I have previously written about Google CoLab which is a way to access Nvidia K80 GPUs for free, but only for 12 hours at a time. After a few months of using Google Cloud instances with GPUs I have run up a substantial bill and have reverted to using CoLab whenever possible. The main problem with CoLab is that the instance is terminated after 12 hours taking all files with it, so in order to use them you need to save your files somewhere.

Until recently I had been saving my files to Google Drive with this method, but while it is easy to save files to Drive it is much more difficult to read them back. As far as I can tell, in order to do this with the API you need to get the file id from Drive and even then it is not so straightforward to upload the files to CoLab. To deal with this I had been uploading files that needed to be accessed often to an AWS S3 bucket and then downloading them to CoLab with wget, which works fine, but there is a much simpler way to do the same thing by using Google Cloud Storage instead of S3.

First you need to authenticate CoLab to your Google account with:

from google.colab import auth


Once this is done you need to set your project and bucket name and then update the gcloud config.
project_id = [project_name]
bucket_name = [bucket_name]
!gcloud config set project {project_id}

After this has been done files can simply and quickly be upload or downloaded from the bucket with the following simple commands:

# download
!gsutil cp gs://{bucket_name}/foo.bar ./foo.bar

# upload
!gsutil cp  ./foo.bar gs://{bucket_name}/foo.bar

I actually have been adding the line to upload the weights to GCS to my training code so it is automatically uploaded every couple epochs, which removes the need for me to manually back them up periodically throughout the day.

Etiketten: coding, python, machine_learning, google, google_cloud
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Mittwoch 26 September 2018

When I first started working with TensorFlow I didn't really like Keras. It seemed like a dumbed down interface to TensorFlow and I preferred having greater control over everything to the ease of use of Keras. However I have recently changed my mind. When you use Keras with a TensorFlow back-end you can still use TensorFlow if you need to tweak something that you can't in Keras, but otherwise Keras just provides an easier to use way to access TensorFlow's functionality. This is especially useful for prototyping models since you can easily make changes without having to write or rewrite a lot of code. I used to write my own functions to do things like make a convolutional layer, but most of that was duplicating functionality that already exists in Keras. 

My original opinion was incorrect, Keras is a valuable tool for creating neural networks, and since you can mix TensorFlow in, there is nothing lost by using it.

Etiketten: machine_learning, tensorflow, keras
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IOU Loss

Mittwoch 29 August 2018

When doing binary image segmentation, segmenting images into foreground and background, cross entropy is far from ideal as a loss function. As these datasets tend to be highly unbalanced, with far more background pixels than foreground, the model will usually score best by predicting everything as background. I have confronted this issue during my work with mammography and my solution was to use a weighted sigmoid cross entropy loss function giving the foreground pixels higher weight than the background.

While this worked it was far from ideal, for one thing it introduced another hyperparameters - the weight - and altering the weight had a large impact on the model. Higher weights favored predicting pixels as positive, increasing recall and decreasing precision, and lowering the weight had the opposite effect. When training my models I usually began with a high weight to encourage the model to make positive predictions and gradually decayed the weight to encourage it to make negative predictions.

For these types of segmentation tasks Intersection over Union tends to be the most relevant metric as pixel level accuracy, precision and recall do not account for the overlap between predictions and ground truth. Especially for this task, where overlap can be the difference between life and death for the patient, accuracy is not as relevant as IOU. So why not use IOU as a loss function?

The reason was because IOU was not differentiable so can not be used for gradient descent. However Wang et al have written a paper - Optimizing Intersection-Over-Union in Deep Neural Networks for Image Segmentation - which provides an easy way to use IOU as a loss function. In addition, this site provides code to implement this loss function in TensorFlow.

The essence of this method is that rather than using the binary predictions to calculate IOU we use the sigmoid probability output by the logits to estimate it which allows IOU to provide gradients. At first I was skeptical of this method, mostly because I understood cross entropy better and it is more common, but after I hit a performance wall with my mammography models I decided to give it a try.

