Why is AWS Batch not giving me an instance, when I have instance type set to ‘optimal’?

When using AWS Batch there can be many reasons why your jobs appear stuck in the runnable status.
So much so, that there’s a whole page in the support section dedicated to it.

I recently had an issue where I had selected the optimal strategy for the instanceTypes on my Compute environment parameters, yet my jobs were appearing stuck in runnable. This didnt make any sense, the reason I choose optimal was to allow AWS to grab any instance for me from a range.

After a chat to the very helpful AWS Support I found out that the documentation was misleading, it says “choose optimal to pick instance types from the C, M, and R instance families”.

What it should says is “Optimal chooses the best fit of M4, C4, and R4 instance types available in the region”.
The CPU and Memory requirements I had set for my container, exceeded what any of those instance types could do.

The AWS support engineer said a ticket has been logged to update the documentation.

References:
https://aws.amazon.com/premiumsupport/knowledge-center/batch-job-stuck-runnable-status
https://stackoverflow.com/questions/48151332/why-are-aws-batch-jobs-stuck-in-runnable
https://stackoverflow.com/questions/52301621/how-do-i-view-aws-batch-compute-environment-errors