Weakly trained ensembles of neural networksApril 24, 2021 | Comments
What if neural networks, but not very good and lots of them?
I recently read A Thousand Brains, by Jeff Hawkins. I’ve been a fan of his since reading the deeply inspirational founding stories of the Palm Pilot (original team: 7 people! 7!). And as an AI weenie, when I discovered what he really wanted to do all along was understand intelligence, I was doubly impressed - and loved his first book, On Intelligence.
Paraphrasing one of the theses of A Thousand Brains: the neocortex is more-or-less uniform and composed of ~150k cortical columns of equivalent structure, wired into different parts of the sensorimotor system. Hawkins suggests that they all build models of the world based on their inputs - so models are duplicated throughout the neocortex - and effectively “vote” to agree on the state of the world. So, for instance, if I’m looking at a coffee cup whilst holding it in my hand, columns receiving visual input and touch input each separately vote “coffee cup”.
The notion of many agents coming to consensus is also prominent in Minsky’s Society of Mind, and in the Copycat architecture which suffuses much of Hofstadter’s work. (I spent a bit of time last year noodling with the latter).
We think the brain doesn’t do backprop (in the same way as neural networks do - Hinton suggests it might do something similar, and some folks from Sussex and Edinburgh have recently proposed how this might work). We do know the brain is massively parallel, and that it can learn quickly from small data sets.
This had me wondering how classical neural networks might behave, if deployed in large numbers (typically called “ensembles”) that vote after being trained weakly - as opposed to being trained all the way to accurate classification in a single network. Could enormous parallelism compensate in some way for either training time or dataset size?
Past work with ensembles
One thing my Master’s (pre-Google) gave me an appreciation for was digging into academic literature. I had a quick look around to see what’s been done previously - there’s no real technical innovation for this kind of exploration, so I expected to find it thoroughly examined. Here’s what I found.
Really large ensembles haven’t been deeply explored:
- Lincoln and Skrzypek trained an ensemble of 5 networks and observed better performance over a single network.
- Donini, Loreggia, Pini and Rossi did experiments with 10.
- Breiman got to 25 (“more than 25 bootstrap replicates is love’s labor lost”), as did Opitz and Maclin
Voting turns up in many places:
- Drucker et al point to many studies of neural networks in committee, initialized with different weights, and suggest an expectation I share: that multiple networks might converge differently and thus have performance improved in combination. They had explored this in previous work (Drucker et al 1993a).
- Donini, Loreggia, Pini and Rossi reach the same conclusion and articulate it thus: “different neural networks can generalize the learning functions in different ways as to learn different areas of the solutions space”. They also explore other voting schemes.
- Clemen notes that “simple combination methods often work reasonably well relative to more complex combinations”, generally in forecasting.
- Kittler, Duin and Matas similarly note that “the sum rules out performs other classifier combinations schemes”.
Breiman’s Bagging (short for “bootstrap aggregating”) seems very relevant. It involves sampling from the same distribution and training a different network on each sample, then combining their results via voting etc. Technically if you sampled fully from that distribution and thus trained all your ensemble of the same dataset, this would be bagging, but it seems a bit over-literal and incompatible in spirit.
In his paper Breiman does not explore the impact of parallelization on small dataset sizes, but does note that “bagging is almost a dream procedure for parallel computing”, given the lack of communication between predictors.
Finally, two other titbits stood out:
- Perrone and Cooper observe that training on hold-out data is possible with an ensemble process without risking overfitting, as we “let the smoothing property of the ensemble process remove any overfitting or we can train each network with a different split of training and hold-out data”. That seems interesting from the POV of maximizing the value of a small dataset.
- Krogh and Vedelsby have an interesting means to formalize a measure of ambiguity in an ensemble. Opitz and Maclin reference them as verifying that classifieds which disagree strongly perform better. I wondered if this means an ensemble mixing networks designed to optimize for individual class recognition might perform better than multiclass classifiers?
Questions to ask
All this left me wanting to answer a few questions:
- What’s the trade-off between dataset size and ensemble size? i.e. would parallelization help a system compensate for having very few training examples, and/or very little training effort?
- Is an ensemble designed to do multiclass classification best served by being formed of homogenous networks, or specialist individual classifiers optimized for each class?
- How might we best optimize the performance of an ensemble?
I chose a classic toy problem, MNIST digit classification, and worked using the Apache MXNet framework. I chose the latter for a very poor reason: I started out wanting to use Clojure because I enjoy writing it, and MXNet seemed like the best option…. but I struggled to get it working and switched to Python. shrug
The MNIST dataset has 60,000 images and MXNet is bundled with a simple neural network for it: three fully connected layers with 128, 64 and 10 neurons each, which get 98% accuracy after training for 10 epochs (i.e. seeing 600,000 images total).
