# Training and Evaluation

By default, Transformers4Rec leverages the Merlin dataloader for GPU-accelerated loading of preprocessed data that is stored as Parquet files. Parquet enables this preprocessed data to be easily structured and queryable. The data in these parquet files are directly loaded to the GPU memory as feature tensors. CPUs are also supported when GPUs are not available.

The following example uses the Merlin Dataloader that is wrapped by the MerlinDataLoader class. The class automatically sets some options from the dataset schema. Optionally, you can use the PyarrowDataLoader as a basic option, but it is slower and works only for small datasets since the full data is loaded to CPU memory.

train_loader = transformers4rec.torch.utils.data_utils.MerlinDataLoader.from_schema(
schema,
paths_or_dataset=train_path,
batch_size=TrainingArguments.train_batch_size,
drop_last=True,
shuffle=True,
)


## PyTorch

To leverage our Transformers4Rec example notebooks that demonstrate how to use Transformers4Rec with PyTorch, refer to Transformers4Rec Example Notebooks.

### Training

For PyTorch, the HF transformers Trainer class is extended while retaining its train() method. Essentially, this means the efficient training implementation from that library is leveraged and manages half-precision (FP16) and multi-GPU training.

PyTorch supports two approaches for multi-GPU training: DataParallel and DistributedDataParallel. DataParallel uses a single process and multiple threads on a single machine. DistributedDataParallel is more efficient for assigning separate processes for each GPU. Transformers4Rec supports the DataParallel approach when using the Merlin dataloader.

The following code block shows how to create an instance of the Trainer class:

from transformers4rec.config.trainer import T4RecTrainingArguments
from transformers4rec.torch import Trainer

training_args = T4RecTrainingArguments(
output_dir="./tmp",
num_train_epochs=3,
fp16=True,
)

recsys_trainer = Trainer(
model=model,
args=training_args,
)

recsys_trainer.train()


You can automatically instantiate the dataloader when you create the Trainer instance by specifying the following:

• The path or dataset of the training and evaluation data in the train_dataset_path and eval_dataset_or_path arguments.

• Specify the schema for the dataset in the schema argument.

training_args = T4RecTrainingArguments(
...,
per_device_train_batch_size=256,
per_device_eval_batch_size=512,
)

# Instantiates the train and eval dataloader
Trainer(
model=model,
schema=schema,
args=training_args,
train_dataset_or_path=train_path,
eval_dataset_or_path=eval_path,
)


### Evaluation

For the item prediction head, top-N metrics and ranking metrics commonly used in [information retrieval](https://en.wikipedia.org/wiki/Evaluation_measures_(information_retrieval) and RecSys are supported for evaluation:

Top-N metrics
• Precision@n - Computes the percentage of the top-N recommended items, which are relevant (labels).

• Recall@n - Computes the percentage of relevant items (labels) that are present among the top-N recommended items.

Ranking metrics
• NDCG@n - Normalized Discounted Cumulative Gain at cut-off N of the recommendation list.

• MAP@n - Mean Average Precision at cut-off N of the recommendation list.

During training, the metrics are computed for each N step for both training and evaluation sets. During evaluation, the metrics are computed for all evaluation batches and averaged.

You can implement incremental evaluation by splitting your data into time windows such as week, day, or hour. A loop, which trains a model or fine-tunes a pre-trained model, can be used with sessions of time window T. This loop evaluates on sessions of time window T+1.

The following example contains a loop that iterates over the days, trains the model or fine-tunes the model pre-trained in the previous day, evaluates with data of the next day, and assumes daily data is split in folders:

# Iterates over parquet files with daily data
for time_index in range(1, 7):
train_paths = glob.glob(f"./data/day-{time_index}/data.parquet")
eval_paths = glob.glob(f"./data/day-{time_index+1}/data.parquet")

print('Training with day {}'.format(time_index))
trainer.train_dataset = train_paths
trainer.reset_lr_scheduler()
trainer.train()

print('Evaluating with day {}'.format(time_index+1))
trainer.eval_dataset = eval_paths
train_metrics = trainer.evaluate(metric_key_prefix='eval')
print(train_metrics)
trainer.wipe_memory()