Presseail: show latency ✎ ⓘ

This simple app gives a way to visualize latency calculations in the rate-synchronous model. The idea is to (i) specify a chain element-by-element, (ii) to manually specify a schedule, and (iii) to see the resulting end-to-end latency values and generated constraints.

1. Select a period for the chain input and click start.
2. Add new links in the chain by specifying the operator, concomitance (‘f’ or ‘b’), and, if necessary, the sampling ratio.
   • direct: write-before-read within a period
   • last: read-before-write within a period
   • any: unconstrained sampling at same period
   • when: fast-to-slow rate transition (write before first read)
   • when last: fast-to-slow rate transition (read before last write)
   • current: slow-to-fast rate transition
3. Clicking on the slots sets the phase offset for the corresponding variable. The gray areas are excluded by dependency and concomitance constraints. The concomitance of each writer and reader is shown above the reader arrows. The backward end-to-end latencies (data age/last-to-last) are shown along the bottom. The forward end-to-end latencies (reaction time/first-to-first) are shown along the top. Click on any of the latencies to highlight the corresponding path through the schedule.
4. Add a latency bound and click update to generate ILP constraints on the end-to-end latency and causality, and to fix the schedule. While normally the constraints restrict possible schedules, here the idea is to validate the results. Click on solve to run glpk.js. Its log is shown at bottom. The left-hand-sides of certain inequalities are shown on the same line in gray. The values of the enabled chainlat constraints should correspond with the end-to-end latencies shown above (forward) or below (backward, exists) the schedule.
5. The exists option generates a single set of constraints so that there need only exist a backward latency chain that satisfies the given bound. The forward option constrains all forward data propagation paths and thus the “reaction time”/ “first-to-first” latency. The backward option constrains all backward data propagation paths and thus the “data age”/ “last-to-last” latency. The simplify option applies simple constant propagation when generating constraints.
6. The small pencil button next to the title gives a way to copy-and-paste latency chains. Click the info button again to hide this text.