yeah. one of the goals was startup time, so It made sense to precreate them. In practice we never ran out of free machines (and if we did, I have a cdk script to make more), and inifnite scaling is a pain in the butt anyways due to having to manage subnets etc.

Cost-wise we're only paying for the EBS volumes for the stopped instances which are like 4GB each, so they cost practically nothing, we spend less than a dollar per month for the whole bunch.

Warm pools are a supported feature in AWS on auto scaling groups. Works as you're describing (have a pool of instances in stopped state ready to use, only pay for EBS volume if relevant) https://aws.amazon.com/blogs/compute/scaling-your-applicatio...

> we spend less than a dollar per month for the whole bunch

This does not change the point, I'm just being pedantic, but:

4GB of gp3 EBS takes $0.32 per month, assuming a 50% discount (not unusual), less than a dollar gives only... 6 instances.

I always thought that stopped instances will cost money as well?!