GAE keep starting new processes at nearly every re

Question

I met a very rare situation today that my GAE project keep starting a new processes even when every request coming, this is really weird, from the server log, it give me below warning so many times in just one minute:

    This request caused a new process to be started for your application, and thus caused 
    your application code to be loaded for the first time. This request may thus take
    longer and use more CPU than a typical request for your application. 

As the starting of a new process takes 3-4 seconds, so the result is really bad, so much latency. I have another project with more traffic than this one but no such issue. Anyone who can tell me what's the cause of this nasty problem and how to solve this? Many thanks.