What is "epoch" in keras.models.Model.fit? Is it one gradient update? If it is more than one gradient update, then what is defining an epoch?
Suppose I am feeding my own batches to fit. I would regard "epoch" as finishing to process entire training set (is this correct)? Then how to control keras for this way? Can I set batch_size equal to x and y size and epochs to 1?
Here is how Keras documentation defines an epoch:
Epoch: an arbitrary cutoff, generally defined as "one pass over the entire dataset", used to separate training into distinct phases, which is useful for logging and periodic evaluation.
So, in other words, a number of epochs means how many times you go through your training set.
The model is updated each time a batch is processed, which means that it can be updated multiple times during one epoch. If batch_size is set equal to the length of x, then the model will be updated once per epoch.
Related
When training a net does it matter if the number of samples in the epoch is not an exact multiple of the batch size? My training code doesnt seem to mind if this is the case, though my loss curve is pretty noisy at the moment (in case that is a related issue).
This would be useful to know, as if it is not an issue it saves on messing around with the dataset to make it quantized by batch size. It may also be less wasteful of captured data.
does it matter if the number of samples in the epoch is not an exact multiple of the batch size
No, it does not. Your number of samples can be say 1000, and your batch size can be 400.
You can decide the total number of iterations (where each iteration = sampling a batch and doing gradient descent) based on the overall number of epochs you want to cover. Say, you want to have roughly 5 epochs, then roughly your number of iterations >= 5 * 1000 / 400 = 13. So you will sample a random batch 13 times to get roughly 5 epochs.
In the context of Convolution Neural Networks (CNN), Batch size is the number of examples that are fed to the algorithm at a time. This is normally some small power of 2 like 32,64,128 etc. During training an optimization algorithm computes the average cost over a batch then runs backpropagation to update the weights. In a single epoch the algorithm is run with $n_{batches} = {n_{examples} \over batchsize} $ times. Generally the algorithm needs to train for several epochs to achieve convergence of weight values. Every batch is normally sampled randomly from the whole example set.
The idea is this: mini-batch optimization wrt (x1,..., xn) is equivalent to consecutive optimization steps wrt x1, ..., xn inputs, because the gradient is a linear operator. This means that mini-batch update equals to the sum of its individual updates. Important note here: I assume that NN doesn't apply batch-norm or any other layer that adds an explicit variation to the inference model (in this case the math is a bit more hairy).
So the batch size can be seen as a pure computational idea that speeds up the optimization through vectorization and parallel computing. Assuming that one can afford arbitrarily long training and the data are properly shuffled, the batch size can be set to any value. But it isn't automatically true for all hyperparameters, for example very high learning rate can easily force the optimization to diverge, so don't make a mistake thinking hyperparamer tuning isn't important in general.
I have been reading this interesting link Linear Regression - SGD
and i have got question on below statement.
" The way this optimization algorithm works is that each training instance is shown to the model one at a time. The model makes a prediction for a training instance, the error is calculated and the model is updated in order to reduce the error for the next prediction. This process is repeated for a fixed number of iterations."
Question:
Is my below pseudo code correct?
for each training input:
1) Input to Model
2) Find the prediction
3) Find the error
4) Update Model.
What i don't understand is "This process is repeated for a fixed number of iterations" . Does it mean step 4) and step 3) is repeated until the error is minimized?
Correct me if i am wrong?
"This process is repeated for a fixed number of iterations." means that you choose the number of epochs or the number of batches send to you network to train it.
When you train your network you have a training dataset. You give your network (with placeholders) iages and labels associated with these inputs (generally you give samples (input + label) by batches).
It makes a prediction for each input and computes the error (the loss function you uses). And then it tunes weights (and biases) to minimize the loss function (it does what is called a gradient descent).
You should tale a look at Gradient Descent here : http://sebastianruder.com/optimizing-gradient-descent/
You are the one deciding how long you want your network to train by fixing the number of time your whole training set is going to be send to your network (what's called an epoch) or the number of batches.
Hope it helps
I was running a random forest classification model and initially divided the data into train (80%) and test (20%). However, the prediction had too many False Positive which I think was because there was too much noise in training data, so I decided to split the data in a different method and here's how I did it.
Since I thought the high False Positive was due to the noise in the train data, I made the train data to have the equal number of target variables. For example, if I have data of 10,000 rows and the target variable is 8,000 (0) and 2,000 (1), I had the training data to be a total of 4,000 rows including 2,000 (0) and 2,000 (1) so that the training data now have more signals.
When I tried this new splitting method, it predicted way better by increasing the Recall Positive from 14 % to 70%.