My models using cross-entropy loss had ceased to improve validation performance so I switched the loss function and trained them for a few more epochs. The validation metrics began to improve, so I decided to train a copy of the model from scratch with the IOU loss. This has been a resounding success. The IOU loss accounts for the imbalanced data, eliminating the need to weight the cross entropy. With the cross entropy loss the models usually began with recall of near 1 and precision of near 0 and then the precision would increase while the recall slowly decreased until it plateaued. With IOU loss they both start near 0 and gradually increase, which to me seems more natural. 

Training with an IOU loss has two concrete benefits for this task - it has allowed the model to detect more subtle abnormalities which models trained with cross entropy loss did not detect; and it has reduced the number of false positives significantly. As the false positives are on a pixel level this effectively means that the predictions are less noisy and the shapes are more accurate.

The biggest benefit is that we are directly optimizing for our target metric rather than attempting to use an imperfect substitute which we hope will approximate the target metric. Note that this method only works for binary segmentation at the moment. It also is a bit slower than using cross entropy, but if you are doing binary segmentation the performance boost is well worth it.


Etiketten: python, machine_learning, mammography, convnets
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Early Stopping

Montag 30 Juli 2018

I recently began using the early stopping feature of Light GBM, which allows you to stop training when the validation score doesn't improve for a certain number of rounds. This is especially useful if you are bagging models, as you don't need to watch each one and figure out when training should stop. The way it works is you specify a number of rounds, and if the validation score doesn't improve during that number of rounds the training is stopped and the round with the best validation score is used.

When working with this I noticed that often the best validation round is a very early round, which has a very good validation score but an incredibly low training score. As an example here is the output from a model I am currently training. Normally the training F1 gets up to the high 0.90s:

Early stopping, best iteration is:
[7]	train's macroF1: 0.525992	valid's macroF1: 0.390373

Out of at least 400 rounds of training, the best performance on the validation set was on the 7th, at which time it was performing incredibly poorly on the training data. This indicates overfitting to the validation set, which is just as bad as overfitting to the training set in that the model is not likely to generalize well.

So what to do about this issue? The obvious solution would be to provide a minimum number of rounds and begin to monitor the validation score for early stopping once that number of rounds has passed, but I don't see any way to do this through the LGB API. 

I am running this code using sklearn's joblib to do parallel processing, so I have create a list of the estimators to fit and then pass that list to the parallel processing which calls a function which fits the estimator to the data and returns it. The early stopping is taken care of by LGB, so what I did is after the estimator is fit I manually get the validation results and the train performance for the best validation round. If the train performance is above a specified threshold I return the estimator as normal. If, however, the train performance is below that threshold I recursively call the function again. 

The downside to this is that it is possible to get into an infinite loop, but if the thresholds are properly tuned this should be easily avoidable. 


Etiketten: coding, data_science, machine_learning, lightgbm
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I recently started looking at a Kaggle Challenge about predicting poverty levels in Costa Rica. I used sklearn train_test_split to split the training data into train and validation sets and fit a few models. The first thing I noticed was that my submissions scored significantly lower than my validation sets: 0.36 on the submission vs. .96 on my validation data.

The data consists of information about individuals with the target as their poverty level. The features include both information relating to that individual as well as information for the household they live in. The data includes multiple individuals from the same household, and some exploratory data analysis indicated that most of the features were on a household level rather than the individual level.

This means that doing a random split ends up including data from the same household in both the train and validation datasets, which will result in the leakage that artificially raised my initial validation scores. This also means that my models were all tuned on a validation dataset which was essentially useless.

To fix this I did the split on unique household IDs, so no household would be included in both datasets. After re-tuning the models appropriately, the validation f1 scores had gone down from 0.96 to 0.65. The submissions scores went up to 0.41, which was not a huge increase, but it was much closer to the validation scores.

The moral of this story is never forget to make sure that your training and validation sets don't contain overlap or leakage, or the validation set becomes useless.

Etiketten: data_science, machine_learning, kaggle
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