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##Training an Object Classifier in Torch-7 on multiple GPUs over ImageNet

In this concise example (1200 lines including a general-purpose and highly scalable data loader for images), we showcase:

  • train AlexNet or Overfeat, VGG and Googlenet on ImageNet
  • showcase multiple backends: CuDNN, CuNN
  • use nn.DataParallelTable to speedup training over multiple GPUs
  • multithreaded data-loading from disk (showcases sending tensors from one thread to another without serialization)

Requirements

Data processing

The images dont need to be preprocessed or packaged in any database. It is preferred to keep the dataset on an SSD but we have used the data loader comfortably over NFS without loss in speed. We just use a simple convention: SubFolderName == ClassName. So, for example: if you have classes {cat,dog}, cat images go into the folder dataset/cat and dog images go into dataset/dog

The training images for imagenet are already in appropriate subfolders (like n07579787, n07880968). You need to get the validation groundtruth and move the validation images into appropriate subfolders. To do this, download ILSVRC2012_img_train.tar ILSVRC2012_img_val.tar and use the following commands:

# extract train data
mkdir train && mv ILSVRC2012_img_train.tar train/ && cd train
tar -xvf ILSVRC2012_img_train.tar && rm -f ILSVRC2012_img_train.tar
find . -name "*.tar" | while read NAME ; do mkdir -p "${NAME%.tar}"; tar -xvf "${NAME}" -C "${NAME%.tar}"; rm -f "${NAME}"; done
# extract validation data
cd ../ && mkdir val && mv ILSVRC2012_img_val.tar val/ && cd val && tar -xvf ILSVRC2012_img_val.tar
wget -qO- https://raw.githubusercontent.com/soumith/imagenetloader.torch/master/valprep.sh | bash

Now you are all set!

If your imagenet dataset is on HDD or a slow SSD, run this command to resize all the images such that the smaller dimension is 256 and the aspect ratio is intact. This helps with loading the data from disk faster.

find . -name "*.JPEG" | xargs -I {} convert {} -resize "256^>" {}

Custom dataset creation

If you have your own dataset to train the CNNs, use this dataset creation script to preprocess images and put them in a ImageNet-training compatible structure. It loads all images from src path and splits into two different sets (training_dir and validation_dir) of directories with the ratio of (1-ratio and ratio) under dst path. Images are resized and cropped into dim-squared images depending on crop mode such as inner_crop or outer crop (boundary padded by offset). This script is independent from the main training script and it has no assumption of source directory structure. It may not create certain class in validation set if the number of samples with respect to the ratio are too few. Using empty directory in the training or validation set will cause error in the main training script.

th create-dataset.lua --dst /path/to/dst --src /path/to/src

Running

The training scripts come with several options which can be listed by running the script with the flag --help

th main.lua --help

To run the training, simply run main.lua By default, the script runs 1-GPU AlexNet with the CuDNN backend and 2 data-loader threads.

th main.lua -data [imagenet-folder with train and val folders]

For 2-GPU model parallel AlexNet + CuDNN, you can run it this way:

th main.lua -data [imagenet-folder with train and val folders] -nGPU 2 -backend cudnn -netType alexnet

Similarly, you can switch the backends to 'cunn' to use a different set of CUDA kernels.

You can also alternatively train OverFeat using this following command:

th main.lua -data [imagenet-folder with train and val folders] -netType overfeat

# multi-GPU overfeat (let's say 2-GPU)
th main.lua -data [imagenet-folder with train and val folders] -netType overfeat -nGPU 2

The training script prints the current Top-1 and Top-5 error as well as the objective loss at every mini-batch. We hard-coded a learning rate schedule so that AlexNet converges to an error of 42.5% at the end of 53 epochs.

At the end of every epoch, the model is saved to disk (as model_[xx].t7 where xx is the epoch number). You can reload this model into torch at any time using torch.load

model = torch.load('model_10.t7') -- loading back a saved model

Similarly, if you would like to test your model on a new image, you can use testHook from line 103 in donkey.lua to load your image, and send it through the model for predictions. For example:

dofile('donkey.lua')
img = testHook({loadSize}, 'test.jpg')
model = torch.load('model_10.t7')
if img:dim() == 3 then
  img = img:view(1, img:size(1), img:size(2), img:size(3))
end
predictions = model:forward(img:cuda())

If you ever want to reuse this example, and debug your scripts, it is suggested to debug and develop in the single-threaded mode, so that stack traces are printed fully.

th main.lua -nDonkeys 0 [...options...]

Model Conversion

After the training, the model definition has a nested-table structure and contains training-specific modules which are not needed in evaluation phase. The model conversion script helps to optimize the model definition by simplifying its structure and removing unnecessary modules. The resulting output of the script is a model definition dependent on torch generic nn library. Users can choose degree of optimization by asserting flags, for example, dropout, batchnorm and softmax.

th convert-model.lua --src /path/to/src/model.t7 --dst /path/to/dst/model.t7

Code Description

  • main.lua (~30 lines) - loads all other files, starts training.
  • opts.lua (~50 lines) - all the command-line options and description
  • data.lua (~60 lines) - contains the logic to create K threads for parallel data-loading.
  • donkey.lua (~200 lines) - contains the data-loading logic and details. It is run by each data-loader thread. random image cropping, generating 10-crops etc. are in here.
  • model.lua (~80 lines) - creates AlexNet model and criterion
  • train.lua (~190 lines) - logic for training the network. we hard-code a learning rate + weight decay schedule that produces good results.
  • test.lua (~120 lines) - logic for testing the network on validation set (including calculating top-1 and top-5 errors)
  • dataset.lua (~430 lines) - a general purpose data loader, mostly derived from here: imagenetloader.torch. That repo has docs and more examples of using this loader.

Plotting charts

In order to plot train and test accuracy and cross-entropy values, cd into the results- folder and into the correct sub-directory. Then, issue the following command

gnuplot ../../tools/plotCharts.plt

The two plots are saved respectively as accuracy.eps and cross-entropy.eps in the current folder. To quit the gnuplot session, press <Return> in the terminal window.

Selecting spcific GPUs to use:

use CUDA_VISIBLE_DEVICES=2,3 before launching your script