Model Parallel

Pipeline Parallelism

Use of Pipelining for GPUs to do training.

https://colossalai.org/docs/concepts/paradigms_of_parallelism/

GPipe paper:

• https://arxiv.org/abs/1811.06965

CS231n 2025 Lec 11 — bubble problem + microbatches

Split the $L$ layers across $N$ GPUs so GPU $i$ holds layers $[iL/N,(i+1)L/N)$. Forward: activations flow GPU 0 → 1 → … → $N-1$. Backward: gradients flow back. Bubble problem: at any instant only one GPU is active — the rest wait on their upstream or downstream neighbor. Max MFU = $1/N$.

Microbatch fix (Huang et al. GPipe, NeurIPS 2019): split the batch into $M$ microbatches and pipeline them. As soon as microbatch 1 finishes on GPU 0, start microbatch 2 there. Forward pass fills the pipeline; backward drains it. With $N$ stages and $M$ microbatches, active-time fraction = $\frac{2NM}{2NM+2(N-1)}$ = $\frac{NM}{NM+N-1}$.

Worked example from the lecture: 4-way PP with 4 microbatches → $16/28\approx 57.1\%$ active time. The $N-1$ bubble on each end is unavoidable; you amortize it by making $M\gg N$.

Source

CS231n 2025 Lec 11 slides ~127–133 (PP bubble diagram, microbatch scheduling, active-time formula, 4-stage × 4-microbatch 57.1% example). 2026 PDF not published — using 2025 fallback.

Related

• Model Parallelism
• Tensor Parallelism
• FSDP
• MFU