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# RStudio AI Weblog: torch for optimization

To this point, all `torch` use instances we’ve mentioned right here have been in deep studying. Nonetheless, its automated differentiation characteristic is beneficial in different areas. One outstanding instance is numerical optimization: We are able to use `torch` to seek out the minimal of a operate.

The truth is, operate minimization is precisely what occurs in coaching a neural community. However there, the operate in query usually is much too advanced to even think about discovering its minima analytically. Numerical optimization goals at increase the instruments to deal with simply this complexity. To that finish, nevertheless, it begins from capabilities which can be far much less deeply composed. As an alternative, they’re hand-crafted to pose particular challenges.

This submit is a primary introduction to numerical optimization with `torch`. Central takeaways are the existence and usefulness of its L-BFGS optimizer, in addition to the impression of working L-BFGS with line search. As a enjoyable add-on, we present an instance of constrained optimization, the place a constraint is enforced through a quadratic penalty operate.

To heat up, we take a detour, minimizing a operate “ourselves” utilizing nothing however tensors. It will turn into related later, although, as the general course of will nonetheless be the identical. All adjustments might be associated to integration of `optimizer`s and their capabilities.

## Operate minimization, DYI strategy

To see how we are able to decrease a operate “by hand”, let’s attempt the enduring Rosenbrock operate. It is a operate with two variables:

[
f(x_1, x_2) = (a – x_1)^2 + b * (x_2 – x_1^2)^2
]

, with (a) and (b) configurable parameters usually set to 1 and 5, respectively.

In R:

``````library(torch)

a <- 1
b <- 5

rosenbrock <- operate(x) {
x1 <- x[1]
x2 <- x[2]
(a - x1)^2 + b * (x2 - x1^2)^2
}``````

Its minimal is situated at (1,1), inside a slender valley surrounded by breakneck-steep cliffs:

Determine 1: Rosenbrock operate.

Our purpose and technique are as follows.

We need to discover the values (x_1) and (x_2) for which the operate attains its minimal. Now we have to start out someplace; and from wherever that will get us on the graph we observe the unfavorable of the gradient “downwards”, descending into areas of consecutively smaller operate worth.

Concretely, in each iteration, we take the present ((x1,x2)) level, compute the operate worth in addition to the gradient, and subtract some fraction of the latter to reach at a brand new ((x1,x2)) candidate. This course of goes on till we both attain the minimal – the gradient is zero – or enchancment is beneath a selected threshold.

Right here is the corresponding code. For no particular causes, we begin at `(-1,1)` . The educational fee (the fraction of the gradient to subtract) wants some experimentation. (Attempt 0.1 and 0.001 to see its impression.)

``````num_iterations <- 1000

# fraction of the gradient to subtract
lr <- 0.01

# operate enter (x1,x2)
# that is the tensor w.r.t. which we'll have torch compute the gradient
x_star <- torch_tensor(c(-1, 1), requires_grad = TRUE)

for (i in 1:num_iterations) {

if (i %% 100 == 0) cat("Iteration: ", i, "n")

# name operate
worth <- rosenbrock(x_star)
if (i %% 100 == 0) cat("Worth is: ", as.numeric(worth), "n")

# compute gradient of worth w.r.t. params
worth\$backward()

# handbook replace
})
}``````
``````Iteration:  100
Worth is:  0.3502924

Iteration:  200
Worth is:  0.07398106

...
...

Iteration:  900
Worth is:  0.0001532408

Iteration:  1000
Worth is:  6.962555e-05

Whereas this works, it actually serves as an example the precept. With `torch` offering a bunch of confirmed optimization algorithms, there is no such thing as a want for us to manually compute the candidate (mathbf{x}) values.

## Operate minimization with `torch` optimizers

As an alternative, we let a `torch` optimizer replace the candidate (mathbf{x}) for us. Habitually, our first attempt is Adam.

With Adam, optimization proceeds lots quicker. Reality be instructed, although, selecting studying fee nonetheless takes non-negligeable experimentation. (Attempt the default studying fee, 0.001, for comparability.)

