To minimize code changes you could add a PETSc viewer format that caused skipping reading in an object. Then each object load would need a skip-read method that mimicked the reading but actually just skipped over the parts of the data of the object (using the correct sizes). For vectors this is trivial since you just skip the single known array. For sparse matrices it is not difficult but you will need to read in the number of nonzeros so you know how much to skip etc.

  Barry


On Oct 4, 2021, at 9:58 AM, Peder Jørgensgaard Olesen via petsc-users <petsc-users@mcs.anl.gov> wrote:

Thank you for your quick reply.


I've had to change away from HDF5 to Binary format at an earlier stage in my work due to the former not working well with what I needed, so I would prefer to stick with the binary format.

I had a quick view at the code for some of the Viewer-routines, but I'm not well versed in gleaning information about header sizes from that. Hints about what I'm looking for there would be appreciated. 


Best regards

Peder

Fra: Matthew Knepley <knepley@gmail.com>
Sendt: 4. oktober 2021 15:37:08
Til: Peder Jørgensgaard Olesen
Cc: petsc-users@mcs.anl.gov
Emne: Re: [petsc-users] Skipping data when reading from binary file
 
On Mon, Oct 4, 2021 at 9:26 AM Peder Jørgensgaard Olesen via petsc-users <petsc-users@mcs.anl.gov> wrote:

Hello

I have a binary file in which a mix of different objects is stored (Vecs, Mats, PetscInts). I can read each element just fine using VecLoad, MatLoad, and PetscIntView, provided they're read in the order in which they were put in the binary. What I would like to do is to skip the reading of any unneeded element, instead proceeding directly to the next one. I tried using PetscBinarySeek() for this, as shown in the attached code. This produces segmentation faults, suggesting that the file pointer isn't going where I want it to.

There is header information you also have to skip for each object.

We can go over the sizes  for that (it is best just to look at the code), but that is fragile. A more robust way to achieve this random
access is to use HDF5 and name the objects.

  Thanks,

     Matt

Any suggestions as to what I'm doing wrong here?


Best regards,

Peder
Peder Jørgensgaard Olesen
PhD student
Department of Mechanical Engineering
 
pjool@mek.dtu.dk
Koppels Allé
Building 403, room 105
2800 Kgs. Lyngby
www.dtu.dk/english



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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.
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