Hi Nicholas,

Assuming ParaView is using the same behavior as upstream hydra

-np 8 should launch 8 processes, 4 on each node
-np 4 should launch 4 processes, 2 on each node
To launch 4 processes on one node, try -hosts compute0:4,compute1:4 -np 4
The number 4 behind each host instructs hydra to assign the first 4 ranks to the host before moving on to another assignment, the default is 1. Hydra uses round-robin assignment policy

Hope it helps,

Hui

From: Nicholas Yue via discuss <discuss@mpich.org>
Sent: Friday, May 6, 2022 11:11 AM
To: discuss@mpich.org <discuss@mpich.org>
Cc: Nicholas Yue <yue.nicholas@gmail.com>
Subject: [mpich-discuss] mpiexec : multi nodes and multi-cores questions

Hi,

I am using mpiexec from Paraview which from the `help` print out, indicates that it is derived from MPICH

```

Please see the intructions provided at
http://wiki.mpich.org/mpich/index.php/Using_the_Hydra_Process_Manager
for further details

```

so I thought I might ask some questions on this mailing list.

I have set up a two node cluster, each compute node has 4 cores

To fully utilise all the cores in both nodes, is this the correct way to launch pvserver ?

```
/shared/ParaView-5.10.1-osmesa-MPI-Linux-Python3.9-x86_64/bin/mpiexec -hosts compute0,compute1 -np 8 /shared/ParaView-5.10.1-osmesa-MPI-Linux-Python3.9-x86_64/bin/pvserver
```

If I change to `-np 4` would mpiexec use only compute0 or would it still run on both compute0 and compute1 but use only 2 cores of each node ?

Cheers
--

Nicholas Yue
https://www.linkedin.com/in/nicholasyue/