``````num_iterations <- 100

x_star <- torch_tensor(c(-1, 1), requires_grad = TRUE)

lr <- 1

for (i in 1:num_iterations) {

if (i %% 10 == 0) cat("Iteration: ", i, "n")

worth <- rosenbrock(x_star)
if (i %% 10 == 0) cat("Worth is: ", as.numeric(worth), "n")

worth\$backward()
optimizer\$step()

}``````
``````Iteration:  10
Worth is:  0.8559565

Iteration:  20
Worth is:  0.1282992

...
...

Iteration:  90
Worth is:  4.003079e-05

Iteration:  100
Worth is:  6.937736e-05

It took us a few hundred iterations to reach at an honest worth. It is a lot quicker than the handbook strategy above, however nonetheless quite a bit. Fortunately, additional enhancements are attainable.

### L-BFGS

Among the many many `torch` optimizers generally utilized in deep studying (Adam, AdamW, RMSprop …), there may be one “outsider”, significantly better recognized in basic numerical optimization than in neural-networks house: L-BFGS, a.ok.a. Restricted-memory BFGS, a memory-optimized implementation of the Broyden–Fletcher–Goldfarb–Shanno optimization algorithm (BFGS).

BFGS is maybe probably the most broadly used among the many so-called Quasi-Newton, second-order optimization algorithms. Versus the household of first-order algorithms that, in deciding on a descent course, make use of gradient data solely, second-order algorithms moreover take curvature data into consideration. To that finish, precise Newton strategies truly compute the Hessian (a pricey operation), whereas Quasi-Newton strategies keep away from that price and, as an alternative, resort to iterative approximation.

Trying on the contours of the Rosenbrock operate, with its extended, slender valley, it’s not troublesome to think about that curvature data may make a distinction. And, as you’ll see in a second, it actually does. Earlier than although, one be aware on the code. When utilizing L-BFGS, it’s essential to wrap each operate name and gradient analysis in a closure (`calc_loss()`, within the beneath snippet), for them to be callable a number of occasions per iteration. You possibly can persuade your self that the closure is, actually, entered repeatedly, by inspecting this code snippet’s chatty output:

``````num_iterations <- 3

x_star <- torch_tensor(c(-1, 1), requires_grad = TRUE)

optimizer <- optim_lbfgs(x_star)

calc_loss <- operate() {

worth <- rosenbrock(x_star)
cat("Worth is: ", as.numeric(worth), "n")

worth\$backward()
worth

}

for (i in 1:num_iterations) {
cat("Iteration: ", i, "n")
optimizer\$step(calc_loss)
}``````
``````Iteration:  1
Worth is:  4

Worth is:  6

...
...

Worth is:  0.04880721

Worth is:  0.0302862

Iteration:  2
Worth is:  0.01697086

Worth is:  0.01124081

...
...

Worth is:  1.111701e-09

Worth is:  4.547474e-12

Iteration:  3
Worth is:  4.547474e-12

Despite the fact that we ran the algorithm for 3 iterations, the optimum worth actually is reached after two. Seeing how effectively this labored, we attempt L-BFGS on a tougher operate, named flower, for fairly self-evident causes.

## (But) extra enjoyable with L-BFGS

Right here is the flower operate. Mathematically, its minimal is close to `(0,0)`, however technically the operate itself is undefined at `(0,0)`, because the `atan2` used within the operate shouldn’t be outlined there.

``````a <- 1
b <- 1
c <- 4

flower <- operate(x) {
a * torch_norm(x) + b * torch_sin(c * torch_atan2(x[2], x[1]))
}``````

We run the identical code as above, ranging from `(20,20)` this time.

``````num_iterations <- 3

x_star <- torch_tensor(c(20, 0), requires_grad = TRUE)

optimizer <- optim_lbfgs(x_star)

calc_loss <- operate() {

worth <- flower(x_star)
cat("Worth is: ", as.numeric(worth), "n")

worth\$backward()

cat("X is: ", as.matrix(x_star), "nn")

worth

}

for (i in 1:num_iterations) {
cat("Iteration: ", i, "n")
optimizer\$step(calc_loss)
}``````
``````Iteration:  1
Worth is:  28.28427
X is:  20 20

...
...

Worth is:  19.33546
X is:  12.957 14.68274

...
...

Worth is:  18.29546
X is:  12.14691 14.06392

...
...

