[petsc-users] MATBDIAG in > 3.1

[Barry Smith]

  My approach will be a million times faster.

   Barry

On Dec 1, 2011, at 8:50 AM, Matthew Knepley wrote:

> On Thu, Dec 1, 2011 at 8:44 AM, Parker, Andrew (UK Filton) <Andrew.Parker2 at baesystems.com> wrote:
> I have tried to use the following, this was before I had both you're
> emails (Barry, Matthew) ;-)
> 
> 1) First I set this up
> 
>      PetscInt size = 7;
>      MatCreate(MPI_COMM_SELF,&LHS);
>      MatSetSizes(LHS,size,size,size,size);
>      MatSetType(LHS,MATSEQDENSE);  //**************NOTE THIS LINE
>      MatAssemblyBegin(LHS,MAT_FINAL_ASSEMBLY);
>      MatAssemblyEnd(LHS,MAT_FINAL_ASSEMBLY);
> 
>      VecCreate(PETSC_COMM_SELF,&RHS);
>      VecSetSizes(RHS,PETSC_DECIDE, size);
>      VecSetFromOptions(RHS);
>      VecAssemblyBegin(RHS);
>      VecAssemblyEnd(RHS);
> 
>      VecCreate(PETSC_COMM_SELF,&Updates);
>      VecSetSizes(RHS,PETSC_DECIDE, size);
>      VecSetFromOptions(Updates);
>      VecAssemblyBegin(Updates);
>      VecAssemblyEnd(Updates);
> 
>      KSPCreate(PETSC_COMM_SELF,&Solver);
>      KSPSetOperators(Solver, LHS, LHS, SAME_NONZERO_PATTERN);
>      KSPSetType(Solver, KSPPREONLY);  //**************NOTE THIS LINE
>      KSPGetPC(Solver, &prec);
>      PCSetType(prec, PCLU);  //**************NOTE THIS LINE
>      KSPSetFromOptions(Solver);
>      KSPSetUp(Solver);
> 
> For each cell I then do:
> 
> 2) I copy out the appropriate data from global structures into LHS and
> RHS
> 
> 3) KSPSolve(Solver, RHS, Updates);
> 
> 4) Stick the local Updates back into my global data structure
> 
> Could I have your thoughts on whether the above is ok?  In terms of
> efficiency and the correct thing to do?  Would I be correct in assuming
> that Barry's method and mine are identical with the exception that I've
> got some copies going on to set up these small 7x7 and 1x7 matrices and
> vectors, but Barry's method does this inline by doing a preonly, lu
> solve DIRECT solve on each 7x7??
> 
> With Barry's method you can do this from the command line so you can easily compare
> with other solvers without changing your code.
> 
>   Matt
>  
> Many thanks,
> Andy
> 
> -----Original Message-----
> From: petsc-users-bounces at mcs.anl.gov
> [mailto:petsc-users-bounces at mcs.anl.gov] On Behalf Of Barry Smith
> Sent: 01 December 2011 14:09
> To: PETSc users list
> Subject: Re: [petsc-users] MATBDIAG in > 3.1
> 
> 
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> 
> On Dec 1, 2011, at 7:05 AM, Matthew Knepley wrote:
> 
> > You can get this effect using PCBJACOBI, setting the number of local
> blocks, and using -sub_pc_type lu -sub_ksp_type preonly.
> 
>   Do not do this. It will give bad performance since it explicitly
> pulls out each block with MatGetSubMatrices() and solves each small
> matrix as a separate PETSc matrix.
> 
>    Instead use the BAIJ matrix format with a block size of 7 and then
> use a PCPBJACOBI (note the P in the name for point block, each point
> block is that little 7 by 7 block that it solves efficiently using a
> tuned kernel) with -ksp_type preonly.  This should be very fast and does
> not do any any neccessary scatters or communication.
> 
>    Barry
> 
> >
> >    Matt
> >
> > On Thu, Dec 1, 2011 at 4:28 AM, Parker, Andrew (UK Filton)
> <Andrew.Parker2 at baesystems.com> wrote:
> > Hi,
> >
> >
> >
> > I've been using PETSc for a conventional full parallel implicit
> solver, currently in MPIBAIJ format.  However, I'd like to understand
> the performance verse memory and speed trade-offs of just having a block
> diagonal system.  In this case I do not scatter to the off diagonal
> blocks.  Each block is 7x7 in size and fully dense.  In my view this
> should be a point implicit solution of the system.  I am looking for the
> best way to solve this in PETSc, clearly because of the lack of coupling
> between neighbouring cells by only having a block diagonal it's actually
> a 7x7 dense linear solve, but to test my theory at present I'd prefer to
> just not scatter to off diagonals, and use:
> >
> >
> >
> > MATBDIAG with my current ksp solver and take it from there.  BDIAG
> seems to have been removed as off 3.0:
> >
> >
> >
> > http://www.mcs.anl.gov/petsc/documentation/changes/300.html
> >
> >
> >
> > Can anybody help me with this, your thoughts on the above and what I
> should use to solve this reduced system would be appreciated??
> >
> >
> > Cheers,
> >
> > Andy
> >
> >
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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
> 
> 
> 
> 
> 
> -- 
> 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