ColossalAI/docs/trainer_engine.md

Build your engine & Customize your trainer

Build your engine

To better understand how Engine class works, let's start from the conception of the process function in common engines. The process function usually controls the behavior over a batch of a dataset, Engine class just controls the process function. Here we give a standard process function in the following code block.

def process_function(dataloader, model, criterion, optim):
    optim.zero_grad()
    data, label = next(dataloader)
    output = model(data)
    loss = criterion(output, label)
    loss.backward()
    optim.setp()

In ignite.engine or keras.engine, the process function is always provided by users. However, it is tricky for users to write their own process functions for pipeline parallelism. Aiming at offering accessible hybrid parallelism for users, we provide the powerful Engine class. This class enables pipeline parallelism and offers one-forward-one-backward non-interleaving strategy. Also, you can use pre-defined learning rate scheduler in the Engine class to adjust learning rate during training.

In order to build your engine, just set variables model, criterion, optimizer, lr_scheduler and schedule. The following code block provides an example. The engine is automatically created from the config file for you if you start with colossalai.initialize.

import torch
import torch.nn as nn
import torchvision.models as models
import colossalai
from colossalai.engine import Engine

model = models.resnet18()
criterion = nn.CrossEntropyLoss()
optimizer = torch.optim.Adam(model.parameters())
schedule = colossalai.engine.NoPipelineSchedule()

MyEngine = Engine(
    model=model,
    criterion=criterion,
    optimizer=optimizer,
    step_schedule=schedule
)

More information regarding the class can be found in the API references.

Customize your trainer

Overview

To learn how to customize a trainer which meets your needs, let's first give a look at the Trainer class. We highly recommend that you read Get Started section and Build your engine first.

The Trainer class enables researchers and engineers to use our system more conveniently. Instead of having to write your own scripts, you can simply construct your own trainer by calling the Trainer class, just like what we did in the following code block.

MyTrainer = Trainer(my_engine)

After that, you can use the fit method to train or evaluate your model. In order to make our Trainer class even more powerful, we incorporate a set of handy tools to the class. For example, you can monitor or record the running states and metrics which indicate the current performance of the model. These functions are realized by hooks. The BasicHook class allows you to execute your hook functions at specified time. We have already created some practical hooks for you, as listed below. What you need to do is just picking the right ones which suit your needs. Detailed descriptions of the class can be found in the API references.

hooks = [
    dict(type='LogMetricByEpochHook'),
    dict(type='LogTimingByEpochHook'),
    dict(type='LogMemoryByEpochHook'),
    dict(type='AccuracyHook'),
    dict(type='LossHook'),
    dict(type='TensorboardHook', log_dir='./tfb_logs'),
    dict(type='SaveCheckpointHook', interval=5, checkpoint_dir='./ckpt'),
    dict(type='LoadCheckpointHook', epoch=20, checkpoint_dir='./ckpt')
]

These hook functions will record metrics, elapsed time and memory usage and write them to log after each epoch. Besides, they print the current loss and accuracy to let users monitor the performance of the model.

Hook

If you have your specific needs, feel free to extend our BaseHook class to add your own functions, or our MetricHook class to write a metric collector. These hook functions can be called at twelve timing in the trainer's life cycle. Besides, you can define the priorities of all hooks to arrange the execution order of them. More information can be found in the API references.

Metric

You can write your own metrics by extending our Metric class. It should be used with the MetricHook class. When your write your own metric hooks, please set the priority carefully and make sure the hook is called before other hooks which might require the results of the metric hook.

We've already provided some metric hooks and we store metric objects in runner.states['metrics']. It is a dictionary and metrics can be accessed by their names.