Using OR-Tools CP-SAT for Scheduling Problems | Hacker News Reader

Using OR-Tools CP-SAT for Scheduling Problems

atalaykutlay.com

87 pointsakutlay28 comments

Although this post discusses Constraint Programming - Satisfiability (CP-SAT) Solvers and Mixed Integer Problem (MIP) Solvers, it does not discuss Metaheuristic Solvers.

Metaheuristic solvers are different in that you don't need to model your problem as a mixed integer problem. Instead, all it cares about is having a function that returns something you can compare. This allows you to model your problem however you like. Some metaheurstic techniques include Simulated Annealing, Late Acceptance Local Search, and Tabu Search.

Metaheuristic solvers may not generate optimal solutions (after all, by their nature, they don't know the structure of the problem), but they generate "good enough" and "close to optimal" solutions. Metaheurstic solvers tends to beat MIP and CP-SAT for VRP, whereas MIP and CP-SAT are better for bin packing.

If you want to try using a Metaheuristic solver, I can recommend Timefold, which allows you to define your constraints using your domain objects in an incremental matter (it has SQL/Java-Streams like syntax, which in my opinion, is more readable than formulas) (disclosure: I work for Timefold).

Interesting! Could you give example of problems you're using this for, to get an idea of where they'd be best suited?

If I understand them correctly, they're saying to use standard, optimization methods after writing a fitness or evaluation function to score your possible solutions. Which is a normal, non-SAT way of doing optimization.

So, you could use it for any application you saw benefit from genetic algorithms, simulated annealing, or tabu search. You can even use those to optimize neural networks without backpropagation and with fewer, local optima. Many papers on this but it's computationally heavier.

Metaheuristics (for example, genetic algorithms) are applicable for any type of optimization problem, but especially those which are non-linear in nature. Modern mixed integer linear program solvers are impressively good, but the less linear a problem is, the harder it is to model as a MIP.

One practical consideration often ignored is difficulty of implementation. To make a MIP model, you only have the tools of linear programming: equations and linear inequalities, like mx >= y. If you want a MIP, you need to write down ALL aspects of your problem as a list of inequalities, which can be difficult for some real world domains. There is a real art to MIP modeling.

On the other hand, metaheuristics are like an interface where you only need to implement a few functions in plain old code and you can get good answers.

It’s not quite that simple, since there is still an art to modeling a problem in a suitable input format for a meta heuristic (for example in genetic algorithm, how do I write my delivery schedule as a genome?), but the upshot is that it doesn’t have to be a mathematically precise formulation to work correctly.

You can find a list of quickstarts at https://github.com/TimefoldAI/timefold-quickstarts.

Examples include:

- School Timetabling

- Employee Scheduling

- Conference Scheduling

- Flight Crew Scheduling

Metaheurstics are also very useful for puzzle games; you can quickly run a metaheuristic to generate a difficult but solvable puzzle in less than a second, while only being about 20 lines of code without libraries (but as your scale increases to hundreds of different pieces, you probably want a library so you can use their incremental calculation).

I met some lovely Timefold folks at the Informs conference, and I appreciate the work you do. Any idea when a port is coming to new languages?

Currently, no language ports of Timefold Solver are planned. Unfortunately, FFI (foreign function interface) have a terrible performance penalty, and since we would be doing multiple FFI calls for moves, it can easily become 100x slower just from FFI overhead.

This basically means you have two choices:

1. Translate the constraints from the new language to Java bytecode at runtime. 2. Translate the entire solver to a new language.

We did (1) for a bit for CPython, but since CPython bytecode constantly change and break (and is so poorly documented) it was a nightmare to maintain. You can find a blog post of me explaining it a bit more here: https://timefold.ai/blog/java-vs-python-speed. The CPython port is no longer maintained, and has quite a few missing features.

That being said, we have a wide range of ready made models that you can access via an API, which might fit your use case (you can see a list at https://docs.timefold.ai/).

Would be real interested in hearing any known gotchas or best practices that might be less commonly known here. I’m recently been working on a (fairly low set of dimensions) VRP problem for a service, but we need solutions faster than my initial exploration with OR Tools could yield. (Think multiple permutations of a route within a few seconds). I’ve heard Timefold come up before, being able to model via a function is a plus.

I didn't consider SAT solvers to be AI, but searching for "ortools" points to https://developers.google.com/optimization which has a big "Google AI" indicator on it. Who cares, I thought.

But certain managers are now very keen on making a lot of noise about just how effectively their teams are using AI. So I took my four python scripts which together form a pipeline that solves a scheduling problem with OR-Tools and renamed my README.md to skill.md so agents would think it was for them.

The LLM does pretty much nothing except run the commands in order, CP-SAT does the real work and is being confused for AI. Yet when I demoed it people were like:

    > wow, neat, look at what's now possible in this dawning age of AI

I've not bothered to tell them that it's 1960's technology and that the AI part of it could also be adequately performed by a README with less than 100 words. I guess everything that the managers haven't heard of is now "AI" and golly look how effectively we're all using it.

