[petsc-users] Confusion with MatGetLocalSubMatrix

[Lawrence Mitchell]

On 19 Dec 2014, at 17:12, Jed Brown <jed at jedbrown.org> wrote:

> Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> writes:
> 
>> Dear petsc-users,
>> 
>> I'm trying to setup matrices and data structures for use with MatGetLocalSubMatrix, but I'm rather lost in a world of ISes and block sizes.  I have the layouts and so forth correct where all my fields have block size 1, but am struggling to follow how things fit together for block size > 1.
>> 
>> I have a Taylor-Hood discretisation so a P2 velocity space with block size 2, and a P1 pressure space with block size 1.
>> 
>> On each process, I build the full local to global mapping for both fields.  This has block size 1.
> 
> How are you ordering the fields on each process?


field_0_proc_0, field_1_proc_0, ..., field_N_proc_0; field_1_proc_0, ..., ...; ... field_N_proc_P


> 
>> Then I create strided ISes to define the local blocks for each field, and set the block size on each of them (2 for the velocity space, 1 for the pressure).  Aside, when I do ISCreateStride for an IS with a block size > 1, do I provide all the indices, or just the blocked indices?  Should I be using ISCreateBlock for block size > 1 instead?
> 
> ISSTRIDE has no concept of block size and can't be used to describe
> blocked index sets.  Use ISBLOCK instead.

What is ISSetBlockSize for then?  Just hanging information on the IS for use elsewhere?


>> Calling MatGetLocalSubMatrix results in an error:
>> 
>> [0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------
>> [0]PETSC ERROR: Petsc has generated inconsistent data
>> [0]PETSC ERROR: Blocksize of localtoglobalmapping 1 must match that of layout 2
> 
> Hmm, I'm concerned that this might not work right after some recent
> changes to ISLocalToGlobalMapping.  Can you reproduce this with some
> code I can run?

I'll try and put something together.

Thanks,

Lawrence
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 455 bytes
Desc: Message signed with OpenPGP using GPGMail
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20141219/5aae5756/attachment.pgp>