Nexus: should default binding option on Summit be -b rs?

[Paul-St-Young]

Describe the bug
Nexus' default bind option on Summit is -b rs.
I observed that -b rs oversubscribes the cores in a pure MPI run.

To Reproduce
Steps to reproduce the behavior:

1. Launch interactive session on Summit
   bsub–W 10 –nnodes1 –P $OLCF_PROJECT_ID –Is $SHELL
2. Load job-step-viewer
   module load job-step-viewer
3. View jsrun with -b rs

Expected behavior
All MPI tasks of a resource set are bound to its first core.
Command: export OMP_NUM_THREAEDS=1; jsrun -n 6 -c 7 -a 7 -b rs -d packed

The problem can be solved by using -b packed:$OMP_NUM_THREADS instead of -b rs
Command: export OMP_NUM_THREAEDS=1; jsrun -n 6 -c 7 -a 7 -b packed:1 -d packed

Is this a real problem or did I misunderstand jsrun?

System:

• summit

[Paul-St-Young]

I should add that -b rs works as intended when each resource set is purely OpenMP
export OMP_NUM_THREAEDS=7; jsrun -n 6 -c 7 -a 1 -b rs -d packed
appropriately binds one thread to each core

[ye-luo]

You may use qmc-check-affinity to access exact affinity. SMT1 not used?

[prckent]

Closing since Summit is no more.