How to use backends in google app engine without wasting cpu resources?
The general advice for optimizing CPU usage is to minimize RPCs, understand how to use the datastore efficiently and use appstats to find your bottlenecks. For specific optimization advice, we would need to see some code.
While backends can be configured to handle public requests, they aren't intended to replace normal instances. Backends are designed for resource-intensive offline processing. Normal instances are created and destroyed automatically in response to request volume; backends have to be configured and instantiated explicitly by an administrator, thus they are not well-suited to handling traffic spikes.
They're also more expensive: keeping a backend instance online for 24 hours will cost you $3.84, whether the instance is handling requests or not.
While processing data in datastore using backends, app engine is using my cpu resources completely. 
How do i process my data without wasting CPU resources?
Can i have the entire app on a backend without wasting cpu resources?
am i missing something..
if the question is too vague, ask me to clarify.. thanks