Compression scheduler

In iterative pruning, we create some kind of pruning regimen that specifies how to prune, and what to prune at every stage of the pruning and training stages. This motivated the design of CompressionScheduler: it needed to be part of the training loop, and to be able to make and implement pruning, regularization and quantization decisions. We wanted to be able to change the particulars of the compression schedule, w/o touching the code, and settled on using YAML as a container for this specification. We found that when we make many experiments on the same code base, it is easier to maintain all of these experiments if we decouple the differences from the code-base. Therefore, we added to the scheduler support for learning-rate decay scheduling because, again, we wanted the freedom to change the LR-decay policy without changing code.

High level overview

Let's briefly discuss the main mechanisms and abstractions: A schedule specification is composed of a list of sections defining instances of Pruners, Regularizers, Quantizers, LR-scheduler and Policies.

  • Pruners, Regularizers and Quantizers are very similar: They implement either a Pruning/Regularization/Quantization algorithm, respectively.
  • An LR-scheduler specifies the LR-decay algorithm.

These define the what part of the schedule.

The Policies define the when part of the schedule: at which epoch to start applying the Pruner/Regularizer/Quantizer/LR-decay, the epoch to end, and how often to invoke the policy (frequency of application). A policy also defines the instance of Pruner/Regularizer/Quantizer/LR-decay it is managing.
The CompressionScheduler is configured from a YAML file or from a dictionary, but you can also manually create Policies, Pruners, Regularizers and Quantizers from code.

Syntax through example

We'll use alexnet.schedule_agp.yaml to explain some of the YAML syntax for configuring Sensitivity Pruning of Alexnet.

version: 1
pruners:
  my_pruner:
    class: 'SensitivityPruner'
    sensitivities:
      'features.module.0.weight': 0.25
      'features.module.3.weight': 0.35
      'features.module.6.weight': 0.40
      'features.module.8.weight': 0.45
      'features.module.10.weight': 0.55
      'classifier.1.weight': 0.875
      'classifier.4.weight': 0.875
      'classifier.6.weight': 0.625

lr_schedulers:
   pruning_lr:
     class: ExponentialLR
     gamma: 0.9

policies:
  - pruner:
      instance_name : 'my_pruner'
    starting_epoch: 0
    ending_epoch: 38
    frequency: 2

  - lr_scheduler:
      instance_name: pruning_lr
    starting_epoch: 24
    ending_epoch: 200
    frequency: 1

There is only one version of the YAML syntax, and the version number is not verified at the moment. However, to be future-proof it is probably better to let the YAML parser know that you are using version-1 syntax, in case there is ever a version 2.

version: 1

In the pruners section, we define the instances of pruners we want the scheduler to instantiate and use.
We define a single pruner instance, named my_pruner, of algorithm SensitivityPruner. We will refer to this instance in the Policies section.
Then we list the sensitivity multipliers, \(s\), of each of the weight tensors.
You may list as many Pruners as you want in this section, as long as each has a unique name. You can several types of pruners in one schedule.

pruners:
  my_pruner:
    class: 'SensitivityPruner'
    sensitivities:
      'features.module.0.weight': 0.25
      'features.module.3.weight': 0.35
      'features.module.6.weight': 0.40
      'features.module.8.weight': 0.45
      'features.module.10.weight': 0.55
      'classifier.1.weight': 0.875
      'classifier.4.weight': 0.875
      'classifier.6.weight': 0.6

Next, we want to specify the learning-rate decay scheduling in the lr_schedulers section. We assign a name to this instance: pruning_lr. As in the pruners section, you may use any name, as long as all LR-schedulers have a unique name. At the moment, only one instance of LR-scheduler is allowed. The LR-scheduler must be a subclass of PyTorch's _LRScheduler. You can use any of the schedulers defined in torch.optim.lr_scheduler (see here). In addition, we've implemented some additional schedulers in Distiller (see here). The keyword arguments (kwargs) are passed directly to the LR-scheduler's constructor, so that as new LR-schedulers are added to torch.optim.lr_scheduler, they can be used without changing the application code.

