multithreading - A question on python GIL

Question

Does the presence of python GIL imply that in python multi threading the same operation is not so different from repeating it in a single thread?.

For example, If I need to upload two files, what is the advantage of doing them in two threads instead of uploading them one after another?.

I tried a big math operation in both ways. But they seem to take almost equal time to complete.

This seems to be unclear to me. Can someone help me on this?. Thanks.

Answer 1

Python's threads get a slightly worse rap than they deserve. There are three (well, 2.5) cases where they actually get you benefits:

• If non-Python code (e.g. a C library, the kernel, etc.) is running, other Python threads can continue executing. It's only pure Python code that can't run in two threads at once. So if you're doing disk or network I/O, threads can indeed buy you something, as most of the time is spent outside of Python itself.

• The GIL is not actually part of Python, it's an implementation detail of CPython (the "reference" implementation that the core Python devs work on, and that you usually get if you just run "python" on your Linux box or something.

  Jython, IronPython, and any other reimplementations of Python generally do not have a GIL, and multiple pure-Python threads can execute simultaneously.

• The 0.5 case: Even if you're entirely pure-Python and see little or no performance benefit from threading, some problems are really convenient in terms of developer time and difficulty to solve with threads. This depends in part on the developer, too, of course.