Antoine, See the attached example. It does sort on rank 0 but puts the permutation results in parallel. Run it with mpirun -n 3 ./ex2 for a demo. Thanks. --Junchao Zhang
On Thu, Jul 9, 2020 at 1:24 PM Antoine Côté <antoine.co...@usherbrooke.ca> wrote: > The Vec containing the scalars represents stresses at nodes points of a > DMDA. My program sorts stresses, and makes calculations according to their > rank. This needs to be done while keeping track of the original positions > of stresses in Vec. Using permutations, I can access the node of my choice > and get its associated rank, without changing the original Vec. > > Regards, > > Antoine > > > ------------------------------ > *De :* Junchao Zhang <junchao.zh...@gmail.com> > *Envoyé :* 9 juillet 2020 12:42 > *À :* Antoine Côté <antoine.co...@usherbrooke.ca> > *Objet :* Re: [petsc-users] Parallel sort in Petsc > > It is easier without permutations. So let me ask first do you need the > permutations or just a (parallel) sorted vector? > > --Junchao Zhang > > > On Thu, Jul 9, 2020 at 11:21 AM Antoine Côté <antoine.co...@usherbrooke.ca> > wrote: > > That is worth trying! > > Say I gather to process 0 as proposed here : > https://www.mcs.anl.gov/petsc/documentation/faq.html#mpi-vec-to-mpi-vec > <https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.mcs.anl.gov%2Fpetsc%2Fdocumentation%2Ffaq.html%23mpi-vec-to-mpi-vec&data=02%7C01%7CAntoine.Cote3%40usherbrooke.ca%7C5b19e2103f434ddd79a308d824270a5c%7C3a5a8744593545f99423b32c3a5de082%7C0%7C0%7C637299097387394607&sdata=4qRikFqiT7IavQKuVjS6rs9L9KsPPIHC23Ekfov1Jvg%3D&reserved=0> > Let Vec A be the vector containing the scalars. If I compute the > permutations I'm looking for, how do I : > - tell the code to run PetscSortRealWithPermutation() *only *on process > 0 ? > - distribute back the permutations on all process (that is, send only the > permutations concerning the local portion of Vec A stored by a given > process) ? > - make sure other processes wait until they receive the permutations > prior proceeding with the rest of the code ? > > Thank you very much! > > Antoine > > ------------------------------ > *De :* Junchao Zhang <junchao.zh...@gmail.com> > *Envoyé :* 7 juillet 2020 10:52 > *À :* Antoine Côté <antoine.co...@usherbrooke.ca> > *Cc :* petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov> > *Objet :* Re: [petsc-users] Parallel sort in Petsc > > Gather all values to rank 0 and do a sequential sort there, then profile > to see whether it is a performance bottleneck in your code. > > --Junchao Zhang > > > On Tue, Jul 7, 2020 at 8:14 AM Antoine Côté <antoine.co...@usherbrooke.ca> > wrote: > > Hi, > > I need to compute the permutations to sort scalar values of a Vec object > in decreasing order. I've been developing on a single process for now, > using VecGetArrayRead() to extract the values > and PetscSortRealWithPermutation() to get the permutations. > > I would like to run the code on multiple processes. From these links, I > can see it's an issue : > https://lists.mcs.anl.gov/pipermail/petsc-users/2013-November/019628.html > <https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.mcs.anl.gov%2Fpipermail%2Fpetsc-users%2F2013-November%2F019628.html&data=02%7C01%7CAntoine.Cote3%40usherbrooke.ca%7C5b19e2103f434ddd79a308d824270a5c%7C3a5a8744593545f99423b32c3a5de082%7C0%7C0%7C637299097387404606&sdata=I1XaEkfaCXpIpHOHH3ErtQyoxMLnRwqFysh1v%2BybfSY%3D&reserved=0> > https://lists.mcs.anl.gov/pipermail/petsc-users/2009-June/004621.html > <https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.mcs.anl.gov%2Fpipermail%2Fpetsc-users%2F2009-June%2F004621.html&data=02%7C01%7CAntoine.Cote3%40usherbrooke.ca%7C5b19e2103f434ddd79a308d824270a5c%7C3a5a8744593545f99423b32c3a5de082%7C0%7C0%7C637299097387404606&sdata=A9i3R3enHTQjWXLY2yiDNSesporutJPUobU1%2F3Nvk9s%3D&reserved=0> > > One work around I've been considering : get the max scalar value using > VecMax() ; compute a scaling factor that makes this max value equals the > largest integer value tolerated by PetscInt ; scale all values of Vec and > convert them to integers ; use PetscParallelSortInt() ; inverse the scaling > on (now sorted) integers values and convert them back to scalars. > > This is only a patch really, the main issues being : (1) I would get a > sorted Vec, not the permutations and (2) there would be a lost of > information when converting scalars to int values. > > Isn't there a way to parallel sort real values using Petsc? > > Thank you very much! > > Antoine Côté > > >
ex2.c
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