Add details on building a pipeline

description2.md

@@ -1,3 +1,25 @@
+## Building a pipeline
+
+### Building Block
+It starts by laying out the passes for a single microbatch, which we call a building
+block. For example, the building block of 1F1B schedule is made of a sequence of forward passes
+followed by backward passes in the reverse order.
+
+### Repeating
+More microbatches are then introduced. The building blocks are repeated and woven
+together to form a pipeline. Notably, legit building blocks are required to repeat without a collision, namely, the
+passes from two building blocks should not overlap with each other.
+
+### Squeezing
+Depending on the building block, there may be redundant bubbles in the pipeline, which
+can be simply removed by squeezing without changing the order of the passes.
+
+### Reordering (optional)
+We can reorder the passes in the warm-up and cool-down phase to further
+improve the computation throughput. Intuitively, the peak of memory happens in the stable phase of
+the pipeline, while in the warm-up and cool-down phases the RAM is under utilized, leaving some
+space for improving the computation throughput without changing peak memory.
+
 ## Alternative schedules
 
 By utilizing the building block, we can search for different types of schedules depending on the need. We illustrate few of them here below: