FSDP for Dummies

Mon, 02 Feb 2026 00:00:00 -0800

I’ve always struggled to understand the intuitions behind Fully Sharded Data Parallel beyond the high level idea of “shard everything.” Without a systems background, the fundamental primitives like “all-reduce” and “reduce-scatter” aren’t in my vocabulary. But FSDP conceptually is not complicated, especially once you state what the goals are (the rest is nearly necessitated by the engineering).

This post is an attempt to deconstruct the algorithm from first principles as a non-systems person. I will bring up the primitives in their specified context, which I think helps reinforces the intuition much better. Most ML researchers have a stronger understanding of the models, params, and optimizer processes than the systems jargon anyways.

Mlsys on Tyler Zhu

FSDP for Dummies