Grid Sample 1d

pytorch cuda extension of grid sample 1d. Since pytorch only supports grid sample 2d/3d, I extend the 1d version for efficiency. The forward pass is 2~3x faster than pytorch grid sample.

setup

• Pytorch == 1.7.1
• CUDA == 10.1

Other versions of pytorch or cuda may work but I haven't test.

you can choose to manually build it or use JIT

Build

python setup.py install

JIT

comment import grid_sample1d_cuda as grid_sample1d in op.py

uncomment

grid_sample1d = load(
    'grid_sample1d_cuda', ['grid_sample1d_cuda.cpp', 'grid_sample1d_cuda_kernel.cu'], verbose=True)

in op.py

Usage

import torch
from grid_sample1d import GridSample1d

grid_sample1d = GridSample1d(padding_mode=True, align_corners=True)
N = 16
C = 256
L_in = 64
L_out = 128
input = torch.randn((N, C, L_in)).cuda()
grids = torch.randn((N, L_out)).cuda()
output = grid_sample1d(input, grids)

Options are

• padding_mode: True for border padding, False for zero padding
• align_corners: same with align_corners in torch.nn.functional.grid_sample

difference

In forward pass, calculation on the channel dim C is parallel, which is serial in torch.nn.functional.grid_sample. Parallel calculation on C may cause round off error in backward. But for now, I found it doesn't influence the forward pass.

Test

Accuracy Test

Since grid sample 1d is a special case of grid sample 2d in most cases (not true when padding_mode & align_corners are both False). I test the accuracy of the implemented grid sample based on torch.nn.functional.grid_sample.

import torch
import torch.nn.functional as F


def gridsample1d_by2d(input, grid, padding_mode, align_corners):
    shape = grid.shape
    input = input.unsqueeze(-1)  # batch_size * C * L_in * 1
    grid = grid.unsqueeze(1)  # batch_size * 1 * L_out
    grid = torch.stack([-torch.ones_like(grid), grid], dim=-1)
    z = F.grid_sample(input, grid, padding_mode=padding_mode, align_corners=align_corners)
    C = input.shape[1]
    out_shape = [shape[0], C, shape[1]]
    z = z.view(*out_shape)  # batch_size * C * L_out
    return z

It is recommended to test on your computer because I only test it on CUDA 10.1 GTX 1080Ti

python test/acc_benchmark.py

Both the forward and the backward results are identical except for align_corners=True, padding_mode=False. It may be caused by round off error when we sum series float numbers in different orders.

Deterministic Test

It is very important to do deterministic test since the associative law is no more applied for the calculation of float numbers on computers.

python test/check_deterministic.py

Note

When padding_mode & align_corners are both False, we cannot regard grid sample 1d as a special case of grid sample 2d in pytorch. I have checked the cuda kernel of grid_sample in Pytorch. When padding_mode & align_corners are both False, the output of torch.nn.functional.grid_sample will be half of the expected. Hope it can be fixed one day.

CPU support

Too lazy to support

speed & memory cost

Here are the speed test results on different size of input

references

• grid sample pytorch
• grid sample cuda
• pytorch C++ doc
• cuda doc