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Benchmarks for RL

A couple years ago, Drew Bagnell and I started the RLBench project to setup a suite of reinforcement learning benchmark problems. We haven’t been able to touch it (due to lack of time) for a year so the project is on hold. Luckily, there are several other projects such as CLSquare and RL-Glue with a similar goal, and we strongly endorse their continued development.

I would like to explain why, especially in the context of criticism of other learning benchmarks. For example, sometimes the UCI Machine Learning Repository is criticized. There are two criticisms I know of:

  1. Learning algorithms have overfit to the problems in the repository. It is easy to imagine a mechanism for this happening unintentionally. Strong evidence of this would be provided by learning algorithms which perform great on the UCI machine learning repository but very badly (relative to other learning algorithms) on non-UCI learning problems. I have seen little evidence of this but it remains a point of concern. There is a natural order to how much we trust performance results:
    1. Competitions. A well run prediction competition provides some of the best evidence available about which learning algorithms work well. There are still some mechanisms for being misled (too many entries, not enough testing data, unlucky choice of learning problem that your algorithm just has the wrong bias for), but they are more controlled than anywhere else.
    2. Benchmarks. Benchmarks provide a “reasonable sanity check” that the algorithm will do something well in practice. Compared to competitions, there are additional ways to be misled and (unfortunately) to mislead. Overfitting becomes a serious issue.
    3. Simulated data Simulated data can be very misleading. Typically, there is little reason to believe that simulated data reflects the structure of real-world problems.

    This ordering should always be kept in mind when considering empirical results.

  2. The repository enforced a certain interface which many natural learning problems did not fit into. Some interface must be enforced in order for a benchmark/repository to be useful. This implies that problems fitting the interface are preferred both in submission of new problems and in choice of algorithms to solve. This is a reasonable complaint, but it’s basically a complaint that the UCI machine learning repository was too succesful. The good news here is that the internet has made it so anyone can setup a repository. Just put up a webserver and announce the dataset.

It is important to consider the benefits of a benchmark suite as well. The standard in reinforcement learning for experimental studies in reinforcement learning algorithms has been showing that some new algorithm does something reasonable on one or two reinforcement learning problems. Naturally, what this implies in practice is that the algorithms were hand tuned to work on these one-or-two problems, and there was relatively little emphasis on building a general purpose reinforcement learning algorithm.

This is strongly at odds with the end goal of the reinforcement learning community! The way to avoid this is by setting up a benchmark suite (the more problems, the better). With a large set of problems that people can easily test on, the natural emphasis in empirical algorithm development shifts towards general purpose algorithms.

Another drawback of the “one or two problems” approach is incomparability of results. When people reimplement simulators for reinforcement learning problems, it turns out these reimplementations typically differ in the details, and these details can radically alter the difficulty of the problem. For example, there are many ways to implement the “car on the hill” problem, and some of them are much more difficult to solve than others. Again, a large set of problems people can easily test on solves this problem because the precise definition of the problem is shared.

One sometimes reasonable view of the world is that if you define a problem publicly, it is as good as solved since there are plenty of smart people out there eager to prove themselves. According to this view of the world, setting up a benchmark is a vital task.

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Debugging Your Brain

One part of doing research is debugging your understanding of reality. This is hard work: How do you even discover where you misunderstand? If you discover a misunderstanding, how do you go about removing it?

The process of debugging computer programs is quite analogous to debugging reality misunderstandings. This is natural—a bug in a computer program is a misunderstanding between you and the computer about what you said. Many of the familiar techniques from debugging have exact parallels.

  1. Details When programming, there are often signs that some bug exists like: “the graph my program output is shifted a little bit” = maybe you have an indexing error. In debugging yourself, we often have some impression that something is “not right”. These impressions should be addressed directly and immediately. (Some people have the habit of suppressing worries in favor of excess certainty. That’s not healthy for research.)
  2. Corner Cases A “corner case” is an input to a program which is extreme in some way. We can often concoct our own corner cases and solve them. If the solution doesn’t match our (mis)understanding, a bug has been found.
  3. Warnings On The compiler “gcc” has the flag “-Wall” which means “turn all warnings about odd program forms on”. You should always compile with “-Wall” as you immediately realize if you compare the time required to catch a bug that “-Wall” finds with the time required to debug the hard way.

    The equivalent for debugging yourself is listening to others carefully. In research, some people have the habit of wanting to solve everything before talking to others. This is usually unhealthy. Talking about the problem that you want to solve is much more likely to lead to either solving it or discovering the problem is uninteresting and moving on.

  4. Debugging by Design When programming, people often design the process of creating the program so that it is easy to debug. The analogy for us is stepwise mastery—first master your understanding of something basic. Then take the next step, the next, etc…
  5. Isolation When a bug is discovered, the canonical early trouble shooting step is isolating the bug. For a parse error, what is the smallest program exhibiting the error? For a compiled program: what are the simplest set of inputs which exhibit the bug? For research, what is the simplest example that you don’t understand?
  6. Representation Change When programming, sometimes a big program simply becomes too unwieldy to debug. In these cases, it is often a good idea to rethink the problem the program is trying to solve. How can you better structure the program to avoid this unwieldiness?

