[petsc-users] questions about the multigrid framework

[Peter Wang]

Thanks a lot, Jed,
 
      I will try the algebric multigrid first. 
  
     In order to  configure PETSc with --download-ml and --download-hypre,which shell file in unix should I modify?
 
     Should I add some line in my current code to run with -pc_type ml or -pc_type hypre, or just use runtime option? 
 
     I am solving a 2-D poisson equation with finite difference scheme. Please find the problem discription as attached if it is necessary. Thanks again.
     
    
 


Date: Wed, 9 Feb 2011 17:54:19 +0100
From: jed at 59A2.org
To: petsc-users at mcs.anl.gov
Subject: Re: [petsc-users] questions about the multigrid framework


On Wed, Feb 9, 2011 at 17:44, Peter Wang <pengxwang at hotmail.com> wrote:


Did you mean All the Krylov methods alone will get worse with increasing grid number?


Yes, the number of Krylov iterations for second order elliptic problems with no preconditioner scales proportional to the number of grid points in any direction. You need a spectrally equivalent preconditioner, usually multigrid of some sort, to prevent this.
 

Since the finer grid has smaller size and more number of grid.
 
   Since I am a new user of PETSc, the easiest way for me is still keep in KSP solver. However, if the solver cannot satisfy the speed reqirement. I am thinking to use MG method. However, I don't have any experience on multigrid. Could you please give me some suggestion on it? 
 
 1, Since I  have built the Matrix and the vector for finite difference scheme in KSP solver, where should I start from to transfer to multigrid?  I studied the example in: src/ksp/ksp/examples/tutorials/ex22f.F. Is it a good prototype to be based on to create my own code?  Is DMMG is the best tool for my problem?



Assuming you currently assemble a matrix, just configure PETSc with --download-ml and --download-hypre, then try running your code with -pc_type ml or -pc_type hypre. You can use geometric multigrid later to improve the constants or handle cases where algebraic multigrid (ML or BoomerAMG from Hypre) are having trouble.


You need to tell us what equations you are solving if you want useful suggestions. 		 	   		  
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