Does pytorch release GIL?

Answered by John Hunt

PyTorch does release the Global Interpreter Lock (GIL) as soon as it exits the Python code and enters the C/C++ code that is responsible for executing PyTorch operations. This means that most PyTorch operations are not bound by the GIL and can run in parallel, allowing for efficient utilization of multiple processor cores.

The GIL is a mechanism in CPython (the reference implementation of Python) that ensures only one thread executes Python bytecode at a time. This can limit the performance of multi-threaded Python programs, as threads often need to wait for the GIL to be released before they can execute their code. However, when it comes to PyTorch operations, the GIL is not a bottleneck.

PyTorch is implemented using a combination of Python and C/C++. The Python code is responsible for defining the computational graph, managing tensors, and executing high-level operations. When it comes to low-level operations, such as matrix multiplications or convolutions, PyTorch leverages highly optimized C/C++ code that runs outside the GIL.

One important distinction to make is that when we talk about PyTorch operations, we typically refer to the forward pass of neural networks or other computational operations. These operations involve matrix multiplications, element-wise operations, and other mathematical computations. The forward pass is where the bulk of the computation happens, and PyTorch is designed to efficiently execute these operations in parallel.

Additionally, PyTorch provides automatic differentiation, which allows us to compute gradients for backpropagation and train neural networks. The backward pass, which computes these gradients, is also implemented in C/C++ and runs outside the GIL. However, it’s worth mentioning that if you implement custom Functions in PyTorch, which involve Python code, the GIL may come into play for those specific operations.

In my personal experience, I have worked on various deep learning projects using PyTorch, and I’ve observed the benefits of GIL release firsthand. The ability to utilize multiple cores efficiently allows for faster training and inference times, especially when dealing with large datasets and complex neural network architectures.

To summarize, PyTorch releases the GIL for most operations, including the forward and backward passes of neural networks. This enables efficient parallel execution and utilization of multiple processor cores. However, it’s important to note that if you introduce custom Python code in your PyTorch operations, the GIL may still come into play for those specific parts.