Hi,
I am afraid this is possibly expected behaviour. The tuning of the threshold when multithreading should be used or not on CPU is qui complex to tweak for the wide range of architecture, core count…
Setting manually torch.set_num_threads() is the right option if you know how many cores your program should use. Especially when working on cpu with large core count, the thresholds can be low.
If by any chance you can provide a simple code sample to reproduce this, we can check if the involved threshold is actually correct or if it is an error.