TorchDynamo is a Python-level JIT compiler designed to make unmodified PyTorch programs faster. TorchDynamo hooks into the frame evaluation API in CPython (PEP 523) to dynamically modify Python bytecode right before it is executed. It rewrites Python bytecode in order to extract sequences of PyTorch operations into an FX Graph which is then just-in-time compiled with an ensemble of different backends and autotuning. It creates this FX Graph through bytecode analysis and is designed to mix Python execution with compiled backends to get the best of both worlds: usability and performance.

Links for more information and development progress updates:

• Update 1: An Experiment in Dynamic Python Bytecode Transformation
• Update 2: 1.48x Geomean Speedup on TorchBench CPU Inference
• Update 3: GPU Inference Edition
• Update 4: LazyTensor & nvFuser Experiments
• Update 5: Improved Capture & Bigger Graphs
• Update 6: Training support with AOTAutograd
• Update 7: Inference with FX2TRT
• (Video) Live deep-dive into TorchDynamo

TorchDynamo is experimental and under active development. You are welcome to try it out and contribute, but should expect to find bugs and rough edges.

Python 3.8 is recommended. Python 3.7 through 3.10 are supported and tested.

PyTorch's main branch contains some fixes that improve TorchDynamo support, so we recommend building PyTorch from source or using PyTorch nightly builds.

For reproducing the experiments in the posts above, use the TorchBenchmark fork found here. This fork contains a few minor fixes that have not yet been merged upstream.

Other development requirements can be installed with:

pip install -r requirements.txt

Install TorchDynamo with:

python setup.py develop

Here is a basic example of how to use TorchDynamo:

from typing import List
import torch
import torchdynamo

def toy_example(a, b):
    x = a / (torch.abs(a) + 1)
    if b.sum() < 0:
        b = b * -1
    return x * b

def my_compiler(gm: torch.fx.GraphModule, example_inputs: List[torch.Tensor]):
    print("my_compiler() called with FX graph:")
    gm.graph.print_tabular()
    return gm.forward  # return a python callable

with torchdynamo.optimize(my_compiler):
    for _ in range(100):
        toy_example(torch.randn(10), torch.randn(10))

Running this example produces the following output:

my_compiler() called with FX graph:
opcode         name     target                                                  args              kwargs
-------------  -------  ------------------------------------------------------  ----------------  --------
placeholder    a        a                                                       ()                {}
placeholder    b        b                                                       ()                {}
call_function  abs_1    <built-in method abs of type object at 0x7f8d259298a0>  (a,)              {}
call_function  add      <built-in function add>                                 (abs_1, 1)        {}
call_function  truediv  <built-in function truediv>                             (a, add)          {}
call_method    sum_1    sum                                                     (b,)              {}
call_function  lt       <built-in function lt>                                  (sum_1, 0)        {}
output         output   output                                                  ((truediv, lt),)  {}

my_compiler() called with FX graph:
opcode         name    target                   args         kwargs
-------------  ------  -----------------------  -----------  --------
placeholder    b       b                        ()           {}
placeholder    x       x                        ()           {}
call_function  mul     <built-in function mul>  (b, -1)      {}
call_function  mul_1   <built-in function mul>  (x, mul)     {}
output         output  output                   ((mul_1,),)  {}

my_compiler() called with FX graph:
opcode         name    target                   args       kwargs
-------------  ------  -----------------------  ---------  --------
placeholder    b       b                        ()         {}
placeholder    x       x                        ()         {}
call_function  mul     <built-in function mul>  (x, b)     {}
output         output  output                   ((mul,),)  {}

Note that the order of the last two graphs is nondeterministic depending on which one is encountered first by the just-in-time compiler.

The example above only covers inference (model.forward).

Torchdynamo supports training, using AotAutograd to capture backwards:

• only the .forward() graph is captured by torchdynamo's python evalframe frontend
• for each segment of .forward() that torchdynamo captures, it uses AotAutograd to generate a backward graph segment
• each pair of forward, backward graph are (optionally) min-cut partitioned to save the minimal state between forward/backwrad
• the forward, backward pairs are wrapped in autograd.function modules
• usercode calling .backward() still triggers eager's autograd engine, which runs each 'compiled backward' graph as if it were one op, also running any non-compiled eager ops' .backward() functions

Current limitations:

• optimizer ops are currently not captured at all, and thus not compiled (under investigation to add support)
• DDP and FSDP, which rely on autograd 'hooks' firing between backward ops to schedule communications ops, may be pessimized by having all communication ops scheduled after whole compiled regions of backwards ops (WIP to fix this)

Example

# nothing special about this part
model = ...
optimizer = ...

with torchdynamo.optimize("aot_nvfuser"):
    for _ in range(100):
        # forward graphs are captured, and AotAutograd generates corresponding backward graphs
        # both forward and backward graphs are compiled at this time
        loss = model(torch.randn(10), torch.randn(10))
        
        # no further compilation happens here, but eager autograd executes compiled backwards graphs from above
        loss.backward()
        
        # dynamo won't compile this, currently
        optimizer.step()

See Troubleshooting.