    The issue of how to represent the problem is perhaps even more important in research since human brains are not as adept as computers at shifting and using representations. Significant initial thought on how to represent a research problem is helpful. And when it’s not going well,
    changing representations can make a problem radically simpler.

Some aspects of debugging a reality misunderstanding don’t have a good analogue for programming because debugging yourself often involves social interactions. One basic principle is that your ego is unhelpful. Everyone (including me) dislikes having others point out when they are wrong so there is a temptation to avoid admitting it (to others, or more harmfully to yourself). This temptation should be actively fought . With respect to others, admitting you are wrong allows a conversation to move on to other things. With respect to yourself, admitting you are wrong allows you to move on to other things. A good environment can help greatly with this problem. There is an immense difference in how people behave under “you lose your job if wrong” and “great, let’s move on”.

What other debugging techniques exist?

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MLTV

As part of a PASCAL project, the Slovenians have been filming various machine learning events and placing them on the web here. This includes, for example, the Chicago 2005 Machine Learning Summer School as well as a number of other summer schools, workshops, and conferences.

There are some significant caveats here—for example, I can’t access it from Linux. Based upon the webserver logs, I expect that is a problem for most people—Computer scientists are particularly nonstandard in their choice of computing platform.

Nevertheless, the core idea here is excellent and details of compatibility can be fixed later. With modern technology toys, there is no fundamental reason why the process of announcing new work at a conference should happen only once and only for the people who could make it to that room in that conference. The problems solved include:

  1. The multitrack vs. single-track debate. (“Sometimes the single track doesn’t interest me” vs. “When it’s multitrack I miss good talks”
  2. “I couldn’t attend because I was giving birth/going to a funeral/a wedding”
  3. “What was that? I wish there was a rewind on reality.”

There are some fears here too. For example, maybe a shift towards recording and placing things on the web will result in lower attendance at a conference. Such a fear is confused in a few ways:

  1. People go to conferences for many more reasons than just announcing new work. Other goals include doing research, meeting old friends, worrying about job openings, skiing, and visiting new places. There also a subtle benefit of going to a conference: it represents a commitment of time to research. It is this commitment which makes two people from the same place start working together at a conference. Given all these benefits of going to a conference, there is plenty of reason for them to continue to exist.
  2. It is important to remember that a conference is a process in aid of research. Recording and making available for download the presentations at a conference makes research easier by solving all the problems listed above.
  3. This is just another new information technology. When the web came out, computer scientists and physicists quickly adopted a “place any paper on your webpage” style even when journals forced them to sign away the rights of the paper to publish. Doing this was simply healthy for the researcher because his papers were more easily readable. The same logic applies to making presentations at a conference available on the web.
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Deadline Season

Many different paper deadlines are coming up soon so I made a little reference table. Out of curiosity, I also computed the interval between submission deadline and conference.

Conference Location Date Deadline interval
COLT Pittsburgh June 22-25 January 21 152
ICML Pittsburgh June 26-28 January 30/February 6 140
UAI MIT July 13-16 March 9/March 16 119
AAAI Boston July 16-20 February 16/21 145
KDD Philadelphia August 23-26 March 3/March 10 166

It looks like the northeastern US is the big winner as far as location this year.

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Automated Labeling

One of the common trends in machine learning has been an emphasis on the use of unlabeled data. The argument goes something like “there aren’t many labeled web pages out there, but there are a huge number of web pages, so we must find a way to take advantage of them.” There are several standard approaches for doing this:

  1. Unsupervised Learning. You use only unlabeled data. In a typical application, you cluster the data and hope that the clusters somehow correspond to what you care about.
  2. Semisupervised Learning. You use both unlabeled and labeled data to build a predictor. The unlabeled data influences the learned predictor in some way.
  3. Active Learning. You have unlabeled data and access to a labeling oracle. You interactively choose which examples to label so as to optimize prediction accuracy.

It seems there is a fourth approach worth serious investigation—automated labeling. The approach goes as follows:

  1. Identify some subset of observed values to predict from the others.
  2. Build a predictor.
  3. Use the output of the predictor to define a new prediction problem.
  4. Repeat…

Examples of this sort seem to come up in robotics very naturally. An extreme version of this is:

  1. Predict nearby things given touch sensor output.
  2. Predict medium distance things given the nearby predictor.
  3. Predict far distance things given the medium distance predictor.

Some of the participants in the LAGR project are using this approach.

A less extreme version was the DARPA grand challenge winner where the output of a laser range finder was used to form a road-or-not predictor for a camera image.

These automated labeling techniques transform an unsupervised learning problem into a supervised learning problem, which has huge implications: we understand supervised learning much better and can bring to bear a host of techniques.

The set of work on automated labeling is sketchy—right now it is mostly just an observed-as-useful technique for which we have no general understanding. Some relevant bits of algorithm and theory are:

  1. Reinforcement learning to classification reductions which convert rewards into labels.
  2. Cotraining which considers a setting containing multiple data sources. When predictors using different data sources agree on unlabeled data, an inferred label is automatically created.

It’s easy to imagine that undiscovered algorithms and theory exist to guide and use this empirically useful technique.