Reducing Activation Recomputation in Large Transformer Models
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This May 2022 paper addresses the challenges of training large transformer models with trillions of parameters, focusing on reducing the memory requirements for storing activations.
As transformer models scale up, model parallelism becomes necessary to distribute model parameters, activations, and optimizer state across devices.
However, tensor-level model parallelism has limitations in terms of communication requirements and performance, while pipeline parallelism cannot reduce activation memory while maintaining high device utilization.
The standard approach to alleviate memory pressure is to use activation recomputation (also known as gradient checkpointing), where activations are not stored but recomputed during the backward pass.
However, this method incurs a significant penalty in training efficiency, with the authors observing a 30-40% execution time overhead when full activation recomputation is used.
To address this issue, the authors present two novel techniques: sequence parallelism and selective activation recomputation. These techniques, when used in conjunction with tensor parallelism, can significantly reduce the need for activation recomputation.
Sequence parallelism is introduced alongside tensor parallelism to prevent redundant storage of activations in regions that are not conducive to standard tensor parallelism.
Unlike the approach in [9], which requires parameters and optimizer state to be replicated on all devices, the authors' technique mixes tensor and sequence parallelism without additional compute, communication, or memory overhead.
By selectively choosing which activations to save and which to recompute, the authors show that much of the cost of recomputation can be eliminated while using only a fraction of the memory compared to when no recomputation is used.
The authors evaluate their approach on language models up to one trillion parameters in scale, demonstrating that their method reduces activation memory by 5× while reducing the execution time overhead from activation recomputation by over 90%.
For example, when training a 530B parameter GPT-3 style model on 2240 NVIDIA A100 GPUs, they achieve a Model Flops Utilization of 54.2%, which is 29% faster than the 42.1% achieved using full activation recomputation.
The paper also discusses related work in model parallelism, including tensor parallelism, pipeline parallelism, and alternative approaches based on data parallelism.
The authors highlight the limitations of these existing techniques and emphasise the advantages of their proposed method, which combines the benefits of tensor and sequence parallelism without the drawbacks of previous approaches.
In summary, this paper presents a significant contribution to the field of large-scale transformer model training by introducing novel techniques that drastically reduce the memory requirements for storing activations and minimize the need for activation recomputation.
The proposed method, which combines sequence parallelism and selective activation recomputation with tensor parallelism, enables more efficient training of trillion-parameter models without incurring additional compute, communication, or memory overhead.
The authors' implementation will be made available in both Megatron-LM and NeMo-Megatron, making it accessible to the wider research community.
In this section, the authors discuss the transformer architecture and derive an approximate formula for the memory required to store activations during the forward pass.
They then explore how different forms of model parallelism impact activation memory requirements and introduce a novel technique called sequence parallelism.
The authors consider a single stack transformer encoder or decoder with L layers.
Input tokens are fed into a word embedding table (size v × h) and combined with learned positional embeddings (size s × h), where s is the sequence length, h is the hidden dimension, and v is the vocabulary size.
The output of the embedding layer is a 3-D tensor of size s × b × h, where b is the micro batch size.
Each transformer layer consists of a self-attention block with a attention heads, followed by an MLP with two layers that increase the hidden size to 4h and then reduce it back to h.
The output from the last transformer layer is projected back into the vocabulary dimension to calculate the cross-entropy loss.
The authors derive an approximate formula for the memory required to store activations in the forward pass, excluding the main model parameters and optimizer state.
They consider the main contributors to memory and ignore small buffers.
Activations are assumed to be stored in 16-bit floating-point format (2 bytes per element), except for dropout masks (1 byte per element).
The memory required to store activations for a single transformer layer is derived as: sbh(34 + 5as/h), where a is the number of attention heads, s is the sequence length, b is the microbatch size, and h is the hidden dimension.
The authors use tensor parallelism developed by Shoeybi et al. to parallelize the attention and MLP blocks.
Tensor parallelism parallelizes model parameters, optimizer states, and activations inside the attention and MLP blocks, but not the input activations to these blocks.
With t-way tensor parallelism, the per-layer activation memory reduces to: sbh(10 + 24/t + 5as/ht).
The authors introduce sequence parallelism to partition the non-tensor parallel regions (layer-norms and dropouts) along the sequence dimension s.
Sequence parallelism reduces the memory required for activations in these regions without introducing additional communication overhead.
New operations g and ¯g are introduced as converters between sequence and tensor parallel regions, replacing the f and ¯f operators from tensor parallelism.
The derivation of g and ¯g is detailed for the MLP block, showing that g is an all-gather in the forward pass and a reduce-scatter in the backward pass, while ¯g is the conjugate.
With tensor and sequence parallelism combined, the per-layer activation memory reduces to: sbh(34/t + 5as/ht), effectively distributing activations among the tensor parallel group.
Pipeline parallelism divides the L layers of the transformer into L/p groups, where p is the pipeline parallel size.
However, pipeline parallelism does not uniformly divide the total activation memory by p due to overlapping introduced to reduce the pipeline bubble.
The first stage of the pipeline must store activations for p microbatches, resulting in a total activation memory of: sbhL(34/t + 5as/ht).
The authors note that the majority of the required activation memory is captured by the equation for the first stage of the pipeline.
Additional activation memory is required for input embeddings, the last layer-norm, and the output layer, but these terms are negligible compared to the main equation.
As a result, the equation sbhL(34/t + 5as/ht) is a good approximation of the total required activation memory.
In summary, this section provides a detailed analysis of the activation memory requirements in transformer models and introduces sequence parallelism as a novel technique to reduce memory consumption when combined with tensor parallelism. The authors derive approximate formulas for activation memory under different parallelism schemes, which serve as the foundation for their proposed optimizations.
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