Q:

The load in some Google App Engine instances and very small in others. Why?

As you can see in the following snapshot the load in some of the dynamic instances is huge (more than 20k requests) while in other are very small. 

Why is this happening? Shouldn't GAE distribute uniformly the load??

A:

This is what I got from a Google App Engine expert:

App Engine request scheduling uses several heuristics for routing requests to application instances. At low QPS it stays in affinity scheduling mode and routes majority of requests to instances that have most recently responded to the health check and handled requests successfully. That would explain why you see this variation in number of requests for each instance. As you ramp up the application traffic, load should even out across all instances.

I also asked what was the policy GAE follow to shut down the instances. I see that many of them are up even if they are not receiving any request

Dynamic instances that are not serving requests get garbage collected eventually. However, you only get billed for 15 additional minutes after they serve the last request. Please refer to this doc for additional information on instance billing.

https://cloud.google.com/appengine/kb/billing#different_on_demand_instance_resident

