Job not getting distributed among nodes

Please try this script (kind of induced parallelism)

#cat stress.sh

#PBS -N stress
#PBS -l select=2:ncpus=4:mpiprocs=4
cd $PBS_O_WORKDIR
total_cores=`cat $PBS_NODEFILE | wc -l `
echo "total_cores=$total_cores"
total_hosts=`cat $PBS_NODEFILE | uniq | wc -l`
echo "total_hosts=$total_hosts"
cores_per_host=$((total_cores / total_hosts))
echo "cores_per_host=$cores_per_host"
echo "running stress"
echo "/opt/pbs/bin/pbsdsh -- stress --cpu $cores_per_host  --timeout 100s"
/opt/pbs/bin/pbsdsh -- stress --cpu $cores_per_host  --timeout 100s
echo "ending stress"

#qsub stress.sh