I would love to hear your feedback if I am doing anything wrong here. I am concerned if I am making my training data biased.
When you have unequal number of data points in each classes in training set, the baseline (random prediction) changes.
By noisy data, I think you want to mean that number of training points for class 1 is more than other. This is not really called noise. It is actually bias.
For ex: You have 10000 data point in training set, 8000 of class 1 and 2000 of class 0. I can predict class 0 all the time and get 80% accuracy already. This induces a bias and baseline for 0-1 classification will not be 50%.
To remove this bias either you can intentionally balance the training set as you did or you can change the error function by giving weight inversely proportional to number of points in training set.
Actually, what you did is right and this process is something similar to "Stratified sampling".
In your first model,where accuracy was very low the model did not get enough correlations between features and target for positive class(1).Also it model might have somewhat over-fitted for negative class.This is called "High bias -High variance" situation.
"Stratified sampling" is nothing but when you are extracting a sample data from a big population,you make sure that all classes will have some what approximately equal proportion to make the model's training assumptions more accurate and reliable.
In the second case model was able to correlate relationships between features and target and positive and negative class characteristics was well distinguishable.
Eliminating noise is a part of data preparation that should be obviously done before putting data into a model.
On Caffe, I am trying to implement a Fully Convolution Network for semantic segmentation. I was wondering is there a specific strategy to set up your 'solver.prototxt' values for the following hyper-parameters:
test_iter
test_interval
iter_size
max_iter
Does it depend on the number of images you have for your training set? If so, how?
In order to set these values in a meaningful manner, you need to have a few more bits of information regarding your data:
1. Training set size the total number of training examples you have, let's call this quantity T.
2. Training batch size the number of training examples processed together in a single batch, this is usually set by the input data layer in the 'train_val.prototxt'. For example, in this file the train batch size is set to 256. Let's denote this quantity by tb.
3. Validation set size the total number of examples you set aside for validating your model, let's denote this by V.
4. Validation batch size value set in batch_size for the TEST phase. In this example it is set to 50. Let's call this vb.
Now, during training, you would like to get an un-biased estimate of the performance of your net every once in a while. To do so you run your net on the validation set for test_iter iterations. To cover the entire validation set you need to have test_iter = V/vb.
How often would you like to get this estimation? It's really up to you. If you have a very large validation set and a slow net, validating too often will make the training process too long. On the other hand, not validating often enough may prevent you from noting if and when your training process failed to converge. test_interval determines how often you validate: usually for large nets you set test_interval in the order of 5K, for smaller and faster nets you may choose lower values. Again, all up to you.
In order to cover the entire training set (completing an "epoch") you need to run T/tb iterations. Usually one trains for several epochs, thus max_iter=#epochs*T/tb.
Regarding iter_size: this allows to average gradients over several training mini batches, see this thread fro more information.
while I'm reading in how to build ANN in pybrain, they say:
Train the network for some epochs. Usually you would set something
like 5 here,
trainer.trainEpochs( 1 )
I looked for what is that mean , then I conclude that we use an epoch of data to update weights, If I choose to train the data with 5 epochs as pybrain advice, the dataset will be divided into 5 subsets, and the wights will update 5 times as maximum.
I'm familiar with online training where the wights are updated after each sample data or feature vector, My question is how to be sure that 5 epochs will be enough to build a model and setting the weights probably? what is the advantage of this way on online training? Also the term "epoch" is used on online training, does it mean one feature vector?
One epoch consists of one full training cycle on the training set. Once every sample in the set is seen, you start again - marking the beginning of the 2nd epoch.
This has nothing to do with batch or online training per se. Batch means that you update once at the end of the epoch (after every sample is seen, i.e. #epoch updates) and online that you update after each sample (#samples * #epoch updates).
You can't be sure if 5 epochs or 500 is enough for convergence since it will vary from data to data. You can stop training when the error converges or gets lower than a certain threshold. This also goes into the territory of preventing overfitting. You can read up on early stopping and cross-validation regarding that.
sorry for reactivating this thread.
im new to neural nets and im investigating the impact of 'mini-batch' training.
so far, as i understand it, an epoch (as runDOSrun is saying) is a through use of all in the TrainingSet (not DataSet. because DataSet = TrainingSet + ValidationSet). in mini batch training, you can sub divide the TrainingSet into small Sets and update weights inside an epoch. 'hopefully' this would make the network 'converge' faster.
some definitions of neural networks are outdated and, i guess, must be redefined.
The number of epochs is a hyperparameter that defines the number times that the learning algorithm will work through the entire training dataset. One epoch means that each sample in the training dataset has had an opportunity to update the internal model parameters.