[MPICH] Asking about some details of mpi in a cluster?

[zyh]

Hi,
  I'd like to know some details about mpich in a cluster:
 
1.Is udp or tcp used to communicate when using mpi_send and mpi_recv?
2.I guess when a mpi programm is run by "mpirun -np process-number mpiprogramme",the mpiprogramme is sent to other computers to run.
Is this right?
3.What does mpi_init do?
4.When using mpi_comm_size,how to get the size?I guess there are two ways:
the first is getting from "mpirun -np process-numbler mpiprogramme";the second is after judging all the computers in the file "hostlist" is alive or not.  Which is right?
4.And when using mpi_comm_rank,how to get the rank of every computer?
 
Any help will be greatly appreciated!Many thanks!
 
 
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