Is this overall runtime or solve time? The former is essentially meaningless as it includes all the startup time (launch, connections, etc). Especially since we are talking about seconds here.
-Nathan > On Aug 28, 2019, at 9:10 AM, Cooper Burns via users > <email@example.com> wrote: > > Peter, > > It looks like: > Node0: > rank0, rank1, rank2, etc.. > Node1: > rank12, rank13, etc > etc > > So the mapping looks good to me. > > Thanks, > Cooper > Cooper Burns > Senior Research Engineer > > > (608) 230-1551 > convergecfd.com > > > >> On Wed, Aug 28, 2019 at 10:50 AM Peter Kjellström <c...@nsc.liu.se> wrote: >> On Wed, 28 Aug 2019 09:45:15 -0500 >> Cooper Burns <cooper.bu...@convergecfd.com> wrote: >> >> > Peter, >> > >> > Thanks for your input! >> > I tried some things: >> > >> > *1) The app was placed/pinned differently by the two MPIs. Often this >> > would probably not cause such a big difference.* >> > I agree this is unlikely the cause, however I tried various >> > configurations of map-by, bind-to, etc and none of them had any >> > measurable impact at all, which points to this not being the cause >> > (as you suspected) >> >> OK, there's still one thing to rule out, which rank was placed on which >> node. >> >> For OpenMPI you can pass "-report-bindings" and verify that the first N >> ranks are placed on the first node (for N cores or ranks per node). >> >> node0: r0 r4 r8 ... >> node1: r1 ... >> node2: r2 ... >> node3: r3 ... >> >> vs >> >> node0: r0 r1 r2 r3 ... >> >> > *2) Bad luck wrt collective performance. Different MPIs have >> > different weak spots across the parameter space of >> > numranks,transfersize,mpi-coll**ective.* This is possible... But the >> > magnitude of the runtime difference seems too large to me... Are >> > there any options we can give to OMPI to cause it to use different >> > collective algorithms so that we can test this theory? >> >> It can certainly cause the observed difference. I've seen very large >> differences... >> >> To get collective tunables from OpenMPI do something like: >> >> ompi_info --param coll all --level 5 >> >> But it will really help to know or suspect what collectives the >> application depend on. >> >> For example, if you suspected alltoall to be a factor you could sweep >> all valid alltoall algorithms by setting: >> >> -mca coll coll_tuned_alltoall_algorithm X >> >> Where X is 0..6 in my case (ompi_info returned: 0 ignore, 1 basic >> linear, 2 bruck, 3 recursive doubling, 4 ring, 5 neighbor exchange, 6: >> two proc only.) >> >> /Peter > _______________________________________________ > users mailing list > firstname.lastname@example.org > https://lists.open-mpi.org/mailman/listinfo/users
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