GUM uses cost-centre profiling internally to record the activity of each PE during reduction. PE activity profiling is performed in virtual time because the area under a PE activity graph represents the percentage time the PE spent on each activity. Any elongation of the time axis caused by using elapsed-time would distort the percentage.
PE activity profiles for two of the PEs in the linear equation solver run described above are given in Figures 5 and 6. Profiles are generated for each PE, and have 4 cost-centres: