Fancy reductions, solving linear systems

As discussed in the previous notebook, KeOps LazyTensors support a wide range of mathematical formulas. Let us now discuss the different operators that may be used to reduce our large M-by-N symbolic tensors into vanilla NumPy arrays or PyTorch tensors.

Note

In this tutorial, we stick to the PyTorch interface; but note that apart from a few lines on backpropagation, everything here can be seamlessly translated to vanilla NumPy+KeOps code.

LogSumExp, KMin and advanced reductions

First, let’s build some large LazyTensors S_ij and V_ij which respectively handle scalar and vector formulas:

import torch

from pykeops.torch import LazyTensor

use_cuda = torch.cuda.is_available()
tensor = torch.cuda.FloatTensor if use_cuda else torch.FloatTensor
M, N = (100000, 200000) if use_cuda else (1000, 2000)
D = 3

x = torch.randn(M, D).type(tensor)
y = torch.randn(N, D).type(tensor)

x_i = LazyTensor(x[:, None, :])  # (M, 1, D) LazyTensor
y_j = LazyTensor(y[None, :, :])  # (1, N, D) LazyTensor

V_ij = (x_i - y_j)  # (M, N, D) symbolic tensor of differences
S_ij = (V_ij ** 2).sum(-1)  # (M, N, 1) = (M, N) symbolic matrix of squared distances

print(S_ij)
print(V_ij)

Out:

KeOps LazyTensor
    formula: Sum(Square((Var(0,3,0) - Var(1,3,1))))
    shape: (100000, 200000)
KeOps LazyTensor
    formula: (Var(0,3,0) - Var(1,3,1))
    shape: (100000, 200000, 3)

As we’ve seen earlier, the pykeops.torch.LazyTensor.sum() reduction can be used on both S_ij and V_ij to produce genuine PyTorch 2D tensors:

print("Sum reduction of S_ij wrt. the 'N' dimension:", S_ij.sum(dim=1).shape)

Out:

Sum reduction of S_ij wrt. the 'N' dimension: torch.Size([100000, 1])

Note that LazyTensors support reductions over both indexing M and N dimensions, which may be specified using the PyTorch-friendly dim or the standard NumPy axis optional arguments:

print("Sum reduction of V_ij wrt. the 'M' dimension:", V_ij.sum(axis=0).shape)

Out:

Sum reduction of V_ij wrt. the 'M' dimension: torch.Size([200000, 3])

Just like PyTorch tensors, pykeops.torch.LazyTensor also support a stabilized log-sum-exp reduction, computed efficiently with a running maximum in the CUDA loop. For example, the following line computes \(\log(\sum_ie^{S_{ij}})\)

print("LogSumExp reduction of S_ij wrt. the 'M' dimension:", S_ij.logsumexp(dim=0).shape)

Out:

LogSumExp reduction of S_ij wrt. the 'M' dimension: torch.Size([200000, 1])

This reduction supports a weight parameter, which can be scalar or vector-valued. For example, the following line computes \(\log(\sum_je^{S_{ij}}V_{ij})\)

print("LogSumExp reduction of S_ij, with 'weight' V_ij, wrt. the 'N' dimension:",
      S_ij.logsumexp(dim=1, weight=V_ij).shape)

Out:

LogSumExp reduction of S_ij, with 'weight' V_ij, wrt. the 'N' dimension: torch.Size([100000, 3])

Going further, the pykeops.torch.LazyTensor.min(), pykeops.torch.LazyTensor.max(), pykeops.torch.LazyTensor.argmin() or pykeops.torch.LazyTensor.argmax() reductions work as expected, following the (sensible) NumPy convention:

print("Min    reduction of S_ij wrt. the 'M' dimension:", S_ij.min(dim=0).shape)
print("ArgMin reduction of S_ij wrt. the 'N' dimension:", S_ij.argmin(dim=1).shape)

print("Max    reduction of V_ij wrt. the 'M' dimension:", V_ij.max(dim=0).shape)
print("ArgMax reduction of V_ij wrt. the 'N' dimension:", V_ij.argmax(dim=1).shape)

Out:

Min    reduction of S_ij wrt. the 'M' dimension: torch.Size([200000, 1])
ArgMin reduction of S_ij wrt. the 'N' dimension: torch.Size([100000, 1])
Max    reduction of V_ij wrt. the 'M' dimension: torch.Size([200000, 3])
ArgMax reduction of V_ij wrt. the 'N' dimension: torch.Size([100000, 3])

To compute both quantities in a single pass, feel free to use the pykeops.torch.LazyTensor.min_argmin() and pykeops.torch.LazyTensor.max_argmax() reductions:

m_i, s_i = S_ij.min_argmin(dim=0)
print("Min-ArgMin reduction on S_ij wrt. the 'M' dimension:", m_i.shape, s_i.shape)

m_i, s_i = V_ij.max_argmax(dim=1)
print("Max-ArgMax reduction on V_ij wrt. the 'N' dimension:", m_i.shape, s_i.shape)

