Automatic Caffe parameter search via Spearmint Bayesian optimisation
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Caffe with Spearmint (CWSM)

Automatic Caffe parameter search via Spearmint Bayesian optimisation. For those familiar with Caffe: now instead of specifying

weight_decay: 0.05

in your solver.prototxt you can write

weight_decay: OPTIMIZE{"type": "FLOAT", "min": 0, "max": 0.2}

and CWSM will use Spearmint to find the best weight_decay for you.

For those not familiar with Caffe: you might have heard that choosing the right parameters for a deep neural network is a painful process. This tool employs the power of Bayesian optimization methods to make this search automatic for the models you describe in Caffe framework.

The OPTIMIZE keyword can be used both in solver.prototxt and trainval.prototxt for as many parameters simultaneuosly as you want (be careful: too many parameters might take too long).

Take a look at experiments/mnist and read the "General Setup" section for a quick start. There are few special tricks which may seem odd at first, but, as you will see below, they are justified. Refer to the "Optimization Parameters" section below for more detailed documentation.

General Setup

STEP 1: Install Caffe

Follow the instructions in the Caffe repository. Once installed set the CAFFE_ROOT variable in the script to point to the Caffe repository root so that CAFFE_ROOT/build/tools/caffe would be the Caffe binary.

STEP 2: Install Spearmint

Follow the instructions in the Spearmint repository. Once installed set the SPEARMINT_ROOT variable in the script to point to the Spearmint repository root so that SPEARMINT_ROOT/spearmint/ is the Spearmint main script.

STEP 3: Prepare experiment directory

The main parameter CWSM's script needs is the location of the experiment directory. Just create empty experiments/myexperiment directory and you are ready to go to the next step, will take care of creating the structure and will tell you what to do next.
For the reference, the final structure will look like this:

  caffeout                  # [created automatically] will be filled with Caffe output
  data                      # holds your data files
    mean_train.binaryproto  # image means computed with Caffe "compute_image_mean"
    train_lmdb              # LMDB directory for the training set
    val_lmdb                # LMDB directory for the validation set
  model                     # here you describe the model
      solver.prototxt       # yes, it has to be named exactly like that
      trainval.prototxt     # this one too
  mongodb                   # [created automatically] Spearmint holds its stuff here
  spearmint                 # [created automatically] Spearmint config and optimization function definition
    output                  # [created automatically] Spearmint output files
  tmp                       # [created automatically] experiment run specific data

STEP 4: Describe your Caffe model

Under experiments/myexperiment/model CWSM will expect to find two (in special cases three) files:

Take a look at experiments/mnist/model/solver.prototxt. Pay attention to the max_iter parameter -- it defines how long each configuration run will take.

This file is used for training, if you are going to optimize accuracy you should have a layer named "accuracy", for example

layer {
  name: "accuracy"
  type: "Accuracy"
  bottom: "fc2"
  bottom: "label"
  top: "accuracy"

If you are optmizing w.r.t. loss then you should have a layer named "loss", something like

layer {
  name: "loss"
  type: "SoftmaxWithLoss"
  bottom: "fc2"
  bottom: "label"
  top: "loss"

This file is needed if you use a performance measure, which is not built-in in Caffe. Currently there is only one such implemented: squared kappa. To use that you will need to create val.prototxt which mimicks trainval.prototxt. The difference is that instead of loss/accuracy layers val.prototxt should have a layer named prob. This layer is used to extract predictions used to calculate kappa.

layer {
  name: "prob"
  type: "Softmax"
  bottom: "fc2"
  top: "prob"

STEP 5: Run the optimization

Make sure you have Caffe in your $PYTHONPATH.
The script has to be started from the repository root.
Assuming CWSM_ROOT is the location of this repository, do the following:


Now you are ready to optimize!

python --experiment experiments/mnist --optimize accuracy --optimizewrt best

where --optimizewrt can be either last, meaning that once a Caffe run is completed the very last accuracy/loss result will be given to Spearmint for evaluation, or best to pick the best result from all the evaluation phases.

STEP 6: Enjoy!

Leave the process running for a night and in the morning have a look at the latest

Minimum expected objective value under model is -0.99590 (+/- 0.08043), at location:

message in STDOUT. There is a chance that it will provide you with the best parameter configuation you've seen so far.

Optimization Parameters

Spearmint supports three types of variables: INT, FLOAT and ENUM. Here is how you use them in CWSM.


Default way is OPTIMIZE{"type": "INT", "min": 2, "max": 5}, for example if you want to try different strides for a convolutional layer.

layer {
  name: "conv1"
  type: "Convolution"
  convolution_param {
    stride: OPTIMIZE{"type": "INT", "min": 2, "max": 5}

There are useful additional tricks for INT, see them below.


Written as OPTIMIZE{"type": "FLOAT", "min": 0, "max": 0.2}. For example

weight_decay: OPTIMIZE{"type": "FLOAT", "min": 0, "max": 0.2}

in your solver.prototxt.
Note that finding the right FLOAT parameter will take longer than finding an INT parameter.


Written as OPTIMIZE{"type": "ENUM", "options": ["2", "17", "100"]}, for example

lr_policy: OPTIMIZE{"type": "ENUM", "options": ["fixed", "inv", "step"]}

in your solver.prototxt.
Note that ENUM loses the relative information between the options: 0.5 is not less than 0.6 anymore, they are completely unrelated values when used with ENUM.

Important Tricks

There are many cases where you would like to keep relative information between the values and avoid using ENUM. For example if you want to try {100, 200, 300} to be the size of a fully connected layer. Or to try learning rates {0.0001, 0.001, 0.01} but avoid using FLOAT because you do not want to waste time on values like 0.0101 etc. For these cases CWSM has a special transform parameter which can be used with INT. Here are examples of what it can do with some intuition why this might be a good idea.


Written as OPTIMIZE{"type": "INT", "transform": "X10", "min": 1, "max": 5}. Here "transform":"X10" acts as a multiplier where instead of 10 you can use any number: "X10", "X100", "X22". The effect of this particular line is that numbers {10, 20, 30, 40, 50} will be tried as the parameter value.
Another example would be

layer {
  name: "fc1"
  type: "InnerProduct"
  inner_product_param {
    num_output: OPTIMIZE{"type": "INT", "transform": "X100", "min": 1, "max": 3}

which will try {100, 200, 300} as the size of the fully connected layer.
Reasonable thing to ask is why not to use ENUM for this: num_output: OPTIMIZE{"type": "ENUM", "options": ["100", "200", "300"]}? Because the Bayesian optimizer will not be able to use the fact that 100 < 200 anymore.


Where [N] is the base of exponentiation. The number which goes to Spearmint is negative of the exponent (example: value 3 with NEGEXP10 means 10^-3 and correpsonds to 0.001). Can be used for learning rate in solver.prototxt:

base_lr: OPTIMIZE{"type": "INT", "transform": "NEGEXP10", "min": 2, "max": 5}

This will try {0.01, 0.001, 0.0001, 0.0001} keeping in mind their order and without the need to try all intermediate values as FLOAT would do.


Number which goes to Spearmint corresponds to log with base [N] of an actual number (example: value 2 of LOG2 corresponds to 4). Would be handy if you want to try 2,4,8,16,... for some parameter.


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