Thursday, 28 January 2021

Final step of PyTorch Gradient Accumulation for small datasets

I am training a BERT model on a relatively small dataset and cannot afford to lose any labelled sample as they must all be used for training. Due to GPU memory constraints, I am using gradient accumulation to train on larger batches (e.g. 32). According to PyTorch documentation, gradient accumulation is implemented as follows:

scaler = GradScaler()

for epoch in epochs:
    for i, (input, target) in enumerate(data):
        with autocast():
            output = model(input)
            loss = loss_fn(output, target)
            loss = loss / iters_to_accumulate

        # Accumulates scaled gradients.
        scaler.scale(loss).backward()

        if (i + 1) % iters_to_accumulate == 0:
            # may unscale_ here if desired (e.g., to allow clipping unscaled gradients)

            scaler.step(optimizer)
            scaler.update()
            optimizer.zero_grad()

However, if you are using e.g. 110 training samples, with batch size 8 and accumulation step 4 (i.e. effective batch size 32), this method would only train the first 96 samples (i.e. 32 x 3), i.e. wasting 14 samples. In order to avoid this, I'd like to modify the code as follows (notice change to the final if statement):

scaler = GradScaler()

for epoch in epochs:
    for i, (input, target) in enumerate(data):
        with autocast():
            output = model(input)
            loss = loss_fn(output, target)
            loss = loss / iters_to_accumulate

        # Accumulates scaled gradients.
        scaler.scale(loss).backward()

        if (i + 1) % iters_to_accumulate == 0 or (i + 1) == len(data):
            # may unscale_ here if desired (e.g., to allow clipping unscaled gradients)

            scaler.step(optimizer)
            scaler.update()
            optimizer.zero_grad()

Is this correct and really that simple, or will this have any side effects? It seems very simple to me, but I've never seen it done before. Any help appreciated!



from Final step of PyTorch Gradient Accumulation for small datasets

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