Python Multi-threading IS NOT in Parallel Execution

1 minute read

I did not realize that Python threading is not in parallel execution until my coworker asked me to check the GIL after I complained about a freeze of the entire GUI caused by a network fetching call, which is truly embarrassing. After googling several articles that detail the infamous GIL: What Is the Python Global Interpreter Lock (GIL)? and Introduction to the Infamous Python GIL, I finally got the truth: Python uses a global mutex in its interpreter to protect the reference count of each object from being messed up among multiple threads, which is called Global Interpreter Lock, aka GIL; They chose to use a single lock just because a lock to each object could cause deadlocks(is it so called pythonic? I don’t know); And Yes, the GIL brings performance penalty to CPU bound programs in multi-threading since they will actually be hyper-threaded in one CPU core; As for why not removing the GIL given plenty of complaints from the developers, the answer seems to be “as such a popular language Python cannot bring a change as significant as removal of GIL without causing backward incompatibility issues”1.

Are there any solutions to overcome the problems caused by GIL? Possibly.

  • To use the Python Multi-Processing Package for your CPU bound programs and be prepared for the issues caused by pickling
  • Try alternative Python implementations such as PyPy, however keep in mind that not all your dependent packages will be supported there.
  • If you implement a python module through C++ binding and there happens to be some blocking operations, remember to give back the GIL at the start and acquire it again at the end in the C++ implementation. (This probably is not a solution but a fix after you find one of your call blocks the entire program, which is exactly what I got today)

Is GIL so bad? Maybe not, they claimed you’ll get a performance boost with it for your single thread only program.