## Diagram: Expert Placement and Computation Pipeline in MoE Layer ### Overview The diagram illustrates the workflow of expert placement and computation in a Mixture of Experts (MoE) layer, comparing two pipeline strategies: **Sequential Offload Pipeline** and **Multi-Stream Overlapping Pipeline**. It uses color-coded blocks to represent expert states (CPU/GPU), compute streams, and communication streams over time. --- ### Components/Axes - **Legend** (top-left): - White: Experts already on CPU - Pink: Experts already on GPU - Purple: Expert fetching (CPU→GPU) - Green: Expert offloading (GPU→CPU) - **X-axis**: Labeled "Time" (left to right). - **Y-axis**: Two labeled streams: 1. **Compute Stream** (top row) 2. **Communication Stream** (bottom row) --- ### Detailed Analysis #### Sequential Offload Pipeline (Top Section) - **Compute Stream**: - **Prev-layer computation** (white block) spans expert 5 (time 5). - **Expert fetching** (purple blocks) for experts 2, 3, 4 (times 2, 3, 4). - **Communication Stream**: - **GPU→CPU** (green blocks) for experts 6, 7, 8 (times 6, 7, 8). - **CPU→GPU** (pink blocks) for experts 2, 3, 4 (times 2, 3, 4). #### Multi-Stream Overlapping Pipeline (Bottom Section) - **Compute Stream**: - **Prev-layer computation** (white block) spans experts 5, 2, 3, 4 (times 5, 2, 3, 4). - **Communication Stream**: - **GPU→CPU** (green blocks) for experts 6, 7, 8 (times 6, 7, 8). - **CPU→GPU** (purple blocks) for experts 2, 3, 4 (times 2, 3, 4). --- ### Key Observations 1. **Sequential Offload Pipeline**: - Experts 5, 6, 7, 8 are prefetched to GPU during prev-layer computation (time 5). - Experts 2, 3, 4 are fetched to GPU after prev-layer computation (times 2–4). - Expert offloading (GPU→CPU) occurs sequentially for experts 6, 7, 8 (times 6–8). 2. **Multi-Stream Overlapping Pipeline**: - Prefetching of experts 2, 3, 4 occurs **during** prev-layer computation (overlap at times 2–4). - Communication streams for experts 6, 7, 8 and 2, 3, 4 overlap in time, enabling parallelism. --- ### Interpretation - **Efficiency Gains**: The Multi-Stream Overlapping Pipeline reduces idle time by overlapping expert fetching/offloading with prev-layer computation. For example, experts 2, 3, 4 are prefetched while prev-layer computation runs for expert 5. - **Routing Path Predictions**: The diagram implies that experts are prefetched based on predicted routing paths during prev-layer computation, enabling proactive resource allocation. - **Color Consistency**: All legend colors (white, pink, purple, green) are strictly adhered to in the diagram, ensuring clarity in expert states and data flow. This visualization highlights how overlapping compute and communication streams in MoE layers can optimize throughput by minimizing sequential bottlenecks.