What’s the trade-off between dataset and ensemble size?
I started like this:
- Taking a small slice of the MNIST data set (100-1000 images out of the 60,000), to approximate the small number of examples a human might see.
- Training on this small dataset for a very small number of epochs (1 to 10), to reflect the fact that humans don’t seem to need machine-learning quantities of re-presented data.
- Repeating the training for 10,000 distinct copies of a simple neural network.
- For increasingly sized subsets of these 10,000 networks, having them vote on what they felt the most likely outcome was, and taking the most voted result as the classification. I tested on subsets to try and understand where the diminishing returns for parallelization might be: 10 networks? 100? 1000?
I ran everything serially, so the time to train and time to return a classification were extremely long: in a truly parallel system they’d likely be 1/10,000th.
I ran two sets of tests:
- With dataset sizes of 200, 500, 1000 and 10000 examples from the MNIST set, all trained for a single epoch. I also ran a test with a completely untrained network that had seen no data at all, to act as a baseline.
- For a dataset of 200 examples, I tried training for 1, 10, and 100 epochs.
It’s worth reiterating: these are very small training data sets (even 10,000 is 1/6th of the MNIST data set).
I expected to see increased performance from larger data sets, and from more training done on the same data set, but I had no intuition over how far I could go (I assumed a ceiling of 0.98, given this is where a well-trained version of the MXNet model got to).
I hoped to see increased performance from larger ensembles I had no intuition about how far this could go.
I expected the untrained model to remain at 0.1 accuracy no matter how large the ensemble, on the basis that it could not have learned anything about the data set, so its guesses would be effectively random.
For dataset sizes trained for a single epoch (data):
- A larger dataset leads to improved accuracy, and faster arrival at peak accuracy: for d=10000 1 network scored 0.705, an ensemble of 50 scored 0.733 and by 300, the ensemble converged on 0.741 where it remained.
- For smaller datasets, parallelization continues to deliver benefits for some time: d=200 didn’t converge near 0.47 (its final accuracy being 0.477) until an ensemble of ~6500 networks.
- An untrained network still saw slight performance improvements (0.0088 with 1 network, to the 0.14 range by 6000.
Looking at the impact of training time (in number of epochs) (data):
- More training means less value from an ensemble: 100 rose from 0.646 accuracy with 1 network to 0.667 by 50 networks, and stayed there.
- Less training means more value from an ensemble: 1 epoch rose from 0.091 accuracy to 0.55 by the time the ensemble reached 4500 networks.
- Parallelization can indeed compensate for either a small dataset or less time training, but not fully: an ensemble trained on 10,000 examples scored 0.744 vs 0.477 for one trained on 200; one trained for 100 epochs scored 0.668 vs 0.477 for one trained for 1 epoch.
- I don’t understand how an untrained network gets better. Is it reflecting some bias in the training/validation data, perhaps? i.e. learning that there are slightly more examples of the digit 1 than 7 etc?
Should individual classifiers be homogenous or heterogeneous?
Instead of training all networks in the ensemble on all classes, I moved to a model where each network was trained on a single class.
To distinguish a network’s target class from others, I experimented with different ratios of true:false training data in the training set (2:1, 1:1, 1:2, and 1:3)
I took the best performing ratio and tried it with ensembles of various sizes, trained for a single epoch on data sets of different sizes. I then compared these to the homogenous networks I’d been using previously.
And finally, I tried different data set sizes with well-trained ensembles, each network being trained for 100 epochs.
Here’s a comparison of those data ratios (data):
I ended up choosing 1:2 - i.e. two random negative examples from different classes presented during training, for each positive one. I wanted to be in principle operating on “minimal amounts of data” and the difference between 1:2 and 1:3 seemed small.
Here’s how a one-class-per-network approach performed (each network trained for a single epoch, (data)):
And then, to answer the question, I compared 1-class-per-network to all-networks-all-classes (data):
Naively, a network trained to classify all classes performed better. But consider the dataset sizes: each all-classes network is trained on 10,000 examples (of all classes), but each per-class network of d=10000 is trained on 1/10 as much data. So a fair comparison is between the d=10000 per-class network and d=1000 all-class network, where per-class networks have the edge.
Here’s the result of well-trained ensembles (data):
This was a red flag for ensembles generally: repeated re-presentation of the same data across multiple epochs reached peak performance very fast. When the network was well trained, using an ensemble didn’t have any noticeable effect. Expanding on the far left of that graph, you can see that in the slowest case (20 examples per dataset) ensembles larger than 50 networks had little effect, but smaller ones did perform better:
How could we optimize the training of an ensemble?
