  Gantt charts are frequently used to explore execution traces of large-scale
parallel programs found in high-performance computing (HPC). In these
visualizations, each parallel processor is assigned a row showing the
computation state of a processor at a particular time. Lines are drawn between
rows to show communication between these processors. When drawn to align
equivalent calls across rows, structures can emerge reflecting communication
patterns employed by the executing code. However, though these structures have
the same definition at any scale, they are obscured by the density of rendered
lines when displaying more than a few hundred processors. A more scalable
metaphor is necessary to aid HPC experts in understanding communication in
large-scale traces. To address this issue, we first conduct an exploratory
study to identify what visual features are critical for determining similarity
between structures shown at different scales. Based on these findings, we
design a set of glyphs for displaying these structures in dense charts. We then
conduct a pre-registered user study evaluating how well people interpret
communication using our new representation versus their base depictions in
large-scale Gantt charts. Through our evaluation, we find that our
representation enables users to more accurately identify communication patterns
compared to full renderings of dense charts. We discuss the results of our
evaluation and findings regarding the design of metaphors for extensible
structures.