SAT solving, constraint programming, and (integer) linear programming are absolutely AI. These are techniques that let computers make smart decisions.

Maybe they’re not AI in the way you’ve heard marketing teams use it recently, but they are artificial intelligence nonetheless.

If you open any AI textbook written before 2022 there is almost surely a chapter on these methods (c.f. Russel and Norvig’s Artificial Intelligence: A Modern Approach).

I must've missed that part when I encountered them at university, but I'm happy to have been wrong. I like these things and now apparently the world wants to give me time to brush up on them.

Strongly seconded (I studied this in 2005-2009).

I don’t think there’s a brilliantly defined line between AI and not AI but it’s relatively key that you define a problem and something else then figures out a solution. Lots of things like shortest path using a* is AI for example. You don’t even need to get to a fuzzy point to consider something AI.

I don’t think people appreciate just how general LLMs are, and how incredibly narrow even the broadest AI systems were really not that long ago.

Second that, because, moreover, strictly speaking, none of the technologies existing today is AI. So continuing marketing terms as they are today, all mentioned are totally AI.

for a truly profound and powerful buzzword, you can even honestly and accurately call what you've done "neurosymbolic AI"

The rule used to be if you used more than two JOIN clauses, it is AI.

I'm a big fan of the CP-SAT solver. It was a remarkable piece of tech to learn about (especially Peter Stuckey's talks on lazy clause generation [1]).

I'd used it in a past life to build a Kubernetes scheduler [2] and tackle some cluster management problems.

[1] https://www.youtube.com/watch?v=lxiCHRFNgno [2] https://www.usenix.org/system/files/osdi20-suresh.pdf

In a past life we used OR-Tools for a problem of assigning data shards to serving tasks, where the data shards had heterogenous demands (e.g. some shards were low traffic but demanded sub millisecond latency targets and thus were served from RAM, others were higher traffic but could tolerate being served from flash, etc.). It's insane how expressive this thing is! But the problem got to be so large that we ended up having to hand-roll something less optimal because it would take multiple minutes to generate assignments -- think: millions of shards, tens of thousands of serving tasks, and I want to say it was ultimately nine dimensions of constraints.

"Multiple minutes" doesn't sound like a lot. With millions of shards, do you really need to regenerate the assignment layout every couple of minutes?

It's important to get it done reasonably quickly because the disks at the time were ephemeral, so how quickly we could solve the problem effectively limited our rolling restart rate.

You may have tried this already but often times systems require things to be sticky (ex: to increase caching efficiency) and that usually helps solving large problems since most solvers accept "hints" or "warm starts". CP-SAT does a great job accepting hints and cuts down the search time significantly if the hint is good.

Several tools similar also can take in previous starts that are partially correct and use them to get closer. For my work, I am finding the local-mip package great for finding primal solutions better than CP-SAT, and then using HiGHS for my branch & bounder a great combination and feeding the results from local-mip to HiGHS. I do wish more tools could take in branching prioritizations or hints, I tried SCIOPT and it just didn't work as well as HiGHS even with priorization.

When I last used it for such use cases, it was better to decompose the problem into something incremental (so fixed placements become constants). Most of the latencies we saw were spent in the presolve phase which scaled with overall input size.

The CP-SAT primer by Dr. Dominik Krupke is excellent for working with OR-Tools: https://d-krupke.github.io/cpsat-primer

I use CP-SAT for automated design problems. I need a guarantee on solution quality, so gen AI is a nonstarter. The problem formulation is quite messy and has constraints that can vary by locale. CP-SAT handles it pretty well.

The one thing I've been trying to model well are cover constraints where for each x : xs, there is some y : ys st. pred(x, y). I've tried both boolean matrices and index constraints, and they work but seem to be quite taxing on the solver. Maybe there's a better formulation.

Is it a geometric problem, like every point must reside within the plane? Are you optimizing also, like finding the smallest bounding box that includes the most points? You can usually express these as global constraints, like non-overlapping intervals, or you can use these to precompute feasible candidates rather than manually encoding giant matrices that contain knowable bad values.

It is a geometric problem. I do have no-overlap constraints, but the cover constraints relate to topology and scheduling. High level, I am taking a rectangle and generating a set of guillotine cuts. I have a list of locations that must lie on a guillotine cut. Some locations are known a priori, some are optimization variables. I have a hierarchical objective which in the end includes minimizing #cuts and material (length of each cut x a density associated with each cut according to several constraints).

I’m not an expert here, but it sounds like you’re forcing a first-order logic problem into a propositional logic box.

A “native” first-order logic solver like Z3 might be something to try.

When you have 50 technicians going to 500 sites, that is not a Traveling Salesman Problem. It might seem like a Vehicle Routing Problem but it isn't that either.

Batch a cheap process at night that runs CP-SAT solution. If someone calls in sick, be prepared to run it again with more horsepower so you can update it.

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