lr_schedulers:
   pruning_lr:
     class: ExponentialLR
     gamma: 0.9

Finally, we define the policies section which defines the actual scheduling. A Policy manages an instance of a Pruner, Regularizer, Quantizer, or LRScheduler, by naming the instance. In the example below, a PruningPolicy uses the pruner instance named my_pruner: it activates it at a frequency of 2 epochs (i.e. every other epoch), starting at epoch 0, and ending at epoch 38.

policies:
  - pruner:
      instance_name : 'my_pruner'
    starting_epoch: 0
    ending_epoch: 38
    frequency: 2

  - lr_scheduler:
      instance_name: pruning_lr
    starting_epoch: 24
    ending_epoch: 200
    frequency: 1

This is iterative pruning:

  1. Train Connectivity

  2. Prune Connections

  3. Retrain Weights

  4. Goto 2

It is described in Learning both Weights and Connections for Efficient Neural Networks:

"Our method prunes redundant connections using a three-step method. First, we train the network to learn which connections are important. Next, we prune the unimportant connections. Finally, we retrain the network to fine tune the weights of the remaining connections...After an initial training phase, we remove all connections whose weight is lower than a threshold. This pruning converts a dense, fully-connected layer to a sparse layer. This first phase learns the topology of the networks — learning which connections are important and removing the unimportant connections. We then retrain the sparse network so the remaining connections can compensate for the connections that have been removed. The phases of pruning and retraining may be repeated iteratively to further reduce network complexity."

Regularization

You can also define and schedule regularization.

L1 regularization

Format (this is an informal specification, not a valid ABNF specification):

regularizers:
  <REGULARIZER_NAME_STR>:
    class: L1Regularizer
    reg_regims:
      <PYTORCH_PARAM_NAME_STR>: <STRENGTH_FLOAT>
      ...
      <PYTORCH_PARAM_NAME_STR>: <STRENGTH_FLOAT>
    threshold_criteria: [Mean_Abs | Max]

For example:

version: 1

regularizers:
  my_L1_reg:
    class: L1Regularizer
    reg_regims:
      'module.layer3.1.conv1.weight': 0.000002
      'module.layer3.1.conv2.weight': 0.000002
      'module.layer3.1.conv3.weight': 0.000002
      'module.layer3.2.conv1.weight': 0.000002
    threshold_criteria: Mean_Abs

policies:
  - regularizer:
      instance_name: my_L1_reg
    starting_epoch: 0
    ending_epoch: 60
    frequency: 1

Group regularization

Format (informal specification):

Format:
  regularizers:
    <REGULARIZER_NAME_STR>:
      class: L1Regularizer
      reg_regims:
        <PYTORCH_PARAM_NAME_STR>: [<STRENGTH_FLOAT>, <'2D' | '3D' | '4D' | 'Channels' | 'Cols' | 'Rows'>]
        <PYTORCH_PARAM_NAME_STR>: [<STRENGTH_FLOAT>, <'2D' | '3D' | '4D' | 'Channels' | 'Cols' | 'Rows'>]
      threshold_criteria: [Mean_Abs | Max]

For example:

version: 1

regularizers:
  my_filter_regularizer:
    class: GroupLassoRegularizer
    reg_regims:
      'module.layer3.1.conv1.weight': [0.00005, '3D']
      'module.layer3.1.conv2.weight': [0.00005, '3D']
      'module.layer3.1.conv3.weight': [0.00005, '3D']
      'module.layer3.2.conv1.weight': [0.00005, '3D']
    threshold_criteria: Mean_Abs

policies:
  - regularizer:
      instance_name: my_filter_regularizer
    starting_epoch: 0
    ending_epoch: 60
    frequency: 1

