Quansight engineers have implemented support for tracing through NumPy code via
`torch.compile`

in PyTorch 2.1. This feature leverages PyTorch’s compiler to
generate efficient fused vectorized code without having to modify your original
NumPy code. Even more, it also allows for executing NumPy code on CUDA
just by running it through `torch.compile`

under `torch.device("cuda")`

!

In this post, we go over how to use this feature and give a few tips and tricks to make the most out of it.

## Compiling NumPy code into Parallel C++

We take as our running example one step in a K-Means algorithm. This piece of code is borrowed from this NumPy book

```
import numpy as np
def kmeans(X, means):
return np.argmin(np.linalg.norm(X - means[:, None], axis=2), axis=0)
```

We create a synthetic dataset with 20M random 2-D points. We can see that, given that the means are chosen appropriately, the function returns the correct cluster for all of them

```
npts = 10_000_000
X = np.repeat([[5, 5], [10, 10]], [npts, npts], axis=0)
X = X + np.random.randn(*X.shape) # 2 distinct "blobs"
means = np.array([[5, 5], [10, 10]])
np_pred = kmeans(X, means)
```

Benchmarking this function gives us a baseline of **1.26s** on an AMD 3970X CPU.

Compiling this function is now as easy as wrapping it with `torch.compile`

and
executing it with the example inputs

```
import torch
compiled_fn = torch.compile(kmeans)
compiled_pred = compiled_fn(X, means)
assert np.allclose(np_pred, compiled_pred)
```

The compiled function yields a 9x speed-up when running it on 1 core. Even
better, as opposed to NumPy, our generated code does take advantage of all the
cores in a processor. As such, when we run it on 32 cores, we get a **57x
speed-up**. Note that PyTorch always uses all the available cores unless
explicitly restricted, so this is the default behavior you get when using
`torch.compile`

.

We may inspect the generated C++ code by running the script with the
environment variable `TORCH_LOGS=output_code`

. When doing so, we can see that
`torch.compile`

was able to compile the broadcasting and the two reductions
into just one for-loop, and parallelize it using OpenMP

```
extern "C" void kernel(const double* in_ptr0, const long* in_ptr1, long* out_ptr0) {
#pragma omp parallel num_threads(32)
#pragma omp for
for(long i0=0L; i0<20000000L; i0+=1L) {
auto tmp0 = in_ptr0[2L*i0];
auto tmp1 = in_ptr1[0L];
auto tmp5 = in_ptr0[1L + (2L*i0)];
auto tmp6 = in_ptr1[1L];
// Rest of the kernel omitted for brevity
```

## Compiling NumPy code into CUDA

Compiling our code so that it runs on CUDA is as simple as setting the default device to be CUDA

```
with torch.device("cuda"):
cuda_pred = compiled_fn(X, means)
assert np.allclose(np_pred, cuda_pred)
```

By inspecting the generated code via `TORCH_LOGS=output_code`

, we see that,
rather than generating CUDA code directly, `torch.compile`

generates rather
readable triton code

```
def triton_(in_ptr0, in_ptr1, out_ptr0, XBLOCK : tl.constexpr):
xnumel = 20000000
xoffset = tl.program_id(0) * XBLOCK
xindex = xoffset + tl.arange(0, XBLOCK)[:]
xmask = xindex < xnumel
x0 = xindex
tmp0 = tl.load(in_ptr0 + (2*x0), xmask)
tmp1 = tl.load(in_ptr1 + (0))
// Rest of the kernel omitted for brevity
```

Running this small snippet on an RTX 2060 gives an **8x speed-up** over the
original NumPy code. This is something, but it is not particularly impressive,
given the speed-ups we have seen on CPU. Let’s have a look into how to squeeze
the most out of our GPU via a couple minor changes.

`float64`

vs `float32`

. Many GPUs, in particular consumer-grade ones, are
rather sluggish when running operations on `float64`

. For this reason, changing
the data generation to `float32`

, the original NumPy code just gets a bit
faster, about a 9%, but our CUDA code gets **40% faster**, yielding a **11x
speed-up** over the plain NumPy code.

`torch.compile`

, by default, respects the NumPy semantics, and as such, it uses
`np.float64`

as its default dtype for all its creation ops. As discussed, this
can hinder performance, so it is possible to change this default by setting

```
from torch._dynamo import config
config.numpy_default_float = "float32"
```

**CPU <> CUDA copies**. An 11x speed-up is good, but it is not even close to
the CPU numbers. This is caused by a small transformation that ```
torch.compile
```

does behind the scenes. The code above takes NumPy arrays and returns NumPy
arrays. All of these arrays are on CPU, but the computations are performed on
the GPU. This means that every time the function is called, `torch.compile`

has
to copy all these arrays from CPU to the GPU, and then copy the result back to
CPU to preserve the original semantics. There is no native solution to this
issue in NumPy, as NumPy does not have the notion of a `device`

. That being
said, we can work around it by creating a wrapper to this function so that it
accepts PyTorch tensors and returns PyTorch tensors.

