set --num_jobs not working during MFA training

[Keshawn Hsieh]

I trained acustic model using command like below

mfa train --num_jobs 200 -v -t $exp_dir/tmp $train_corpus_data $lexicon $am_model

However,  though I set the --num_jobs parameter, the top comand shows that the MFA still run in single thread mode and the CPU load is very low. How can I run MFA training in multi-thread mode?