CPU Credits allow T2 instances to have CPU performance beyond the baseline performance provided by EC2. One CPU Credit is equal to one minute of a full CPU core.
T2 instances earn CPU credits continuously. The rate at which credits are earned depends on the instance size. A t2.nano instance earns 3 credits per hour while a t2.2xlarge earns 81 credits per hour.
When a t2 instance runs out of CPU credits, it is limited to its baseline performance. Running out of CPU credits during peak usage can cause application slowness and should be avoided. It is possible to enable T2Unlimited on a t2 instance, which allows an instance to use surplus credits that are later billed based on usage in a rolling 24-hour window.
Blue Matador will monitor cpu credit usage on instances which do not have T2Unlimited enabled and warn when credits are projected to run out soon. When credits are depleted, the issue becomes an alert.