Fill and drain
Beginner
The start and end of a job on a grid, when the numbers are still marching in or the last answers are marching out, so some cells sit idle.
Novice
The cycles at the start of a pass while data is still reaching the far PEs (fill) and at the end while the last results are still leaving (drain). During them only part of the array is busy.
Expert
For an array a pass of streamed steps takes about cycles, so the overhead is fixed per pass and shrinks relative to work as grows. Overlapping passes (double buffering) hides part of it.
Explained in Systolic arrays (Architectures).
See also: Array utilization, Skewed input (wavefront).