What is the pipeline bubble and how do you reduce it?

The pipeline bubble represents idle time for GPUs during the warm-up and cool-down phases of a pipeline schedule. For a standard 1F1B schedule, the bubble fraction is (p-1)/m. To reduce this, you can implement an interleaved schedule with v chunks, which reduces the fraction to (p/v - 1)/m, though this increases the number of P2P sends per micro-batch. Alternatively, increasing the number of micro-batches (m) per flush is an effective way to reduce the bubble without changing the number of chunks.