Job executing problem on RBPi with Raspbian Stretch in _parallel.py

[Tomev]

Informations

• Qiskit Terra version: 0.6.1
• Python version: 3.6
• Operating system: Raspbian Stretch

What is the current behavior?

When trying to execute job on my Raspberry Pi B I get following error in _parallel.py:

"... _parallel.py", line 109, in parallel_map
if platform.system() != 'Windows' and num_processes > 1
TypeError: '>' not supported between instances of 'NoneType' and 'int'

From what I understand initialization of num_processes (or rather it's default value - CPU_COUNT = local_hardware_info()['cpus']) is a problem on selected os.

Steps to reproduce the problem

Try to execute a job on Raspbian Stretch on RBPi.

What is the expected behavior?

The job gets executed.

Suggested solutions

Check if num_processes is an initialized int before comparison.

[nonhermitian]

It seems that psutil cannot determine the physical cpu count on your system. The solution is to probably fall back to num_processes=1 if it cannot be determined

[charlesastaylor]

I can make this fix I think.

Checking psutil.cpu_count on my pi I noticed that cpu_count(logical=False) returns None but cpu_count(logical=True) returns 4. Is it worth trying value of logical=True when logical=False returns None or just default straight to 1?

[mtreinish]

Opened an upstream issue in psutil for this: giampaolo/psutil#1359