the auto-scaling feature can take a while to react to increased load, which makes it hard to run tests against. While you might have already found these tips, I’d definitely recommend 1) running your load test for at least a few minutes and 2) making sure your [services.concurrency] block is set to type “requests” as an initial troubleshooting step.
We’d love if autoscale were a bit more responsive; we’re currently working on making some changes to how we schedule instances which we think will help a great deal with autoscale performance.
on a somewhat related note, we’re running some scheduled maintenance today which involves rebooting a significant number of hosts in our fleet. I don’t think that’s interfering with autoscaling, but if you’re doing testing today you might notice a few minutes of downtime with your app’s individual VMs.