Feature: set OpenMP thread num to a proper default value

[caic99]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #3701

Unit Tests and/or Case Tests for my changes

• A unit test is added for each new feature or bug fix.

What's changed?

ABACUS will try to set a proper OpenMP threading value if OMP_NUM_THREADS is not set, and exit with error if the value set is beyond hardware availability (which should not happen since the value has been automatically set).

[caic99]

Could someone please help me on this error building tests?

[QuantumMisaka]

@caic99 I have some test by using this repo. It worked well , but if the machine has hyper-thread (like WSL or bohrium image), the product of OMP number and MPI number will include hyper-thread and lead to lower parallel efficiency than the hardware-core (hyper-thread / 2) situation.

[caic99]

@caic99 I have some test by using this repo. It worked well , but if the machine has hyper-thread (like WSL or bohrium image), the product of OMP number and MPI number will include hyper-thread and lead to lower parallel efficiency than the hardware-core (hyper-thread / 2) situation.

It's hard to determine with standard c++ API whether hyperthreading is used.

[mohanchen]

Any updates?