Worth is:  9.853705
X is:  5.763702 8.895616

Worth is:  2635.866
X is:  -1949.697 -1773.551

Iteration:  2
Worth is:  1333.113
X is:  -985.4553 -897.5367

Worth is:  30.16862
X is:  -21.02814 -21.72296

Worth is:  1281.39
X is:  964.0121 843.7817

Worth is:  628.1306
X is:  475.7051 409.7372

Worth is:  4965690
X is:  -3721262 -3287901

Worth is:  2482306
X is:  -1862675 -1640817

Worth is:  8.61863e+11
X is:  645200412672 571423064064

Worth is:  430929412096
X is:  322643460096 285659529216

Worth is:  Inf
X is:  -2.826342e+19 -2.503904e+19

Iteration:  3
Worth is:  Inf
X is:  -2.826342e+19 -2.503904e+19 ``````

This has been much less of a hit. At first, loss decreases properly, however all of a sudden, the estimate dramatically overshoots, and retains bouncing between unfavorable and optimistic outer house ever after.

Fortunately, there’s something we are able to do.

Taken in isolation, what a Quasi-Newton technique like L-BFGS does is decide the very best descent course. Nonetheless, as we simply noticed, course shouldn’t be sufficient. With the flower operate, wherever we’re, the optimum path results in catastrophe if we keep on it lengthy sufficient. Thus, we want an algorithm that fastidiously evaluates not solely the place to go, but in addition, how far.

For that reason, L-BFGS implementations generally incorporate line search, that’s, a algorithm indicating whether or not a proposed step size is an effective one, or must be improved upon.

Particularly, `torch`’s L-BFGS optimizer implements the Sturdy Wolfe situations. We re-run the above code, altering simply two strains. Most significantly, the one the place the optimizer is instantiated:

``optimizer <- optim_lbfgs(x_star, line_search_fn = "strong_wolfe")``

And secondly, this time I discovered that after the third iteration, loss continued to lower for some time, so I let it run for 5 iterations. Right here is the output:

``````Iteration:  1
...
...

Worth is:  -0.8838741
X is:  0.09035123 -0.03220009

Worth is:  -0.928809
X is:  0.06564617 -0.026706

Iteration:  2
...
...

Worth is:  -0.9991404
X is:  0.0006493925 -0.0002656128

Worth is:  -0.9992246
X is:  0.0007130796 -0.0002947929

Iteration:  3
...
...

Worth is:  -0.9997789
X is:  0.0002042478 -8.457939e-05

Worth is:  -0.9998025
X is:  0.0001822711 -7.553725e-05

Iteration:  4
...
...

Worth is:  -0.9999917
X is:  -6.320081e-06 2.614706e-06

Worth is:  -0.9999923
X is:  -6.921942e-06 2.865841e-06

Iteration:  5
...
...

Worth is:  -0.9999999
X is:  -7.267168e-08 3.009783e-08

Worth is:  -0.9999999
X is:  -7.404627e-08 3.066708e-08 ``````

It’s nonetheless not excellent, however lots higher.

Lastly, let’s go one step additional. Can we use `torch` for constrained optimization?

### Quadratic penalty for constrained optimization

In constrained optimization, we nonetheless seek for a minimal, however that minimal can’t reside simply wherever: Its location has to satisfy some variety of extra situations. In optimization lingo, it must be possible.

As an instance, we stick with the flower operate, however add on a constraint: (mathbf{x}) has to lie outdoors a circle of radius (sqrt(2)), centered on the origin. Formally, this yields the inequality constraint

[
2 – {x_1}^2 – {x_2}^2 <= 0
]

A strategy to decrease flower and but, on the similar time, honor the constraint is to make use of a penalty operate. With penalty strategies, the worth to be minimized is a sum of two issues: the goal operate’s output and a penalty reflecting potential constraint violation. Use of a quadratic penalty, for instance, leads to including a a number of of the sq. of the constraint operate’s output:

``````# x^2 + y^2 >= 2
# 2 - x^2 - y^2 <= 0
constraint <- operate(x) 2 - torch_square(torch_norm(x))

penalty <- operate(x) torch_square(torch_max(constraint(x), different = 0))``````

A priori, we are able to’t know the way large that a number of must be to implement the constraint. Due to this fact, optimization proceeds iteratively. We begin with a small multiplier, (1), say, and improve it for so long as the constraint continues to be violated:

``````penalty_method <- operate(f, p, x, k_max, rho = 1, gamma = 2, num_iterations = 1) {

for (ok in 1:k_max) {
cat("Beginning step: ", ok, ", rho = ", rho, "n")

decrease(f, p, x, rho, num_iterations)

cat("Worth: ",  as.numeric(f(x)), "n")
cat("X: ",  as.matrix(x), "n")

current_penalty <- as.numeric(p(x))
cat("Penalty: ", current_penalty, "n")
if (current_penalty == 0) break

rho <- rho * gamma
}

}``````

`decrease()`, known as from `penalty_method()`, follows the same old proceedings, however now it minimizes the sum of the goal and up-weighted penalty operate outputs:

