Q-Logic IB6054601-00 D 用户手册

3 – Using InfiniPath MPI

Getting Started with MPI

IB6054601-00 D

3-3

Q

Here

./cpi

 designates the executable of the example program in the working 

directory. The -np parameter to 

mpirun

 defines the number of processes to be 

used in the parallel computation. Now try it with four processes:

$ mpirun -np 4 -m mpihosts ./cpi

Process 3 on hostname1

Process 0 on hostname2

Process 2 on hostname2

Process 1 on hostname1

pi is approximately 3.1416009869231249, 

Error is 0.0000083333333318

wall clock time = 0.000603 

If you run the program several times with the same value of the 

-np

 parameter, you 

may get the output lines in different orders. This is because they are issued by 
independent asynchronous processes, so their order is non-deterministic.

The number of processes can be greater than the number of nodes. In this 
four-process example, the 

mpihosts

 file listed only two hosts, hostname1 and 

hostname2. Generally, 

mpirun

 will try to distribute the specified number of 

processes evenly among the nodes listed in the 

mpihosts

 file, but if the number of 

processes exceeds the number of nodes listed in the 

mpihosts

 file, then some 

nodes will be assigned more than one instance of the program.    

Up to a limit, the number of processes can even exceed the total number of 
processors on the specified set of nodes, although it is usually detrimental to 
performance to have more than one node program per processor. This limit is eight 
processes per node with the QHT7140, and four processes per node with the 
QLE7140. See 

 for further discussion.

Details on alternate means of specifying the 

mpihosts

 file are given in 

. 

Further information on the 

mpirun

 and 

This section gives more examples, one for Fortran77, one for Fortran90, and one 
for C++. Fortran95 usage will be similar to that for Fortran90.

fpi.f

 is a Fortran77 program that computes pi in a way similar to 

cpi.c

. Compile 

and link it with:

$ mpif77 -o fpi3 fpi3.f

and run it with:

$ mpirun -np 2 -m mpihosts ./fpi3

pi3f90.f90

 in the same directory is a Fortran90 program that does essentially the 

same computation. Compile and link it with:

$ mpif90 -o pi3f90 pi3f90.f90