Mixing it up

You can mix pruning and regularization.

version: 1
pruners:
  my_pruner:
    class: 'SensitivityPruner'
    sensitivities:
      'features.module.0.weight': 0.25
      'features.module.3.weight': 0.35
      'features.module.6.weight': 0.40
      'features.module.8.weight': 0.45
      'features.module.10.weight': 0.55
      'classifier.1.weight': 0.875
      'classifier.4.weight': 0.875
      'classifier.6.weight': 0.625

regularizers:
  2d_groups_regularizer:
    class: GroupLassoRegularizer
    reg_regims:
      'features.module.0.weight': [0.000012, '2D']
      'features.module.3.weight': [0.000012, '2D']
      'features.module.6.weight': [0.000012, '2D']
      'features.module.8.weight': [0.000012, '2D']
      'features.module.10.weight': [0.000012, '2D']


lr_schedulers:
  # Learning rate decay scheduler
   pruning_lr:
     class: ExponentialLR
     gamma: 0.9

policies:
  - pruner:
      instance_name : 'my_pruner'
    starting_epoch: 0
    ending_epoch: 38
    frequency: 2

  - regularizer:
      instance_name: '2d_groups_regularizer'
    starting_epoch: 0
    ending_epoch: 38
    frequency: 1

  - lr_scheduler:
      instance_name: pruning_lr
    starting_epoch: 24
    ending_epoch: 200
    frequency: 1

Quantization

Similarly to pruners and regularizers, specifying a quantizer in the scheduler YAML follows the constructor arguments of the Quantizer class (see details here). Note that only a single quantizer instance may be defined per YAML.

Let's see an example:

quantizers:
  dorefa_quantizer:
    class: DorefaQuantizer
    bits_activations: 8
    bits_weights: 4
    bits_overrides:
      conv1:
        wts: null
        acts: null
      relu1:
        wts: null
        acts: null
      final_relu:
        wts: null
        acts: null
      fc:
        wts: null
        acts: null
  • The specific quantization method we're instantiating here is DorefaQuantizer.
  • Then we define the default bit-widths for activations and weights, in this case 8 and 4-bits, respectively.
  • Then, we define the bits_overrides mapping. In the example above, we choose not to quantize the first and last layer of the model. In the case of DorefaQuantizer, the weights are quantized as part of the convolution / FC layers, but the activations are quantized in separate layers, which replace the ReLU layers in the original model (remember - even though we replaced the ReLU modules with our own quantization modules, the name of the modules isn't changed). So, in all, we need to reference the first layer with parameters conv1, the first activation layer relu1, the last activation layer final_relu and the last layer with parameters fc.
  • Specifying null means "do not quantize".
  • Note that for quantizers, we reference names of modules, not names of parameters as we do for pruners and regularizers.

Defining overrides for groups of layers using regular expressions

Suppose we have a sub-module in our model named block1, which contains multiple convolution layers which we would like to quantize to, say, 2-bits. The convolution layers are named conv1, conv2 and so on. In that case we would define the following:

bits_overrides:
  'block1\.conv*':
    wts: 2
    acts: null
  • RegEx Note: Remember that the dot (.) is a meta-character (i.e. a reserved character) in regular expressions. So, to match the actual dot characters which separate sub-modules in PyTorch module names, we need to escape it: \.

Overlapping patterns are also possible, which allows to define some override for a groups of layers and also "single-out" specific layers for different overrides. For example, let's take the last example and configure a different override for block1.conv1:

bits_overrides:
  'block1\.conv1':
    wts: 4
    acts: null
  'block1\.conv*':
    wts: 2
    acts: null
  • Important Note: The patterns are evaluated eagerly - first match wins. So, to properly quantize a model using "broad" patterns and more "specific" patterns as just shown, make sure the specific pattern is listed before the broad one.

