fix: do not consider odd 'K' values when constructing a Fat Tree#8
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matt-dbs wants to merge 1 commit into
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fix: do not consider odd 'K' values when constructing a Fat Tree#8matt-dbs wants to merge 1 commit into
matt-dbs wants to merge 1 commit into
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A non-obvious property of the formulas for computing a Fat Tree topology is that the constant `K` must be even. If you select an odd value of `K`, the math may appear to work out but the resulting tree has unexpected properties that make later routing calculations not work out. The htsim code for constructing a Fat Tree was considering every > 0 possible value of `K`. Apart from being inefficient, this resulted in constructing a malformed Fat Tree for node counts like 6, as well as accidentally missing the guard condition that any tree must be fully populated. The user-visible effect of this was puzzling assertion failures during routing at runtime: htsim_uec: htsim/sim/route.h:22: void Route::push_back(PacketSink*): Assertion `sink != NULL' failed.
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This is fixing a latent problem that bit us because we were writing new connection matrices, some of which accidentally induced an odd
K. Whether this is relevant to your work or not, I am not sure.A non-obvious property of the formulas for computing a Fat Tree topology is that the constant
Kmust be even. If you select an odd value ofK, the math may appear to work out but the resulting tree has unexpected properties that make later routing calculations not work out.The htsim code for constructing a Fat Tree was considering every > 0 possible value of
K. Apart from being inefficient, this resulted in constructing a malformed Fat Tree for node counts like 6, as well as accidentally missing the guard condition that any tree must be fully populated. The user-visible effect of this was puzzling assertion failures during routing at runtime: