CPU Credits

Traditional Amazon EC2 instance types provide fixed performance, while T2 instances provide a baseline level of CPU performance with the ability to burst above baseline. The baseline performance and ability to burst are governed by CPU credits. Each T2 instance continuously receives CPU credits at a set rate, depending on the instance size. T2 instances accrue CPU credits when they are idle, and use CPU credits when they are active. A CPU credit provides the performance of a full CPU core for one minute. Each T2 instance starts with a healthy balance of CPU credits.

For example, a t2.small instance continuously receives credits at a rate of 12 CPU credits per hour. This capability provides baseline performance equivalent to 20 percent of a CPU core. If at any moment the instance does not need the credits it receives, it stores them in its CPU credit balance for up to 24 hours. If and when your t2.small instance needs to burst to more than 20 percent of a core, it draws from its CPU credit balance. Over time, if you find that your workload needs more CPU credits, or that your instance does not maintain a positive CPU credit balance, we recommend using a larger T2 size, such as the t2.medium, or using a fixed performance instance type.

CPU credits are spent bursting above baseline in different ways depending on the number of vCPUs provided in the instance. One CPU credit can be spent by one vCPU bursting to 100 percent of a CPU core for one minute. Other combinations of vCPUs, utilization, and time will also spend one CPU credit; such as one vCPU running at 50 percent for two minutes, or two vCPUs (for example, on t2.medium instances) running at 25 percent each for two minutes.

The following table lists the initial CPU credit allocation received at launch, the rate at which CPU credits are received, the baseline performance level as a percentage of a full core performance, and the maximum CPU credit balance that an instance can accrue.

The t2.micro and t2.small instance types launch with an initial balance of 30 CPU credits, and the t2.medium instance type launches with 60 CPU credits. This initial credit balance is designed to provide a good startup experience.

When your T2 instance doesn’t use the credits it receives, CPU credits are added to its CPU credit balance up to a specific point. The maximum credit balance for an instance is equal to the number of CPU credits received per hour times 24 hours. For example, a t2.micro instance earns 6 CPU credits per hour and can accumulate a maximum CPU credit balance of 144 CPU credits. When your instance requires more than the base performance CPU level, it simply uses CPU credits in the credit balance to burst to the required performance level.

If your instance uses all of its CPU credit balance, performance remains at the baseline performance level. If your instance is running low on credits, your instance’s CPU credit consumption (and therefore CPU performance) is gradually lowered to the base performance level over a 15-minute interval, so you will not experience a sharp performance drop-off when your CPU credits are depleted.

The CPU credit balance for an instance does not persist between instance stops and starts; stopping an instance causes it to lose its credit balance entirely, and when it restarts it will have an empty credit balance.