Ok, I could-not see the num_workers. But now, given that num_workers=8, explains why you there are 9 processes running. One process is the main process, which creates 8 other subprocesses for loading the data.
Well, there is no specific reason to set num_workers to 8, and I know that the more workers i know,
the faster I / O processes can handle them.
(I refer to the following article.)
I checked as you told me, and I’ve confirmed that the number of num_workers+1 (main process + subprocess num_workers) pid is allocated. Is the usage wrong?
The code looks fine. But whether or not you get any improved performance in I/O speed depends on the amount of work each worker has to do. For example, if the work load is not much, using num_workers=2 and num_workers=8 may have similar speed.