A friend suggested I experiment with the learning rate for small datasets - reasoning that we want individual networks to converge as quickly as possible, even if imperfectly, and rely on voting to smooth out the differences. The default learning rate in MXNet was 0.02; I compared this to 0.1, 0.2, 0.3, and 0.4, all for networks shown few examples and given a single epoch of training.
Finally, I wondered how performance changed during training: imagine a scenario where each network is trained on a single extra example (+ negatives) and then the ensemble is tested. How does performance of the ensemble change as the number of examples grows? This might be a good approximation for an ensemble that learns very slowly and naturally in-the-wild, in line with the kinds of biological plausibility that originally interested me.
A learning rate of 0.2 seemed to give the best results (data):
And here’s how that gradual learning worked out, as each ensemble size saw more examples (data):
An ensemble of 500 networks gets to ~0.75 accuracy by the time it’s seen 200 examples. Earlier results suggest it doesn’t go much further than that though.
Pulling it all together
Phew. This all took a surprising amount of time to actually run; I was doing it all on my Mac laptop, after finding that Colab instances would time out before the runs completed - some of them took a week to get through.
My conclusions from all this:
- Large ensembles didn’t seem useful for getting to high accuracy classifications. Nothing I did got near to the 0.98 accuracy that this MXNet example could get to, well trained.
- They did compensate for a dearth of training data and/or training time, to some degree. Getting to 0.75 accuracy with just 100 examples of each digit, just by doing it lots of times and voting, seemed… useful in theory. In practice I’m struggling to think of situations where it’d be easier to run 1000 ensembles than iterate over the training data a network has already seen.
In retrospect this might be explained as follows: a network is initialized with random weights, training it with a few examples would bias a set of these weights towards some features in the examples, but a slightly different set each time because of the randomness of the starting position. Thus across many networks you’d end up slightly biasing towards different aspects of the training data, and thus be able, in aggregate, to classify better.
Things I didn’t quite get to try:
- Different voting schemes: I was super-naive throughout, and in particular wonder if I could derive some idea of confidence from different networks in an ensemble, just pick the confident ones?
- MNist is a useful toy example, but I wonder if these results would replicate for other problems.
- Applying the measure of ambiguity from Krogh and Vedelsby to my ensembles.
NumboAugust 01, 2020 | Comments
I finally got around to finishing GEB. I think I bought a copy 10-15 years ago and either gave up or got distracted during a couple of attempts to finish it; at my birthday last year I vowed to complete the thing, and I kept my promise a few months back.
Many of Hofstadter’s then-novel ideas (e.g. perception as the basis of cognition) seem more mainstream nowadays, but I got interested in some projects from his lab (FARG, the Fluid Analogies Research Group) around cognitively plausible architectures operating in microdomains.
A follow-up book, Fluid Concepts and Creative Analogies, goes into these in more detail, and I thought I’d have a go at reimplementing of one of their projects, initially for giggles but then to play with some of the underlying ideas a bit more.
Numbo is a simple number puzzle solver (UK readers: it’s the number puzzle from Countdown): given a set of 5 integers, apply mathematical operations to them to make a 6th target value. Classically a computer scientist might treat this as a search problem, but FARG was more interested in cognitive plausibility. You can find Daniel Defays’ original paper on Numbo here. I’m sure I’ve seen an OCR scan of the original source code at some point, but can’t find it now.
Numbo uses an architecture they called Copycat (from another project, called err Copycat, which investigated analogies like “abc is to pqr as bcd is to…?”). Numbo’s version of the Copycat architecture has 3 parts:
- The Pnet, a network of static information: numbers, operations and basic calculations of a kind a child might learn by rote e.g. 2 + 2 = 4, 7 * 10 = 70.
- The coderack, a store of priority-weighted processes (called codelets) which is sampled from probabilistically, and thus run in a slightly random order, effectively emulating parallelism even when run serially. Codelets operate on the cytoplasm and can create other codelets. They are independent processes and do not communicate with one another directly.
- The cytoplasm, a working memory which is operated on by codelets sampled from the coderack, and stores current theories and constituent numbers.
My implementation is written in Clojure, partly because the original was Common Lisp but more because I enjoy writing Clojure. You can find it here; it can be run from the command-line, or there’s a GUI to help visualize a single run, plus a script which tries to solve each of 10 puzzles 100 times, which I’m using to track the effectiveness of changes I make. Because there’s a lot of random processes underlying Numbo, different runs can and do produce different results - or sometimes no results.
