Environment

• Qiskit Terra version: 0.19.2
• Python version: 3.8
• Operating system: Linux

What is happening?

There is a large overhead when doing parallel transpilation for circuits targeting larger systems. In the case below they are targeting the 127Q system, but the circuits themselves consist of only x gates and are already the correct width. Thus transpilation should just be a pass-through. However, in parallel mode (quad-core machine) it takes 330sec to transpile. If I turn off parallel transpile it takes 3sec.

from qiskit import *
from mthree.circuits import balanced_cal_strings, balanced_cal_circuits

b_strs = balanced_cal_strings(127)
cal_circs = balanced_cal_circuits(b_strs)

backend = provider.backend.ibm_washington
trans_circs = transpile(cal_circs, backend, optimization_level=0)

How can we reproduce the issue?

Run above

What should happen?

Parallel should not give 100x overhead.

Any suggestions?

I am guessing this is copying overhead from forking when the systems are large.