[petsc-users] Ghost points in DMSwarm

[Matthew Knepley]

On Sun, Feb 25, 2024 at 7:21 AM MIGUEL MOLINOS PEREZ <mmolinos at us.es> wrote:

> Dear Matthew,
>
> Thank you for answer me out of office hours and for the suggestion. If I
> interpret it correctly, your point is using the background mesh halo to
> identify and create ghosts particles in the overlapping region using
> DMSwarmMigrate().
>
> Then two questions hits me:
>
>    - If I change a variable in the “real” particle (say the position), it
>    should be updated in the replicas on the overlapped regions, right? This
>    sort of communicator is included in DMSwarmMigrate()?
>
> No, the particles that are migrated are copies. There is no current
analogue of DMGlobalToLocal(). It would need to be constructed. It is
possible, but would take writing some code. All applications we have
supported to date do not use replicated particles.

>
>    - The function DMSwarmCreateLocalVectorFromField() does not include
>    extra space for ghost particles. Therefore, I should resize the local size
>    for each rank to accommodate the replicas manually. Right?
>
> It uses all particles on the process. If some of those are replicas, they
will be included.

  Thanks,

     Matt


> Anyway, I think this this will take me more time I expected originally.
> Meanwhile,  I will go ahead with my “humble” parallelization scheme which
> consist in using AllReduce to create a serial vectors with the variables I
> need to evaluate rank-wise the residual equations of my problem  (please,
> don’t judge me, publish or perish...)
>
> Many thanks,
> Miguel
>
>
> On Feb 24, 2024, at 4:24 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Fri, Feb 23, 2024 at 5:14 PM MIGUEL MOLINOS PEREZ <mmolinos at us.es>
> wrote:
>
>> Dear all, I am struggling on how to include ghost points in DMSwarm local
>> vectors. According to PETSc documentation it seems “straightforward” for a
>> DMDA mesh. However, I am not so sure on how to do it for a DMSwarm. In
>> fact, if I add the result
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>> Dear all,
>>
>> I am struggling on how to include ghost points in DMSwarm local vectors.
>> According to PETSc documentation it seems “straightforward” for a DMDA
>> mesh. However, I am not so sure on how to do it for a DMSwarm. In fact,
>> if I add the result of DMSwarmGetLocalSize for each rank, the result is
>> the exact number of particles in the system. Which means that the halo
>> is zero.
>>
>> Since there is a function called DMSwarmCreateLocalVectorFromField,I was
>> wandering if there is a function already implemented in petsc (and I’m
>> missing it) to include ghost points in DMSwarm and therefore don’t have
>> to reinvent the wheel. If so, is there any example out there I can follow?
>>
>
> DMSwarm is currently different from the other DMs. There is no idea of
> identity for particles, so there is no idea of a shared particle. You can
> create this by adding an ID to your particle fields, but it would be a
> manual process. DMSwarmMigrate() can duplicate particles if you have an
> overlapping mesh, which would mimic shared particles.
>
>   THanks,
>
>      Matt
>
>
>> Thanks in advance.
>>
>> Regards,
>> Miguel
>>
>> petsc.org
>> <https://urldefense.us/v3/__https://petsc.org/release/manual/vec/__;!!G_uCfscf7eWS!f6U_z6q8Rk6XrOxvbmE8KyNErlmqYwpGzkxCQ56xX0agWaCG0tLVLh1Cml6fTtqvve0aL3HGiAhZn-hDgIoH5w$>
>>
>> <https://urldefense.us/v3/__https://petsc.org/release/manual/vec/__;!!G_uCfscf7eWS!f6U_z6q8Rk6XrOxvbmE8KyNErlmqYwpGzkxCQ56xX0agWaCG0tLVLh1Cml6fTtqvve0aL3HGiAhZn-hDgIoH5w$>
>> <https://urldefense.us/v3/__https://petsc.org/release/manual/vec/__;!!G_uCfscf7eWS!f6U_z6q8Rk6XrOxvbmE8KyNErlmqYwpGzkxCQ56xX0agWaCG0tLVLh1Cml6fTtqvve0aL3HGiAhZn-hDgIoH5w$>
>>
>>
>>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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>
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://urldefense.us/v3/__https://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!bRyZjT28s2cAZIFj69F3Bmv2X5NDZvT-prHmzArFuDq6aqqCaDpfaHH0ip-K53DfXi574ejpx04DurcseuAb$  <https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!bRyZjT28s2cAZIFj69F3Bmv2X5NDZvT-prHmzArFuDq6aqqCaDpfaHH0ip-K53DfXi574ejpx04Dukm3lyRJ$ >
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