How to manage process distribution in mpich - parallel-processing

Question

I am running mpi code on host1(quad core) and host2(dual core)
mpiexec -hosts host1,host2 -n 6 ./mytask
I want to assign 4 processes for host1 and 2 for host2. I tried --map-by core but I found that the processes are distributed 3 for each.
This is the mpiexec help output
mpiexec -h
Usage: ./mpiexec [global opts] [local opts for exec1] [exec1] [exec1 args] : [local opts for exec2] [exec2] [exec2 args] : ...
Global options (passed to all executables):
Global environment options:
-genv {name} {value} environment variable name and value
-genvlist {env1,env2,...} environment variable list to pass
-genvnone do not pass any environment variables
-genvall pass all environment variables not managed
by the launcher (default)
Other global options:
-f {name} file containing the host names
-hosts {host list} comma separated host list
-wdir {dirname} working directory to use
-configfile {name} config file containing MPMD launch options
Local options (passed to individual executables):
Local environment options:
-env {name} {value} environment variable name and value
-envlist {env1,env2,...} environment variable list to pass
-envnone do not pass any environment variables
-envall pass all environment variables (default)
Other local options:
-n/-np {value} number of processes
{exec_name} {args} executable name and arguments
Hydra specific options (treated as global):
Launch options:
-launcher launcher to use (ssh rsh fork slurm ll lsf sge manual persist)
-launcher-exec executable to use to launch processes
-enable-x/-disable-x enable or disable X forwarding
Resource management kernel options:
-rmk resource management kernel to use (user slurm ll lsf sge pbs cobalt)
Processor topology options:
-topolib processor topology library (hwloc)
-bind-to process binding
-map-by process mapping
-membind memory binding policy
Checkpoint/Restart options:
-ckpoint-interval checkpoint interval
-ckpoint-prefix checkpoint file prefix
-ckpoint-num checkpoint number to restart
-ckpointlib checkpointing library (none)
Demux engine options:
-demux demux engine (poll select)
Other Hydra options:
-verbose verbose mode
-info build information
-print-all-exitcodes print exit codes of all processes
-iface network interface to use
-ppn processes per node
-profile turn on internal profiling
-prepend-rank prepend rank to output
-prepend-pattern prepend pattern to output
-outfile-pattern direct stdout to file
-errfile-pattern direct stderr to file
-nameserver name server information (host:port format)
-disable-auto-cleanup don't cleanup processes on error
-disable-hostname-propagation let MPICH auto-detect the hostname
-order-nodes order nodes as ascending/descending cores
-localhost local hostname for the launching node
-usize universe size (SYSTEM, INFINITE, <value>)
Please see the intructions provided at
http://wiki.mpich.org/mpich/index.php/Using_the_Hydra_Process_Manager
for further details

Answer 1

There are several options.
pi#RPi:~ $ mpiexec -H rpi,rpi,rpi,rpi5,rpi7,rpi7 -np 6 helloworld.py
Hello World! I am process 3 of 6 on RPi5.
Hello World! I am process 5 of 6 on RPi7.
Hello World! I am process 4 of 6 on RPi7.
Hello World! I am process 0 of 6 on RPi.
Hello World! I am process 2 of 6 on RPi.
Hello World! I am process 1 of 6 on RPi.
The hostfile with the -hostfile filename.
pi#RPi:~ $ cat filename
RPi slots=4 max_slots=4
RPi5 slots=2 max_slots=2
RPi7 slots=4 max_slots=4
Also, use the -nooversubscribe option.