Embracing The GIL¶
- Railed on GIL at PyCon, thought it deserved some Love
- Godwin’s Law of Python
- Fun hard systems problem
- Likes to break GILs as a hobby
Threads are Useful¶
- People love to hate on threads...
- Because they are being used.
- They solve tricky problems.
In A Nutshell¶
- Python code -> VM instructions
- Can’t execute VM instructions concurrently, therefore locking
- Keep things safe * Ref counts * Mutable types * Internal bookkepping * Thread safety
- All low level.
An Experiment in Messaging¶
Comes up in a lot of contexts
Foundation for working around the GIL
- Shows an experiment in messaging using 5 implementations
- C + ZeroMQ
- Python + ZeroMQ (C extensions)
- Python + multiprocessing
- Python + blocking sockets
- Python + nonblocking sockets
Tested on xlarge EC2 instance.
- Unloaded server
- Expect ~10 seconds (10 seconds of sleep in there)
- All roughly the same (~13 seconds for each)
- Shows a real time example
- Implement a thread to calculate Fib(200) (Referencing Node.js is a cancer)
- C version is barely affected.
- Python blocking goes to 142 seconds.
- Real time example takes a long time.
- Try Pypy
- Test on Pypy (6k seconds)
- fixed in trunk
- Try 2.7
- Within 2x
- Uses Idle with threads (esp CPU bound)
- Kills performance to the point of completely unusable
- Can barely type into Idle
- GIL aquisition based on timeout
- Thread that want the GIL must wait 5ms
- Causes a problem on release
- 5ms delays build up
What’s Really Happening¶
- Before send and recv, acquire GIL
- After release
How to Fix¶
- Thread priorities
- Was in the original “New GIL” patch
- Should be revisited
- Has an experimental python 3.2 with priorities
- Really minimal
- Threads can set their priority
- Performance that is comparable to version without threads.
- Makes GUI completely usable.
- Tried with 1.4k threads.
- Huge boost in performance with few modifications
- Not the only way to improve the GIL
- Example: Should the GIL release on nonblocking IO?
- Currently releases on every IO
- If you are doing nonblocking IO, you aren’t blocking.
- Python programmers should be interested in improving the GIL
- Doesn’t have to be huge, incremental.
- Code available via version control
- Did you do an academic paper on this?
- No, but I think there is room for it.
- The interesting question is if the OS thread library gives enough help to languages with a GIL.
- Could it cooperate to tell the thread that it will be context switched.
- At Pycon, OS kernel hackers came to talk about this.
- Should say “Fixing the GIL is impossible”.
- Is priority code production runnable?
- Threads cannot quit.