I have a t2.micro instance running Ubuntu Linux on ec2 in Ireland, it's running batch jobs and I've tuned them so that the max CPU is pretty low - it never exceeds 10%.
The docs say that:
When a T2 instance uses fewer CPU resources than its base performance level allows (such as when it is idle), the unused CPU credits (or the difference between what was earned and what was spent) are stored in the credit balance for up to 24 hours, building CPU credits for bursting. When your T2 instance requires more CPU resources than its base performance level allows, it uses credits from the CPU credit balance to burst up to 100% utilization.
The same page also says that "Base performance (CPU utilization)" for this type of instance is 10%.
But when CPU usage is around the 6% level - always with the max remaining below 10% - the instance consumes CPU credits. Which is not what I expected, because I understand the the docs (quoted above) to say that when the instance is at or below its base performance level, it doesn't consume any CPU credits. (I'm getting the CPU usage figure from Cloudwatch basic monitoring).
So why is the instance consuming CPU credits?