On 2020-06-05 16:32, Mark Shannon wrote:
Hi,
There have been a lot of changes both to the C API and to internal implementations to allow multiple interpreters in a single O/S process.
These changes cause backwards compatibility changes, have a negative performance impact, and cause a lot of churn.
While I'm in favour of PEP 554, or some similar model for parallelism in Python, I am opposed to the changes we are currently making to support it.
What are sub-interpreters? --------------------------
A sub-interpreter is a logically independent Python process which supports inter-interpreter communication built on shared memory and channels. Passing of Python objects is supported, but only by copying, not by reference. Data can be shared via buffers.
Here's my biased take on the subject: Interpreters are contexts in which Python runs. They contain configuration (e.g. the import path) and runtime state (e.g. the set of imported modules). An interpreter is created at Python startup (Py_InitializeEx), and you can create/destroy additional ones with Py_NewInterpreter/Py_EndInterpreter. This is long-standing API that is used, most notably by mod_wsgi. Many extension modules and some stdlib modules don't play well with the existence of multiple interpreters in a process, mainly because they use process-global state (C static variables) rather than some more granular scope. This tends to result in nasty bugs (C-level crashes) when multiple interpreters are started in parallel (Py_NewInterpreter) or in sequence (several Py_InitializeEx/Py_FinalizeEx cycles). The bugs are similar in both cases. Whether Python interpreters run sequentially or in parallel, having them work will enable a use case I would like to see: allowing me to call Python code from wherever I want, without thinking about global state. Think calling Python from an utility library that doesn't care about the rest of the application it's used in. I personally call this "the Lua use case", because light-weight, worry-free embedding is an area where Python loses to Lua. (And JS as well—that's a relatively recent development, but much more worrying.) The part I have been involved in is moving away from process-global state. Process-global state can be made to work, but it is much safer to always default to module-local state (roughly what Python-language's `global` means), and treat process-global state as exceptions one has to think through. The API introduced in PEPs 384, 489, 573 (and future planned ones) aims to make module-local state possible to use, then later easy to use, and the natural default. Relatively recently, there is an effort to expose interpreter creation & finalization from Python code, and also to allow communication between them (starting with something rudimentary, sharing buffers). There is also a push to explore making the GIL per-interpreter, which ties in to moving away from process-global state. Both are interesting ideas, but (like banishing global state) not the whole motivation for changes/additions. It's probably possible to do similar things with threads or subprocesses, sure, but if these efforts went away, the other issues would remain. I am not too fond of the term "sub-interpreters", because it implies some kind of hierarchy. Of course, if interpreter creation is exposed to Python, you need some kind of "parent" to start the "child" and get its result when done. Also, due to some practical issues you might (sadly, currently) need some notion of "the main interpreter". But ideally, we can make interpreters entirely independent to allow the "Lua use case". In the end-game of these efforts, I see Py_NewInterpreter transparently calling Py_InitializeEx if global state isn't set up yet, and similarly, Py_EndInterpreter turning the lights off if it's the last one out.