Chimera

Chimera is a pipeline model parallelism scheme which combines bidirectional pipelines for efficiently training large-scale models. The key idea of Chimera is to combine two pipelines in different directions (down and up pipelines).

Denote $N$ as the number of micro-batches executed by each worker within a training iteration, and $D$ the number of pipeline stages (depth), and $P$ the number of workers.

The Figure shows an example with four pipeline stages (i.e. $D=4$). Here we assume there are $D$ micro-batches executed by each worker within a training iteration, namely $N=D$, which is the minimum to keep all the stages active.

In the down pipeline, stage$_{0}$∼stage$_{3}$ are mapped to $P_{0}∼P_{3}$ linearly, while in the up pipeline the stages are mapped in a completely opposite order. The $N$ (assuming an even number) micro-batches are equally partitioned among the two pipelines. Each pipeline schedules $N/2$ micro-batches using 1F1B strategy, as shown in the left part of the Figure. Then, by merging these two pipelines together, we obtain the pipeline schedule of Chimera. Given an even number of stages $D$ (which can be easily satisfied in practice), it is guaranteed that there is no conflict (i.e., there is at most one micro-batch occupies the same time slot on each worker) during merging.