``````decrease <- operate(f, p, x, rho, num_iterations) {

calc_loss <- operate() {
worth <- f(x) + rho * p(x)
worth\$backward()
worth
}

for (i in 1:num_iterations) {
cat("Iteration: ", i, "n")
optimizer\$step(calc_loss)
}

}``````

This time, we begin from a low-target-loss, however unfeasible worth. With yet one more change to default L-BFGS (particularly, a lower in tolerance), we see the algorithm exiting efficiently after twenty-two iterations, on the level `(0.5411692,1.306563)`.

``````x_star <- torch_tensor(c(0.5, 0.5), requires_grad = TRUE)

optimizer <- optim_lbfgs(x_star, line_search_fn = "strong_wolfe", tolerance_change = 1e-20)

penalty_method(flower, penalty, x_star, k_max = 30)``````
``````Beginning step:  1 , rho =  1
Iteration:  1
Worth:  0.3469974
X:  0.5154735 1.244463
Penalty:  0.03444662

Beginning step:  2 , rho =  2
Iteration:  1
Worth:  0.3818618
X:  0.5288152 1.276674
Penalty:  0.008182613

Beginning step:  3 , rho =  4
Iteration:  1
Worth:  0.3983252
X:  0.5351116 1.291886
Penalty:  0.001996888

...
...

Beginning step:  20 , rho =  524288
Iteration:  1
Worth:  0.4142133
X:  0.5411959 1.306563
Penalty:  3.552714e-13

Beginning step:  21 , rho =  1048576
Iteration:  1
Worth:  0.4142134
X:  0.5411956 1.306563
Penalty:  1.278977e-13

Beginning step:  22 , rho =  2097152
Iteration:  1
Worth:  0.4142135
X:  0.5411962 1.306563
Penalty:  0 ``````

## Conclusion

Summing up, we’ve gotten a primary impression of the effectiveness of `torch`’s L-BFGS optimizer, particularly when used with Sturdy-Wolfe line search. The truth is, in numerical optimization – versus deep studying, the place computational velocity is rather more of a problem – there may be hardly a purpose to not use L-BFGS with line search.

We’ve then caught a glimpse of the right way to do constrained optimization, a activity that arises in lots of real-world purposes. In that regard, this submit feels much more like a starting than a stock-taking. There’s a lot to discover, from basic technique match – when is L-BFGS effectively suited to an issue? – through computational efficacy to applicability to completely different species of neural networks. For sure, if this conjures up you to run your personal experiments, and/or in the event you use L-BFGS in your personal initiatives, we’d love to listen to your suggestions!

Thanks for studying!

## Appendix

### Rosenbrock operate plotting code

``````library(tidyverse)

a <- 1
b <- 5

rosenbrock <- operate(x) {
x1 <- x[1]
x2 <- x[2]
(a - x1)^2 + b * (x2 - x1^2)^2
}

df <- expand_grid(x1 = seq(-2, 2, by = 0.01), x2 = seq(-2, 2, by = 0.01)) %>%
rowwise() %>%
mutate(x3 = rosenbrock(c(x1, x2))) %>%
ungroup()

ggplot(knowledge = df,
aes(x = x1,
y = x2,
z = x3)) +
geom_contour_filled(breaks = as.numeric(torch_logspace(-3, 3, steps = 50)),
present.legend = FALSE) +
theme_minimal() +
scale_fill_viridis_d(course = -1) +
theme(side.ratio = 1)``````

### Flower operate plotting code

``````a <- 1
b <- 1
c <- 4

flower <- operate(x) {
a * torch_norm(x) + b * torch_sin(c * torch_atan2(x[2], x[1]))
}

df <- expand_grid(x = seq(-3, 3, by = 0.05), y = seq(-3, 3, by = 0.05)) %>%
rowwise() %>%
mutate(z = flower(torch_tensor(c(x, y))) %>% as.numeric()) %>%
ungroup()

ggplot(knowledge = df,
aes(x = x,
y = y,
z = z)) +
geom_contour_filled(present.legend = FALSE) +
theme_minimal() +
scale_fill_viridis_d(course = -1) +
theme(side.ratio = 1)``````

Photograph by Michael Trimble on Unsplash

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