Yes point to point communication is OK with quad prec. and one-way
reductions as well. I also tried my sample code on another platform
(which sports AMD opterons instead of Intel CPUs) with the same compilers,
and get the same *wrong* results with the call to MPI_ALLREDUCE in quad
prec, so it does not seem to be a machine bug. Also altering a
bit my sample code so as to replace MPI_SUM by MPI_MAX in the call to
MPI_ALLREDUCE works perfectly well in quad prec !!!
On Tue, 28 Oct 2008, Terry Frankcombe wrote:
I assume you've confirmed that point to point communication works
happily with quad prec on your machine? How about one-way reductions?
On Tue, 2008-10-28 at 08:47 +0000, Julien Devriendt wrote:
Thanks for your suggestions.
I tried them all (declaring my variables as REAL*16 or REAL(16)) to no
avail. I still get the wrong answer with my call to MPI_ALLREDUCE.
I think the KINDs are compiler dependent. For Sun Studio Fortran, REAL*16
and REAL(16) are the same thing. For Intel, maybe it's different. I don't
know. Try running this program:
double precision xDP
real(16) x16
real*16 xSTAR16
write(6,*) kind(xDP), kind(x16), kind(xSTAR16), kind(1.0_16)
end
and checking if the output matches your expectations.
Jeff Squyres wrote:
I dabble in Fortran but am not an expert -- is REAL(kind=16) the same as
REAL*16? MPI_REAL16 should be a 16 byte REAL; I'm not 100% sure that
REAL(kind=16) is the same thing...?
On Oct 23, 2008, at 7:37 AM, Julien Devriendt wrote:
Hi,
I'm trying to do an MPI_ALLREDUCE with quadruple precision real and
MPI_SUM and open mpi does not give me the correct answer (vartemp
is equal to vartored instead of 2*vartored). Switching to double
precision
real works fine.
My version of openmpi is 1.2.7 and it has been compiled with ifort v10.1
and icc/icpc at installation
Here's the simple f90 code which fails:
program test_quad
implicit none
include "mpif.h"
real(kind=16) :: vartored(8),vartemp(8)
integer :: nn,nslaves,my_index
integer :: mpierror
call MPI_INIT(mpierror)
call MPI_COMM_SIZE(MPI_COMM_WORLD,nslaves,mpierror)
call MPI_COMM_RANK(MPI_COMM_WORLD,my_index,mpierror)
nn = 8
vartored = 1.0_16
vartemp = 0.0_16
print*,"P1 ",my_index,vartored
call MPI_ALLREDUCE
(vartored,vartemp,nn,MPI_REAL16,MPI_SUM,MPI_COMM_WORLD,mpierror)
print*,"P2 ",my_index,vartemp
stop
end program test_quad
Any idea why this happens?
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