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points by gopalv 2 days ago | 0 comments

Python is harder to JIT because of the way __dict__ works.

If all python had was __slots__ then we would not be struggling to JIT python.

Tight numerical loops is where the JIT adds performance.

Most performance sensitive features are already available in python between numpy, numba and Cython without really leaving the language core.

You'll mostly be speeding up the slow parts of python, not supercharging the fast parts.

[–] hermitdev 2 days ago

> Python is harder to JIT because of the way __dict__ works.

Heh, how about how `__class__` works? Not that I recommend actually doing this, but you can literally change the type of an already instantiated object by reassigning `__class__`:

    >>> class Dog:
    ...     def hi(self):
    ...             print("woof")
    ...
    >>> class Cat:
    ...     def hi(self):
    ...             print("meow")
    ...
    >>> dog = Dog()
    >>> dog.hi()
    woof
    >>> dog.__class__ = Cat
    >>> dog.hi()
    meow
    >>> isinstance(dog, Dog)
    False
    >>> isinstance(dog, Cat)
    True
    >>>

[–] samatman 2 days ago

Lua has the same property with metatables. LuaJIT, which is very fast, has a solution: if you change a metatable after assigning it, or reassign a metatable, Mike Pall turns his back on you. Your code dies slowly, in the outer darkness of the interpreter loop, never to be JIT’ted upon again.

[–] sakesun 2 days ago

I've been doing things in Python for over 20 years. Only recently I learn that I can do this.

    >>> Dog.__bases__ = (Cat,)
    >>> isinstance(dog, Dog)
    True
    >>> isinstance(dog, Cat)
    True
    >>>

[–] pjmlp 1 day ago

Smalltalk, Self, Common Lisp, Ruby as well, no big deal in regards to JIT code.

[–] gregdeon 2 days ago

This is wild. Is it part of Python's specification, or an implementation detail of CPython?

[–] sakesun 2 days ago

As far as I understand, CPython implementation is THE specification. Some billions dollar ERP system firmly rely on this behavior, btw.

[–] gregdeon 1 day ago

Yeah, I was afraid you would say that.

[–] sakesun 1 day ago

Just check with pypy and Graalpy. Both support this as well.

[–] dcrazy 1 day ago

Is the question whether multiple base classes are part of Python’s spec, or if the ability to assign to `__class__` is part of the spec?

[–] esprehn 2 days ago

That's also possible in JS with __proto__. The prototypes of the classes in the chain can be mutated at runtime too. How to handle these cases is well understood for JITs though and python could do the same.

See also the warning here:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

And this related article:

https://mathiasbynens.be/notes/prototypes

[–] mirashii 2 days ago

The thing that actually makes it harder in Python is threading, because it breaks all ability to reason locally about what might happen and optimize. In Python you have to assume that another thread may do these things at any point mid-execution.

[–] lefra 1 day ago

I see no issue here.

Modifying a variable that is read by another thread needs special care in all multithreaded languages I know. If you don't use a synchronization primitive to tell the other thread "hey, this thing here may have changed, read it again from memory"; the other thread is free to use its old value for as long as it whishes.

[–] pjmlp 2 days ago

Smalltalk, Self and Lisp Machines enter the bar, with live image changes.

[–] cpburns2009 2 days ago

I've never bought this argument. Underneath all of the magic inheritance in Python is just the MRO (method resolution order). That's not characteristically any different from JavaScript's prototype chain. There's no reason a Python JIT couldn't optimize everything JavaScript does, and if you do specifically access `self.__dict__`, that can be a cold, unoptimizable path.

[–] hinkley 2 days ago

One of the tricks here is to treat the happy path as a quasi-invariant and leave a guard that branches to slow code if the invariant didn’t hold.

I saw this presented as new tech at either OOPSLA or OSCon almost exactly 20 years ago.

CPU branch prediction is still pretty good even with the loss of function in recent years.

[–] Sesse__ 2 days ago

That works fine as long as nobody actually uses the unhappy path. If major libraries and frameworks do it all the time, you lose. So there is a certain aspect of culture outside what the language itself is capable of doing.

[–] hinkley 15 hours ago

As cpburns2009 says, the benefit of inlining code (which this technique often applies to) is that you make copies at multiple call sites and so they don’t have to share the same hottest branch.

[–] cpburns2009 1 day ago

If there's common real world paths that need to be supported, you can tailor optimizations to target those uses cases.

[–] gobdovan 2 days ago

I'd say that's about right. The way people usually code in Python, mro is quite stable and most lookups would be cache hits. You can still get more weird invalidation in Python cuz they let you change stuff like `__bases__`, or you get some weird `__getattribute__` change in some place that would force you to invalidate more than you'd initially think should be necessary.

[–] rurban 2 days ago

Same as for the common lisp or raku mop. You check for the default, and use the fast path if so. And if you can guarantee that nobody changed those keys or values, lift that check to the front.

[–] pjmlp 2 days ago

See Self.