Amazon EC2 recently transitioned from hourly to per-second billing. Previously, as your jobs completed, AWS Batch would retain idle compute resources for the majority of the billing hour in order to avoid the cost associated with additional instance launches within the hour. With per-second billing, AWS customers are able to focus on their applications instead of maximizing usage to the hour. As a result, AWS Batch will now evaluate compute resources more frequently and immediately scale down any idle instances when there are no more runnable jobs in your job queues.