[mpich-discuss] How to handle complex variables

[Antonin@LPS]

Hi Jayesh,

Thanks for your reply.
I'm using MPICH2-1.2.1p1 (not the latest...) on a Unix 64 with ifort 
11.1. I get no error message when I use the basic compilation "mpif90 
MPItest.f90" with no further options.
Here I paste the output of the code, using complex(8) and a ring of 2 
processes:

  BCAST with count=1*MPI_COMPLEX: (1.50000000000000,0.500000000000000)
  Proc.           1  received (1.50000000000000,0.000000000000000E+000)
  BCAST with count=2*MPI_COMPLEX: (1.50000000000000,0.500000000000000)
  Proc.           1  received (1.50000000000000,0.500000000000000)
  BCAST with count=1*MPI_DOUBLE_COMPLEX: 
(1.50000000000000,0.500000000000000)
  Proc.           1  received (1.50000000000000,0.500000000000000)

  REDUCE with count=1*MPI_DOUBLE_COMPLEX
  Proc.           0  has (1.50000000000000,0.500000000000000)
  Proc.           1  has (2.50000000000000,1.50000000000000)
... and here the program hangs...

You can see that, when using count=1 and datatype=MPI_COMPLEX on a 
complex(8) variable, only the real part is broadcasted (that is, the 
first data in memory that has the length of a double. The whole 
complex(8) variable, which is twice larger than a double, is only 
broadcasted when using count=2 with datatype=MPI_COMPLEX, or count=1 
with datatype=MPI_DOUBLE_COMPLEX.
Declaring the variable as "double complex" instead of "complex(8)" is 
exactly equivalent.
If I declare a "complex(16)", which is twice larger than a complex(8), 
nothing is broadcasted when using count=1*MPI_COMPLEX, and only the real 
part is broadcasted when using count=2*MPI_COMPLEX or 
count=1*MPI_DOUBLE_COMPLEX.

It looks like something is wrong with the "size" of the complex 
variables, doesn't it?
I hope these indications are helpful...

Regards,
Antonin


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