Introduction || What is DDP || Single-Node Multi-GPU Training || Fault Tolerance || Multi-Node training || minGPT Training
What is Distributed Data Parallel (DDP)¶
Authors: Suraj Subramanian
How DDP works under the hood
What is the DistributedSampler
How gradients are synchronized across GPUs
Familiarity with basic non-distributed training in PyTorch
Follow along with the video below or on youtube.
This tutorial is a gentle introduction to PyTorch DistributedDataParallel (DDP) which enables data parallel training in PyTorch. Data parallelism is a way to process multiple data batches across multiple devices simultaneously to achieve better performance. In PyTorch, the DistributedSampler ensures each device gets a non-overlapping input batch. The model is replicated on all the devices; each replica calculates gradients and simultaneously synchronizes with the others using the ring all-reduce algorithm.
This illustrative tutorial provides a more in-depth python view of the mechanics of DDP.
Why you should prefer DDP over DataParallel (DP)¶
DataParallel is an older approach to data parallelism. DP is trivially simple (with just one extra line of code) but it is much less performant. DDP improves upon the architecture in a few ways:
DataParallel |
DistributedDataParallel |
|---|---|
More overhead; model is replicated and destroyed at each forward pass |
Model is replicated only once |
Only supports single-node parallelism |
Supports scaling to multiple machines |
Slower; uses multithreading on a single process and runs into Global Interpreter Lock (GIL) contention |
Faster (no GIL contention) because it uses multiprocessing |
Further Reading¶
Multi-GPU training with DDP (next tutorial in this series)
