Google App Engine - tasks per service instance

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I have an Google App Engine module with max_instances: 15. I want task queues that work on that service to be concurrent, eg: now only 15 tasks are running at once. How can i let app engine now that more tasks could be run in the same time, not one per instance?

The way it works now is 1/instance so if i set max_instances to 150 it would fire up 150 instances and work on 150 tasks. But those tasks are not CPU heavy so multiple ones could be run on the same instance.

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