transformers/docs/source/en/grad_accumulation.md

Gradient accumulation

Large batches produce large activations that exhaust GPU memory. Gradient accumulation lets you train with a larger effective batch size by spreading gradient computation across multiple mini-batches.

Gradients accumulate across n mini-batches before the optimizer updates the weights. For example, with a per-device batch size of 8 and 4 accumulation steps, the effective batch size is 32.

Step 1: mini-batch 1 → forward → backward → grads = G₁
Step 2: mini-batch 2 → forward → backward → grads = G₁ + G₂
Step 3: mini-batch 3 → forward → backward → grads = G₁ + G₂ + G₃
Step 4: mini-batch 4 → forward → backward → grads = G₁ + G₂ + G₃ + G₄
        → optimizer.step()  ← same update as if batch_size × 4
        → zero_grad()

Use gradient accumulation only when a larger batch doesn't fit in memory. It doesn't improve throughput over training with a true large batch.

Accumulate gradients for gradient_accumulation_steps across per_device_train_batch_size.

from transformers import TrainingArguments

args = TrainingArguments(
    ...,
    per_device_train_batch_size=8,
    gradient_accumulation_steps=4,
)

Loss scaling

For a custom loss function, include num_items_in_batch so [Trainer] divides the loss by the number of prediction targets across all mini-batches. This normalizes by tokens rather than a fixed step count with gradient_accumulation_steps. Otherwise, [Trainer] divides loss by gradient_accumulation_steps.

import torch.nn.functional as F

def compute_loss(outputs, labels, num_items_in_batch=None):
    logits = outputs["logits"]
    loss = F.cross_entropy(logits, labels, reduction="sum")
    return loss / num_items_in_batch

For causal LM models, num_items_in_batch counts the shifted labels. The loss shifts labels so the prediction at position i targets the token at position i + 1, which leaves position 0 of every sequence without a target. [Trainer] excludes those positions and counts over labels[..., 1:], so the denominator matches the number of prediction targets the loss uses. When a data collator supplies shift_labels directly, such as a padding-free collator, [Trainer] counts over that tensor instead. Other loss types, like masked LM and classification, count the full label tensor.

Next steps

• Read the GPU memory usage doc to understand what is driving memory usage on the GPU during training.
• See the Gradient checkpointing guide to learn how to reduce activation memory by recomputing activations instead of caching them.
• See the Mixed precision training guide to learn how to use lower precision data types to reduce memory and speed up training.
• Read the Gradient Accumulation Fix blog post to learn how gradient accumulation is computed.