u/hconel

Hi everyone,

I regularly run multiple parallel MPI jobs simultaneously on my workstations. I have two systems:

• Intel i7-12700 (12 cores: 8 P-cores + 4 E-cores), OS: Ubuntu 20.04
• AMD Threadripper 3960X (24 cores, 48 threads), OS: Ubuntu 18.04

I wrote a simple C++ MPI test program that runs with mpirun -np 2. On both machines, a single instance finishes in about 12 seconds.

The problem appears when I run two instances at the same time (both mpirun -np 2):

• Threadripper: Both finish in ~12 seconds (no slowdown)
• Intel: Both take ~30 seconds (significant slowdown)

I tried pinning processes to specific cores using taskset and --cpu-set in mpirun. The processes do land on the correct cores (I verified with ps), but the slowdown persists.

Is this expected behavior for Alder Lake? Could the hybrid P-core/E-core architecture be causing memory bandwidth contention? Or am I missing something else?

I'm trying to figure out if my Intel system is performing normally or if I should be hunting for a configuration issue.

Additional notes:

• My code shows reasonable&normal speed-up with increasing core numbers on both systems
• The Intel PC has only one memory stick
• The AMD PC has multiple memory sticks
• My test code is not memory intensive (mostly CPU math)

I can provide more details if needed. I'm not super knowledgeable about CPU architectures, so apologies in advance.

Thanks for any insights!