The QuantizationPolicy, which controls the quantization procedure during training, is actually quite simplistic. All it does is call the prepare_model() function of the Quantizer when it's initialized, followed by the first call to quantize_params(). Then, at the end of each epoch, after the float copy of the weights has been updated, it calls the quantize_params() function again.

policies:
    - quantizer:
        instance_name: dorefa_quantizer
      starting_epoch: 0
      ending_epoch: 200
      frequency: 1

Important Note: As mentioned here, since the quantizer modifies the model's parameters (assuming training with quantization in the loop is used), the call to prepare_model() must be performed before an optimizer is called. Therefore, currently, the starting epoch for a quantization policy must be 0, otherwise the quantization process will not work as expected. If one wishes to do a "warm-startup" (or "boot-strapping"), training for a few epochs with full precision and only then starting to quantize, the only way to do this right now is to execute a separate run to generate the boot-strapped weights, and execute a second which will resume the checkpoint with the boot-strapped weights.

Knowledge Distillation

Knowledge distillation (see here) is also implemented as a Policy, which should be added to the scheduler. However, with the current implementation, it cannot be defined within the YAML file like the rest of the policies described above.

To make the integration of this method into applications a bit easier, a helper function can be used that will add a set of command-line arguments related to knowledge distillation:

import argparse
import distiller

parser = argparse.ArgumentParser()
distiller.knowledge_distillation.add_distillation_args(parser)

(The add_distillation_args function accepts some optional arguments, see its implementation at distiller/knowledge_distillation.py for details)

These are the command line arguments exposed by this function:

Knowledge Distillation Training Arguments:
  --kd-teacher ARCH     Model architecture for teacher model
  --kd-pretrained       Use pre-trained model for teacher
  --kd-resume PATH      Path to checkpoint from which to load teacher weights
  --kd-temperature TEMP, --kd-temp TEMP
                        Knowledge distillation softmax temperature
  --kd-distill-wt WEIGHT, --kd-dw WEIGHT
                        Weight for distillation loss (student vs. teacher soft
                        targets)
  --kd-student-wt WEIGHT, --kd-sw WEIGHT
                        Weight for student vs. labels loss
  --kd-teacher-wt WEIGHT, --kd-tw WEIGHT
                        Weight for teacher vs. labels loss
  --kd-start-epoch EPOCH_NUM
                        Epoch from which to enable distillation

Once arguments have been parsed, some initialization code is required, similar to the following:

# Assuming:
# "args" variable holds command line arguments
# "model" variable holds the model we're going to train, that is - the student model
# "compression_scheduler" variable holds a CompressionScheduler instance

args.kd_policy = None
if args.kd_teacher:
    # Create teacher model - replace this with your model creation code
    teacher = create_model(args.kd_pretrained, args.dataset, args.kd_teacher, device_ids=args.gpus)
    if args.kd_resume:
        teacher, _, _ = apputils.load_checkpoint(teacher, chkpt_file=args.kd_resume)

    # Create policy and add to scheduler
    dlw = distiller.DistillationLossWeights(args.kd_distill_wt, args.kd_student_wt, args.kd_teacher_wt)
    args.kd_policy = distiller.KnowledgeDistillationPolicy(model, teacher, args.kd_temp, dlw)
    compression_scheduler.add_policy(args.kd_policy, starting_epoch=args.kd_start_epoch, ending_epoch=args.epochs,
                                     frequency=1)

Finally, during the training loop, we need to perform forward propagation through the teacher model as well. The KnowledgeDistillationPolicy class keeps a reference to both the student and teacher models, and exposes a forward function that performs forward propagation on both of them. Since this is not one of the standard policy callbacks, we need to call this function manually from our training loop, as follows:

if args.kd_policy is None:
    # Revert to a "normal" forward-prop call if no knowledge distillation policy is present
    output = model(input_var)
else:
    output = args.kd_policy.forward(input_var)

To see this integration in action, take a look at the image classification sample at examples/classifier_compression/compress_classifier.py.