I’ve /just/ managed to get Numbo to a point where it solves 7/10 of the original sample puzzles, which in the spirit of “launching early enough to be embarrassed by my first version” seems like the right time for a push to GitHub. I have some ideas about the remaining 3 and other improvements - I suspect that speedy puzzle-solving is influenced by some of the decisions around decay rates of nodes in the Pnet and cytoplasm. There are a few other jumping-off points for future work:
- The contents of the Pnet are important; I’m interested in working out how you might construct a viable Pnet from e.g. educational materials;
- I wonder how codelets might be evolved rather than hand-coded;
- I wonder how transferable the architecture might be to a new micro domain.
Lots to think about! The process of writing this was also quite fun. It’s the largest program I’ve written since my Master’s dissertation on superoptimization (code, dissertation, paper). A few observations:
- I wrote the cytoplasm 3 times: the first one was too simple, the second too complex (but I got to learn about Clojure zippers) and the third one OK so far - but I think I’ve just found a bug which points to a error in my data model (around how secondary targets are treated)
- Writing a GUI to examine the state of runs was a good move: it’s saved me so a ton of time in debugging, as I can visually run through the evolving state of the cytoplasm, identify odd points by eye and dive in to debug them. Swing is still a PITA, even with Seesaw wrapping it.
- After starting to read Paul Graham’s On Lisp, I regret not having pulled key abstractions beyond library functions into macros. My codelets contain a ton of boilerplate and whilst I pulled out some functions to handle probabilistic sampling, I feel like this could be made into a part of the language a bit more.
- In a lovely demonstration of Conway’s Law, the needs of real life meant I had to write Numbo in tiny non-cooperating chunks which made individual sense. I left messages to future-me on what to do next but otherwise frequently lost state when I returned to work on it. Given all this, I became a set of tiny discrete processes acting towards a greater goal…
Donkey no moreAugust 01, 2020 | Comments
My attempt to resurrect this site in 2017 didn’t go very far. I had intended to start writing more about my experiments with the Donkey car, but they hit a bit of a wall when I started in X and had real work to do there, also parenting. I spent a year in X working for what’s now been revealed as the Everyday Robot project, and then returned to my previous team in Google Research, where I’m still working today.
In the last 3 years I think I resurrected the Donkey car 2 or 3 times, and even managed a short habit of heading to one of the DeepRacing get-togethers on a Saturday, or to DIY Robocars races; but it turns out that working full time and then taking half a day out at weekends to go play with cars isn’t totally compatible with Life. After a short attempt to get a track set up in my attic (tl;dr it’s big but needs to be bigger to be useful), I threw in the towel… for now.
Apologies for the spare look. I wanted to get something up fast, if I get around to it I’ll play with look and feel later…
Resurrecting the weblogJuly 11, 2017 | Comments
So it’s been a little while since I posted here - nearly 5 years. I’ve had very little to say in public, work and life have been a lot of fun, and I’ve not missed writing.
A lot has changed in that time: I joined Google in London, turned 40, got married, relocated to San Francisco, and at the end of last year our daughter arrived.
A lot has stayed the same too. When I look at my writing I’m happy to see some continuity with my work at Google. There I ended up tugging on a thread of “user interfaces for local intelligence” which manifested across many products I worked on: Auto Awesome Movies, the Android launcher, Smart Text Selection, and Federated Learning (not an exhaustive list). I’ve particularly enjoyed working with a mix of designers, engineers and researchers.
And to hammer home the point about continuity, my last post here touched on the tortoises of W. Grey Walter; fittingly, in August I’m moving to X to work in robotics. More interfaces, more intelligence, a new domain.
This is why I’ve resurrected this site: to get some hands-on experience I’ve been playing at the edges of the DIY Robocars movement, building a Donkey and trying to get it running reliably around The Panhandle, a nearby park. I want somewhere to share what I’m doing; this site will do nicely, thank you.
Turing, Tortoises, and Lancaster BombersOctober 02, 2012 | Comments
The Science Museum in London is currently showing an exhibition about Alan Turing. Kate and I wandered up there on Saturday. I found the exhibition itself a little superficial - which isn't so surprising given the breadth of material the curators had to draw on between his personal life and death, contributions to computing, the war effort and Bletchley park, and his work on morphogenesis.
But there were two little gems in there which I focused on: the first, one of the tortoises of W. Grey Walter: beautiful and tremendously simplistic devices which exhibit eerily animalistic behaviours. And secondly, a bombsight computer from a Lancaster bomber, on which I was chuffed to discover the manufacturer's mark of Sperry. For it was a Sperry machine which D P Henry used to create his spirograph...