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coll/circ_graph: fix schedules for p > 32 - #8045
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gen_rsched now follows allblocks() in Traff's paper: the picked r' must strictly decrease from round to round, and a path is backed off when the shorter path without its last skip still falls in the range. Previously, for some p > 32 (e.g. 33, 65-73, 129-153), it could pick the same r' twice via different paths, or a deeper path with a smaller baseblock, producing receive schedules that broadcast blocks a process does not have yet. model: Opus 5.5
gen_ssched now follows sendschedule() (Algorithm 6) in Traff's paper. It tracks the end of the region r' is in, and uses it rather than Skip[k+1] to decide when a send has to be looked up from the receive schedule of the target. Previously, for some p > 32 (e.g. 33, 65-73, 129-153), the send schedule disagreed with the receive schedule of the target. model: Opus 5.5
model: Opus 5.5
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Pull Request Description
The circ_graph broadcast schedules are wrong for many process counts above 32 (172 values of p up to 600, starting at p=33, e.g. 33, 65-73, 129-153). Processes send blocks before they have them, and sends disagree with the receivers' schedules. Both schedule generators had departed from Träff's algorithms (TOPC 2025, arXiv 2407.18004):
gen_rsched(Algorithms 4-5): the chosen intermediate process r' must strictly decrease from round to round, and the shortest path in range must win. Without these rules it could pick the same r' twice through different paths.gen_ssched(Algorithm 6): the region-end boundeis now tracked in place ofSkip[k+1], so the cases that need a lookup in the target's receive schedule are detected correctly.After the fix, both functions match the paper's algorithms for every rank at every p ≤ 2100.
Additional change:
test/units/circ_graph.unitnow checks every p up to 300 instead of 32.Author Checklist
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