2 ms·

One big difference is the c/c++ standards say that messing with a variable in multiple threads without locks is undefined behaviour, while Python claims there a

One big difference is the c/c++ standards say that messing with a variable in multiple threads without locks is undefined behaviour, while Python claims there at least won't be an interpreter crash.

vlovich123 8y ago
I fail to see how what you said is relevant. All you've said is that Python & C/C++'s memory models are different from each other, which yes they are & no one has stated otherwise. My interpretation of the claims made were: 1. Python doesn't have thread-local variables 2. Python's memory model around threading is what prevents any optimization. Both are clearly incorrect. For example, take Java. You have the same behaviour as Python: your VM won't crash & your code won't even generate a panic. Java clearly has thread-local variables & an optimizing compiler. Python's inhibition around optimizing is partially around the guarantees they make around APIs that do self introspection but mainly around keeping the interpreter simple. For a counterpoint see PyPy, Cython, JPython, IronPython etc which optimize Python just fine. They mostly trip over module extensions being tied directly to the non-standardized CPython module API & that API is tied intimately to implementation decisions of CPython.
Animats 8y ago
Right. The options in this area are: - Don't check. Programs will crash if a thread-local variable is passed outside the thread (C, C++) - Naive interpreter. Everything works, slowly, because the worst case code is used for everything. (CPython) - Really clever just-in-time recompilation when what seemed to be thread-local suddenly gets accessed from another thread. (PyPy, Java?) - Compile time checking to prevent this. (Rust) - No threads (classic Javascript) - Explicit shared areas into which only certain types can be placed (ECMAscript 2018)

2 ms·

One big difference is the c/c++ standards say that messing with a variable in multiple threads without locks is undefined behaviour, while Python claims there a

by CJefferson 8y ago

One big difference is the c/c++ standards say that messing with a variable in multiple threads without locks is undefined behaviour, while Python claims there at least won't be an interpreter crash.

vlovich123 8y ago
I fail to see how what you said is relevant. All you've said is that Python & C/C++'s memory models are different from each other, which yes they are & no one has stated otherwise. My interpretation of the claims made were: 1. Python doesn't have thread-local variables 2. Python's memory model around threading is what prevents any optimization. Both are clearly incorrect. For example, take Java. You have the same behaviour as Python: your VM won't crash & your code won't even generate a panic. Java clearly has thread-local variables & an optimizing compiler. Python's inhibition around optimizing is partially around the guarantees they make around APIs that do self introspection but mainly around keeping the interpreter simple. For a counterpoint see PyPy, Cython, JPython, IronPython etc which optimize Python just fine. They mostly trip over module extensions being tied directly to the non-standardized CPython module API & that API is tied intimately to implementation decisions of CPython.
Animats 8y ago
Right. The options in this area are: - Don't check. Programs will crash if a thread-local variable is passed outside the thread (C, C++) - Naive interpreter. Everything works, slowly, because the worst case code is used for everything. (CPython) - Really clever just-in-time recompilation when what seemed to be thread-local suddenly gets accessed from another thread. (PyPy, Java?) - Compile time checking to prevent this. (Rust) - No threads (classic Javascript) - Explicit shared areas into which only certain types can be placed (ECMAscript 2018)