activations

Four all-reduces per transformer block, on activations, on the critical path. Inside the NVLink domain they cost 8% of a step; across the NIC they cost more than the step itself. The derivation for a 70B, and the second reason TP stops that has nothing to do with the network.

Activations are the term the 16-bytes-per-parameter rule leaves out, and at 8k context they are bigger than the weights: about 34 × sequence × hidden bytes per layer, 2.3 GB, 182 GB per micro-batch across 80 layers. The chain, what FlashAttention removes, and what checkpointing does to the number and the compute.