is idle), the unspent CPU credits are accrued in the CPU credit balance. If a T2 instance needs to burst above the baseline performance level, it spends the accrued credits. The more credits a T2 instance has accrued, the more time it can burst beyond its baseline when more performance is needed.

The following table lists the rate at which CPU credits are earned per hour, the maximum number of earned CPU credits that an instance can accrue, the number of vCPUs per instance, and the baseline performance level as a percentage of a full core performance (using a single vCPU).

Instance type

CPU credits earned per hour

Maximum earned credits that can be accrued*

vCPUs

Baseline performance per vCPU

t2.nano

3

72

1

5%

t2.micro

6

144

1

10%

t2.small

12

288

1

20%

t2.medium

24

576

2

20%**

t2.large

36

864

2

30%**

t2.xlarge

54

1296

4

22.5%**

t2.2xlarge

81

1944

8

17%**

* The number of credits that can be accrued is equivalent to the number of credits that can be earned in a 24-hour period. For T2 Standard, launch credits can also be accrued, and do not count towards to the maximum earned credits that can be accrued.

** t2.medium and larger instances have more than one vCPU. The baseline performance in the table is per vCPU. To calculate the baseline CPU utilization for the instance, multiply the vCPU percentage by the number of vCPUs. For example, a t2.large instance has two vCPUs, which provides a baseline CPU utilization for the instance of 60% (2 vCPUs x 30% baseline performance of one vCPU). In CloudWatch, CPU utilization is shown per vCPU; therefore the CPU utilization for a t2.large instance operating at the baseline performance is shown as 30% in CloudWatch CPU metrics.

CPU Credit Earn Rate

The number of CPU credits earned per hour is determined by the instance size. For example, a t2.nano earns three credits per hour, while a t2.small earns 12 credits per hour. The preceding table lists the credit earn rate for all T2 instances.

CPU Credit Accrual Limit

While earned credits never expire on a running instance, there is a limit to the number of earned credits an instance can accrue. The limit is determined by the CPU credit balance limit. Once the limit is reached, any new credits that are earned are discarded, indicated by the following image: the full bucket indicates the CPU credit balance limit, and the spillover indicates newly earned credits that exceed the limit.

The CPU credit balance limit differs for each T2 instance size. For example, a t2.micro instance can accrue a maximum of 144 earned CPU credits in the CPU credit balance. The preceding table lists the maximum number of earned credits that each T2 instance can accrue.

Launch credits do not count towards the CPU credit balance limit. If a T2 Standard instance has not spent its launch credits, and remains idle over a 24-hour period while accruing earned credits, its CPU credit balance appears as over the limit. For more information, see Launch Credits.

Accrued CPU Credits Are Lost When an Instance Is Stopped

CPU credits on a running instance do not expire. However, the CPU credit balance does not persist between instance stops and starts; if you stop an instance, the instance loses all its accrued credits. For more information, see CPUCreditBalance in the CloudWatch metrics table.

Baseline Performance

The number of credits an instance earns per hour can be expressed as a percentage of CPU utilization, and is known as the baseline performance, and sometimes just as the baseline. For example, a t2.nano earns three credits per hour resulting in a baseline performance of 5% (3/60 minutes). A t2.large, with two vCPUs, earns 36 credits per hour, resulting in a baseline performance of 30% (18/60 minutes) per vCPU.