@Alexey_Demyanchuk ,
Thank you!
For the first 2 columns, the RAM is 16G. when I changed num_worker=4, there is OOM issue. So, I add a new 32G RAM to my machine. So, those 4 columns are run in the same machine.
You explanation above is the only possibility that we could think of.
what is your machine configuration?
what is your test data? for example, what is the time needed for each iteration/batch?