Hi Hui,
Here are the outputs. I tried the following commands
mpirun --version
./cpi
[vy57456@maya-usr1 em_real]$mpirun --version
HYDRA build details:
Version: 3.2.1
Release Date: Fri Nov 10 20:21:01 CST 2017
CC: gcc
CXX: g++
F77: gfortran
F90: gfortran
Configure options: '--disable-option-checking' '--prefix=/umbc/xfs1/zzbatmos/users/vy57456/application/gfortran/mpich-3.2.1' 'CC=gcc' 'CXX=g++' 'FC=gfortran' 'F77=gfortran' '--cache-file=/dev/null' '--srcdir=.' 'CFLAGS= -O2' 'LDFLAGS=' 'LIBS=-lpthread ' 'CPPFLAGS= -I/home/vy57456/zzbatmos_user/application/gfortran/source_code/mpich-3.2.1/src/mpl/include -I/home/vy57456/zzbatmos_user/application/gfortran/source_code/mpich-3.2.1/src/mpl/include -I/home/vy57456/zzbatmos_user/application/gfortran/source_code/mpich-3.2.1/src/openpa/src -I/home/vy57456/zzbatmos_user/application/gfortran/source_code/mpich-3.2.1/src/openpa/src -D_REENTRANT -I/home/vy57456/zzbatmos_user/application/gfortran/source_code/mpich-3.2.1/src/mpi/romio/include' 'MPLLIBNAME=mpl'
Process Manager: pmi
Launchers available: ssh rsh fork slurm ll lsf sge manual persist
Topology libraries available: hwloc
Resource management kernels available: user slurm ll lsf sge pbs cobalt
Checkpointing libraries available:
Demux engines available: poll select
[vy57456@maya-usr1 examples]$./cpi
Process 0 of 1 is on maya-usr1
pi is approximately 3.1415926544231341, Error is 0.0000000008333410
wall clock time = 0.000066
[vy57456@maya-usr1 examples]$mpirun ./cpi
Process 0 of 1 is on maya-usr1
pi is approximately 3.1415926544231341, Error is 0.0000000008333410
wall clock time = 0.000095
[vy57456@maya-usr1 examples]$mpirun -np 1 ./cpi
Process 0 of 1 is on maya-usr1
pi is approximately 3.1415926544231341, Error is 0.0000000008333410
wall clock time = 0.000093
There is no error.
Zhifeng