Python's GIL
Dec 10 2015
5 minutes read

Recently I got very interested to explore about the GIL or the Global Interpreter Lock in Python that has been a topic of flame wars since forever. Global interpreter lock is a lock on the python interpreter, that a thread must acquire in order to run. Because of this, effectively any python program can have only one thread running at a time on one instance of python interpreter. However, it has been a common belief that threaded programming in Python doesn’t make any sense at all due to GIL, but that is not actually true. I might use process and thread in context of a single python interpreter instance interchangeably in this post.

Not all kind of processes are affected by GIL heavily. I/O bound process or in general any kind of Blocking process don’t always1 perform poorly, in-fact their performance can be as good as a C application. Since python threads are actual OS threads (Posix or pthreads in *nix based systems) and thus they are scheduled by the operating system (OS) scheduler. Depending on the OS scheduler, the performance can be good or bad in multi-threaded situations. In Linux, if more than one threads are waiting for a lock, there is no specific order in which those threads would be woken up when the lock becomes available. Usually, all the threads are allowed to wake up and then the scheduler randomly (probably? not sure about this on linux) picks up a process to run. It is possible, and very often happens, that the thread that just released the lock acquires it again increasing the wait time for other threads. This behavior is different on Mac or Windows or FreeBSD operating systems and is dependent on the scheduler. It looks like Mac OSX tries to do the fair scheduling and thus the max wait time for a thread would vary linearly depending on the number of threads running in Mac OSX. Windows and FreeBSD do have priority based scheduling, but how performance on these servers are affected when using a multi-threaded python application as compared to a sequential one is something I am not sure about. If they implement priority scheduling, it is possible they would fairly schedule the threads.

Before Python 3.2 the GIL was allowed to be retained with a thread for 100 instructions, after that the thread had to give up the GIL. But, the point here is, they just had to give up the GIL, no one is stopping them from acquiring it again. Due to this, the threads had to wait to much longer time than 100 instructions and would result in poor performance. Also, notice that instruction is not a fair unit of CPU time for processes, as time taken by each instruction might vary. Notice that I am talking about CPU time and not actual time taken by the thread, that depends on several other factors like the CPU load.

So, in Python 3.2, a new version of GIL was introduced which tried to fix some of the issues that I mentioned above. Firstly, the amount of time a process was allowed to hold the GIL was fixed to 5 ms. I am not sure if this was inspired by Ruby’s GIL which allows each thread to have the lock for 10ms. The choice of time for which a thread is allowed to hold the lock depends on performance and lock switching overhead. If the time is too less, the lock would be acquired and released so often that it would increase the overhead for switching and very less actual work would be done as the none of the process would get sufficient time to do any piece of work before they are preempted and forced to give up the GIL. If the lock is acquired by a single process for too long, other process would have to wait for longer and this would kill the performance. the choice of 5 ms was probably based on some statistical experiment that would maximize the productivity with the minimum lock switching overhead.

Another thing that was added in the new GIL was how the locks are switched. If the threads don’t give up the GIL on their own before their time is up, they are forced to give up the GIL. Which threads gets the GIL next depends on the OS scheduler and is not controlled by Python. The way the process is forced to give up the lock is multi-step mechanism. However, a new concept of priority request was also introduced. Priority request would immediately force the running process to give up the GIL and would acquire it. This whole process makes sure the original thread can not acquire the lock again just after releasing it. While this might sound like the right thing to do (it did sound to me), in some cases this brings down productivity.

I am not going to get into the implementation specific details of GIL, probably in some another post. But, I would like to mention that it is a more complex than just a simple mutex lock from pthreads. It uses a combination of mutex and conditional variables.

Footnotes:

1: This is true only when there is no CPU bound process running at the same time on the same Python interpreter instance. It would cause the CPU bound process to run whenever the I/O bound process enters a wait state for an external I/O and then make them wait for longer time than they actually need to.

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