```
@torch.compile
def tensor_fn(X, means):
X, means = X.numpy(), means.numpy()
ret = kmeans(X, means)
return torch.from_numpy(ret)
def cuda_fn(X, means):
with torch.device("cuda"):
return tensor_fn(X, means)
```

This function now takes tensors in CUDA memory and returns tensors in CUDA
memory, but the function itself is written in NumPy! `torch.compile`

uses the
`numpy()`

and the `from_numpy()`

calls as hints, and optimizes them away, and
internally it simply works with PyTorch tensors without moving the memory at
all. When we keep the tensors in CUDA and perform the computations in
`float32`

, we see a **200x speed-up** over the initial NumPy implementation on
`float32`

arrays.

**Mixing NumPy and PyTorch**. In this example, we had to write a small adaptor
to convert tensors to ndarrays and then back to tensors. In programs that mix
PyTorch and NumPy converting a tensor into an ndarray is often implemented as
`x.detach().cpu().numpy()`

, or simply `x.numpy(force=True)`

. Since when running
under `torch.compile`

we can run NumPy code in CUDA, we can implement this
conversion pattern as call to `x.numpy()`

, as we did above. Doing so and
running the resulting code under `device("cuda")`

will generate efficient CUDA
code from original NumPy calls without copying the data from CUDA to CPU at
all. Note that the resulting code does not run without `torch.compile`

. For it
to run in eager mode one would need to rollback to `x.numpy(force=True)`

.

## Further Speed-up tricks

**General advice**. The CUDA code we have shown is already quite efficient, but
it is true that the running example is rather short. When dealing with larger
programs, we may need to tweak parts of it to make it more efficient. A good
place to start is the multiple tutorials and FAQs for torch.compile.
This showcases a number of ways to inspect the tracing process, and how to
identify problematic code that may cause slowdowns.

**Advice when compiling NumPy code**. NumPy, even if rather similar to PyTorch,
is often used very differently. It is rather common to perform computations in
NumPy and then do an if/else depending on values within the array, or perform
operations in-place, perhaps via boolean masks. These constructions, while
supported by `torch.compile`

, hamper its performance. Changes like writing the
code in a branchless way to avoid graph breaks, or avoiding in-place ops can go
a long way.

To write fast NumPy code, it is best to avoid loops, but sometimes they are
unavoidable. When tracing through a loop, `torch.compile`

will try to fully
unroll it. This is sometimes desirable, but sometimes it may not even be
possible, like when we have a dynamic stopping condition, like in a while loop.
In these cases, it may be best to just compile the body of the loop, perhaps a
few iterations at a time (loop unrolling).

**Debugging NumPy code**. Debugging is rather tricky when a compiler is
involved. To figure out whether an error you are hitting is a ```
torch.compile
```

error, or an error from the program, you can execute your NumPy program without
`torch.compile`

by replacing the NumPy import by `import torch._numpy as np`

.
This is should just be used for **debugging purposes** and is in no way a
replacement for the PyTorch API, as it is **much slower** and, as a private API,
**may change without notice**. See also this FAQ for other tricks.

## Differences between NumPy and `torch.compile`

NumPy

**NumPy scalars**. NumPy returns NumPy scalars in almost any case where PyTorch
would return a 0-D tensor (e.g. from `np.sum`

). Under `torch.compile`

, NumPy
scalars are treated as 0-D arrays. This is just fine in most cases. The only
case when their behavior diverges is when NumPy scalars are implicitly used as
Python scalars. For example,

```
>>> np.asarray(2) * [1, 2, 3] # 0-D array is an array-like
array([2, 4, 6])
>>> u = np.int32(2)
>>> u * [1, 2, 3] # scalar decays into a Python int
[1, 2, 3, 1, 2, 3]
>>> torch.compile(lambda: u * [1, 2, 3])()
array([2, 4, 6]) # acts as a 0-D array, not as a scalar ?!?!
```

If we compile the first two lines, we see that `torch.compile`

treats `u`

as a
0-D array. To recover the eager semantics, we just need to make the casting
explicit

```
>>> torch.compile(lambda: int(u) * [1, 2, 3])()
[1, 2, 3, 1, 2, 3]
```

**Type promotion and versioning**. NumPy’s type promotion rules may be, at
times, a bit surprising

```
>>> np.zeros(1, dtype=np.int8) + 127
array([127], dtype=int8)
>>> np.zeros(1, dtype=np.int8) + 128
array([128], dtype=int16)
```

NumPy 2.0 is changing these rules to follow others that are closer to those
PyTorch. The relevant technical document is NEP 50.
`torch.compile`

went ahead and implemented NEP 50 rather than the about-to-be-deprecated rules.

In general, NumPy within torch.compile follows NumPy 2.0 pre-release.

## Beyond NumPy: SciPy and scikit-learn

In parallel to this effort of making `torch.compile`

understand NumPy code,
other Quansight engineers have designed and proposed a way to support PyTorch
tensors within scikit-learn and SciPy. This was received enthusiastically by
other maintainers from these libraries, as it was shown that using PyTorch as a
backend would often yield considerable speed-ups. Both projects have now merged
initial support for PyTorch tensors across a number of APIs and submodules.

This sets the stepping stone to move towards a future where PyTorch tensors can be used within other libraries in the Python data ecosystem. Even more, this will enable running these other libraries on GPUs and even compiling code mixing these libraries and PyTorch, similar to what we have been discussed in this post.

If you want to learn more about this effort, how to use it, or how to help moving it forward, see this other blogpost.

## Conclusion

PyTorch has committed since its inception to be a framework compatible with the rest of the Python ecosystem. Enabling compiling NumPy programs, and establishing the tools necessary to do the same for other prominent libraries are two more steps in this direction. Quansight and Meta continue working hand on hand, improving the compatibility between PyTorch and the rest of the ecosystem.

From Quansight, we would like to thank Mengwei, Voz, and Ed for their
invaluable help in integrating our work with `torch.compile`

. We would also
like to thank Meta for funding this project as well as previous work on
improving NumPy compatibility within PyTorch, and the project that led to
supporting PyTorch within scikit-learn and SciPy. These are giant leaps towards
consolidating PyTorch as the framework of choice within the open source Python
data ecosystem.