One could replace my_compiler() with something that generates faster code, for example one using optimize_for_inference:

def my_compiler(gm: torch.fx.GraphModule, example_inputs: List[torch.Tensor]):
    scripted = torch.jit.trace(gm, example_inputs)
    return torch.jit.optimize_for_inference(scripted)

TorchDynamo also includes many backends, which can be found in backends.py or torchdynamo.list_backends(). Note many backends require installing additional packages. You can combine these backends together with code like:

from torchdynamo.optimizations import BACKENDS

def my_compiler(gm: torch.fx.GraphModule, example_inputs: List[torch.Tensor]):
    trt_compiled = BACKENDS["tensorrt"](gm, example_inputs)
    if trt_compiled is not None:
        return trt_compiled
    # first backend failed, try something else...

    cudagraphs_compiled = BACKENDS["cudagraphs"](gm, example_inputs)
    if cudagraphs_compiled is not None:
        return cudagraphs_compiled

    return gm.forward

If you just want to use an existing backend, you can pass a string containing the backend name to torchdynamo.optimize(). torchdynamo.optimize() can also be used as a decorator on functions, methods, or nn.Modules(). So a shorter version of using optimize_for_inference on toy_example would be:

@torchdynamo.optimize("ofi")
def toy_example(a, b):
    ...

TorchDynamo operates just-in-time and specializes graphs based on dynamic properties. For example, the first graph above has the following guards:

GUARDS:
 - local 'a' TENSOR_MATCH
 - local 'b' TENSOR_MATCH
 - global 'torch' FUNCTION_MATCH

If any of those guards fail, the graph will be recaptured and recompiled. The interesting guard type there is TENSOR_MATCH, which checks the following torch.Tensor properties:

• Python class of the tensor (tensor subclassing, etc)
• dtype
• device
• requires_grad
• dispatch_key (with thread-local includes/excludes applied)
• ndim
• sizes* (optional)
• strides* (optional)

*For sizes/strides you can disable this specialization by setting:

torchdynamo.config.dynamic_shapes = True

The full specialization mode allows the backend compiler to assume an entirely static graph. Unfortunately, most backends require this. Operators which return dynamic shapes will trigger a graph break when not in dynamic shape mode.

In some cases, you may not want unexpected compiles after a program has warmed up. For example, if you are serving production traffic in a latency critical application. For this, TorchDynamo provides an alternate mode where prior compiled graphs are used, but no new ones are generated:

with torchdynamo.run():
    toy_example(torch.randn(10), torch.randn(10))

In some cases, you may want to ensure there are no graph breaks in your program to debug performance issues. You can turn graph breaks into errors by setting nopython=True:

with torchdynamo.optimize(my_compiler, nopython=True):
    toy_example(torch.randn(10), torch.randn(10))

Which will trigger the following error in the example program above:

Traceback (most recent call last):
  ...
torchdynamo.exc.Unsupported: generic_jump TensorVariable()
Processing original code:
  File "example.py", line 7, in toy_example
    if b.sum() < 0:

If you want to understand better what TorchDynamo is doing, you can set:

torchdynamo.config.debug = True

which triggers useful (but spammy) printouts.

For example, the printouts for the first graph in the toy_example above are:

__compiled_fn_0 <eval_with_key>.1
opcode         name     target                                                  args              kwargs
-------------  -------  ------------------------------------------------------  ----------------  --------
placeholder    a        a                                                       ()                {}
placeholder    b        b                                                       ()                {}
call_function  abs_1    <built-in method abs of type object at 0x7f9ca082f8a0>  (a,)              {}
call_function  add      <built-in function add>                                 (abs_1, 1)        {}
call_function  truediv  <built-in function truediv>                             (a, add)          {}
call_method    sum_1    sum                                                     (b,)              {}
call_function  lt       <built-in function lt>                                  (sum_1, 0)        {}
output         output   output                                                  ((truediv, lt),)  {}

ORIGINAL BYTECODE toy_example example.py 9
 10           0 LOAD_FAST                0 (a)
              2 LOAD_GLOBAL              0 (torch)
              4 LOAD_METHOD              1 (abs)
              6 LOAD_FAST                0 (a)
              8 CALL_METHOD              1
             10 LOAD_CONST               1 (1)
             12 BINARY_ADD
             14 BINARY_TRUE_DIVIDE
             16 STORE_FAST               2 (x)

