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Subject: [OMPI users] Low cpu utilization due to high IO operations of openmpi
From: Mahmood Naderan (nt_mahmood_at_[hidden])
Date: 2012-10-21 16:40:57


Dear all,
We have a diskless cluster with these specs:
1) A server which has some disks. Root directories (/usr, /lib, ...) are

on /dev/sda while /home is on /dev/sdb and these are two physical

hard drives.

2) Some compute nodes. These don't have any disk drive instead they

are connected through a 10/100/1000 switch to the server

3) Nodes uses a NFS directory for booting which resides on /dev/sda.

In our cluster we use openmpi with openfoam. Both were compiled using

default options. Problem is when the openfoam solver with openmpi is

sent to a compute node, the opnempi uses a lot of *WRITE* operations
which causes a low cpu utilization and hence the processes are mainly

in 'D' state. A brief description of our tests are:

We ssh to the compute node and run the application there

Test 1) One process of openfoam is launched without openmpi. Everything

is fine and cpu is utilized 100%
Test 2) Two processes of openfoam is launched (mpirun -np 2 ....). Two

openfoam processes has about 30% cpu utilization and they are in 'D'

state most of the time.

Is there any suggestion on that. That is a really poor performance

Regards,
Mahmood