I want to know when AM apply for resource from RM, how does AM know to ask for what amount resource? I have tried to read the code, but it is hard for me to find the answers. Thanks for all your help.
The ApplicationMaster can request whatever it feels like. It is up to the ResourceManager to decide what to actually grant it. In many cases you specify how much it should request. E.g. in spark-submit --driver-memory 5G