 11          18 LOAD_FAST                1 (b)
             20 LOAD_METHOD              2 (sum)
             22 CALL_METHOD              0
             24 LOAD_CONST               2 (0)
             26 COMPARE_OP               0 (<)
             28 POP_JUMP_IF_FALSE       38

 12          30 LOAD_FAST                1 (b)
             32 LOAD_CONST               3 (-1)
             34 BINARY_MULTIPLY
             36 STORE_FAST               1 (b)

 13     >>   38 LOAD_FAST                2 (x)
             40 LOAD_FAST                1 (b)
             42 BINARY_MULTIPLY
             44 RETURN_VALUE

MODIFIED BYTECODE
  9           0 LOAD_GLOBAL              3 (__compiled_fn_0)
              2 LOAD_FAST                0 (a)
              4 LOAD_FAST                1 (b)
              6 CALL_FUNCTION            2
              8 UNPACK_SEQUENCE          2
             10 STORE_FAST               2 (x)
             12 POP_JUMP_IF_FALSE       24
             14 LOAD_GLOBAL              4 (__resume_at_30_1)
             16 LOAD_FAST                1 (b)
             18 LOAD_FAST                2 (x)
             20 CALL_FUNCTION            2
             22 RETURN_VALUE
        >>   24 LOAD_GLOBAL              5 (__resume_at_38_2)
             26 LOAD_FAST                1 (b)
             28 LOAD_FAST                2 (x)
             30 CALL_FUNCTION            2
             32 RETURN_VALUE

GUARDS:
 - local 'a' TENSOR_MATCH
 - local 'b' TENSOR_MATCH
 - global 'torch' FUNCTION_MATCH

At the top you can see the FX graph (which we already shared above). Next you see the original bytecode of the function, followed by the modified bytecode generated by TorchDynamo. Finally, you see the guards which we covered above.

In the modified bytecode __compiled_fn_0 is the return value of my_compiler() (the compiled graph). __resume_at_30_1 and __resume_at_38_2 are both generated continuation functions that pick up execution after a graph break (at bytecode offsets 30 and 38). Each of these functions take the form:

__resume_at_<offset>:
    ... restore stack state if needed ...
    JUMP_ABSOLUTE <offset> into toy_example
    ... original bytecode of toy_example ...

By generating these resume_at function we force the remainder of the function to be executed in a new Python frame which recursively will trigger TorchDynamo to re-start its capture once execution reaches that point for the first time.

As background reading, I'd suggest looking at the PyTorch, functorch, and TorchBench setup docs. Since these projects work together in different ways.

The following instructions use Miniconda.

conda create --name=torchdynamo python=3.8
conda activate torchdynamo

# install pytorch nightlies
# for CUDA version, replace `cpuonly` with `cudatoolkit=11.3`
conda install pytorch torchvision torchaudio torchtext cpuonly -c pytorch-nightly

git clone [email protected]:pytorch/torchdynamo.git
cd torchdynamo
pip install -r requirements.txt

# check if everything works
make test

If see errors about missing symbols from guards.so, that may mean your C++ compiler is incompatible CUDA and/or with the one used to compile PyTorch. You may need to change your compiler version or build PyTorch from source.

To add TorchBench, which is useful for benchmarking and more extensive testing:

# you should still be in the conda env from before

cd ..  # if still in torchdynamo/

# download everything
git clone [email protected]:jansel/benchmark.git torchbenchmark
cd torchbenchmark
python install.py

cd ../torchdynamo

# fix the version of black so `make format` / `make lint` work
make lint-deps

# make sure it works
./torchbench.py --fast

Run tests with

pytest tests

To debug a specific test (with more debug prints) do:

pytest -vsk <test name>

Test on torchbenchmark models with:

python torchbench.py

To reproduce the performance measurements shared in the posts above, run either make offline-autotune-cpu or make offline-autotune-gpu. These targets will run something like the following:

# cleanup leftover state
rm -rf subgraphs

# initial run to record all the graphs to ./subgraphs/*
python torchbench.py -dcuda --speedup -n1

# autotune each graph and store which backend is best on disk
python autotune.py

# measure the speedups using the autotuned backend choices
python torchbench.py -dcuda --speedup -n100

# results are in ./speedups.csv

The baselines can be run with make baseline-cpu or make baseline-gpu. Which both string together a lot of calls to ./torchbench.py and generate *.csv files. See ./torchbench.py --help for more options.

Install format/linter deps with pip install -r requirements.txt, then:

make format  # reformat all files (in-place)
make lint    # run the linters

TorchDynamo has a BSD-style license, as found in the LICENSE file.