Out:

Min-ArgMin reduction on S_ij wrt. the 'M' dimension: torch.Size([200000, 1]) torch.Size([200000, 1])
Max-ArgMax reduction on V_ij wrt. the 'N' dimension: torch.Size([100000, 3]) torch.Size([100000, 3])

More interestingly, KeOps also provides support for the pykeops.torch.LazyTensor.Kmin(), pykeops.torch.LazyTensor.argKmin() and pykeops.torch.LazyTensor.Kmin_argKmin() reductions that may be used to implement an efficient K-nearest neighbor algorithm :

K = 5
print("KMin    reduction of S_ij wrt. the 'M' dimension:", S_ij.Kmin(K=K, dim=0).shape)
print("ArgKMin reduction of S_ij wrt. the 'N' dimension:", S_ij.argKmin(K=K, dim=1).shape)

Out:

KMin    reduction of S_ij wrt. the 'M' dimension: torch.Size([200000, 5])
ArgKMin reduction of S_ij wrt. the 'N' dimension: torch.Size([100000, 5])

It even works on vector formulas!

K = 7
print("KMin    reduction of V_ij wrt. the 'M' dimension:", V_ij.Kmin(K=K, dim=0).shape)
print("ArgKMin reduction of V_ij wrt. the 'N' dimension:", V_ij.argKmin(K=K, dim=1).shape)

Out:

KMin    reduction of V_ij wrt. the 'M' dimension: torch.Size([200000, 7, 3])
ArgKMin reduction of V_ij wrt. the 'N' dimension: torch.Size([100000, 7, 3])

Finally, the pykeops.torch.LazyTensor.sumsoftmaxweight() reduction may be used to computed weighted SoftMax combinations

\[a_i = \frac{\sum_j \exp(s_{i,j})\,v_{i,j} }{\sum_j \exp(s_{i,j})},\]

with scalar coefficients \(s_{i,j}\) and arbitrary vector weights \(v_{i,j}\):

a_i = S_ij.sumsoftmaxweight(V_ij, dim=1)
print("SumSoftMaxWeight reduction of S_ij, with weights V_ij, wrt. the 'N' dimension:",
      a_i.shape)

Out:

SumSoftMaxWeight reduction of S_ij, with weights V_ij, wrt. the 'N' dimension: torch.Size([100000, 3])

Solving linear systems

Inverting large M-by-M linear systems is a fundamental problem in applied mathematics. To help you solve problems of the form

\[\begin{split}& & a^{\star} & =\operatorname*{argmin}_a \| (\alpha\operatorname{Id}+K_{xx})a -b\|^2_2, \\\\ &\text{i.e.}\quad & a^{\star} & = (\alpha \operatorname{Id} + K_{xx})^{-1} b,\end{split}\]

KeOps pykeops.torch.LazyTensor support a simple LazyTensor.solve(b, alpha=1e-10) operation that can be used as follows:

x = torch.randn(M, D, requires_grad=True).type(tensor)  # Random point cloud
x_i = LazyTensor(x[:, None, :])  # (M, 1, D) LazyTensor
x_j = LazyTensor(x[None, :, :])  # (1, M, D) LazyTensor

K_xx = (- ((x_i - x_j) ** 2).sum(-1)).exp()  # Symbolic (M, M) Gaussian kernel matrix

alpha = .1  # "Ridge" regularization parameter
b_i = torch.randn(M, 4).type(tensor)  # Target signal, supported by the x_i's
a_i = K_xx.solve(b_i, alpha=alpha)  # Source signal, supported by the x_i's

print("a_i is now a {} of shape {}.".format(type(a_i), a_i.shape))

Out:

a_i is now a <class 'torch.Tensor'> of shape torch.Size([100000, 4]).

As expected, we can now check that:

\[(\alpha \operatorname{Id} + K_{xx}) \,a \simeq b.\]

c_i = alpha * a_i + K_xx @ a_i  # Reconstructed target signal

print("Mean squared reconstruction error: {:.2e}".format(((c_i - b_i) ** 2).mean()))

Out:

Mean squared reconstruction error: 1.64e-06

Please note that just like (nearly) all the other LazyTensor methods, pykeops.torch.LazyTensor.solve() fully supports the torch.autograd module:

[g_i] = torch.autograd.grad((a_i ** 2).sum(), [x])

print("g_i is now a {} of shape {}.".format(type(g_i), g_i.shape))

Out:

g_i is now a <class 'torch.Tensor'> of shape torch.Size([100000, 3]).

Warning

As of today, the pykeops.torch.LazyTensor.solve() operator only implements a conjugate gradient descent under the assumption that K_xx is a symmetric, positive-definite matrix. To solve generic systems, you could either interface KeOps with the routines of the SciPy package or implement your own solver, mimicking our reference implementation.