Evaluating Communication Pattern Representations in Execution Trace Gantt Charts

Gantt charts are frequently used to explore execution traces of large-scale parallel programs. 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, visual patterns can emerge reflecting communication behavior of the executing code. However, though these patterns have the same definition at any scale, they can be obscured by the density of rendered lines when displaying more than a few hundred processors. We seek to understand the effectiveness of various strategies for recognizing these patterns in Gantt charts. Specifically, we conduct a pre-registered user study comparing recognition of patterns when viewing all processors, a subset of processors, or a set of abstracted glyphs overlaid on the chart. We find that all strategies have limitations when scaling, motivating further designs. Our results further indicate that for simple patterns, the glyphs are more effective in general pattern recognition while the zoomed subsets provide nuance to specific characteristics, such as offsets, in patterns. These results suggest the development of a combined approach may be appropriate to enable pattern comprehension in large-scale Gantt charts.