Instead of calling into the Python dispatcher for EVERY dispatcher
call, we now have a two step process. First, we
getattr(op: OpOverload, dispatch_key) to "load" the handler for the
function. This can either be a conventional function (in which
case we will call it, in the same way the old Python dispatcher
worked), or it can be a DispatchKey, in which case we will directly
call that DispatchKey in C++, bypassing marshalling between Python
and C++ entirely. OpOverload.__getattr__ is carefully written so
that it will cache the
A further optimization would be to define __slots__ on OpOverload,
and ensuring that the DispatchKey strings are interned.
The resulting Python dispatcher is less flexible: after the first
lookup, the handler is cached and we won't recompute it. Furthermore,
by default, dispatches will not go into Python, and so you won't
get stack frames for the Python dispatcher by default. But we get
a huge performance improvement: on the following microbenchmark
we go from 2.5s to 1.9s.
```
import time
import torch
from functorch import make_fx
def f(x):
for i in range(1000):
x = x * x
return x
begin = time.time()
res = make_fx(f, tracing_mode="symbolic")(torch.randn(10, 20))
print(time.time()-begin)
```
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85133
Approved by: https://github.com/wconstab
Signed-off-by: Edward Z. Yang <ezyangfb.com>
From @ezyang's original PR:
There are a number of situations where we have non-backend kernels (e.g., CompositeImplicitAutograd, batching rules) which we would like to port to Python, but we have no way to integrate these ports with the overall system while using preexisting C++ registrations otherwise. This PR changes that by introducing a Python dispatcher (which can have its own kernels directly in Python), which can be interpose over ordinary C++ dispatch. The ingredients:
We introduce a new PythonDispatcher dispatch key, that has the same tenor as FuncTorchDynamicLayerFrontMode: it works by getting triggered before every other dispatch key in the dispatch key, and shunting to a Python implementation
The Python dispatcher is a per-interpreter global object that is enabled/disabled via the guard EnablePythonDispatcher/DisablePythonDispatcher. We don't make it compositional as I have no idea what a compositional version of this feature would look like. Because it is global, we don't need to memory manage it and so I use a simpler SafePyHandle (newly added) to control access to this pointer from non-Python C++. Like __torch_dispatch__, we use PyInterpreter to get to the Python interpreter to handle the dispatch.
I need to reimplement dispatch table computation logic in Python. To do this, I expose a lot more helper functions for doing computations on alias dispatch keys and similar. I also improve the pybind11 handling for DispatchKey so that you can either accept the pybind11 bound enum or a string; this simplifies our binding code. See https://github.com/pybind/pybind11/issues/483#issuecomment-1237418106 for how this works; the technique is generally useful.
I need to be able to call backend fallbacks. I do this by permitting you to call at a dispatch key which doesn't have a kernel for the operator; if the kernel doesn't exist, we check the backend fallback table instead.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84826
Approved by: https://github.com/ezyang
