[Trilinos-Users] AztecOO and Problem Size

[Ammar T. Al-Sayegh]

Hi Again,

I just figured out that the first issue is caused by not
defining the right preconditioner for the solver. I set
AZ_precond to AZ_dom_decomp, and AZ_subdomain_solve to
AZ_icc. This seems to have taken care of the couple of
the "matrix not be symmetric" warning problem. I also
realized that covergence does depend on the number of
processors after I enabled full output and saw the note
about convergence on the solution header.

I'm still trying to resolve the second issue, though.
That is the "Epetra ERROR -3, AztecOO.cpp, line 824".
I can see from the output notes that this maybe be
caused by the big difference between Actual Residual
and Recursive Residual. However, even though the
difference is in order of 10^7 magnitude, the two
numbers are trivially small (Actual residual = 
4.2508e-07,   Recursive residual =  6.1268e-14),
and both are well below my threshold of 10e-6. So
which of these two residuals is more accurate? I
assume that the actual is. If that's the case, why
would we bother with the recursive residual value?
Can we tell AztecOO to set convergence at Actual
Residual only and ignore Recursive Residual without
giving any errors?

Thanks.


-ammar



----- Original Message ----- 
From: "Ammar T. Al-Sayegh" <alsayegh at purdue.edu>
To: <trilinos-users at software.sandia.gov>
Sent: Saturday, November 19, 2005 6:27 PM
Subject: [Trilinos-Users] AztecOO and Problem Size


> Hi All,
> 
> I'm using AztecOO to solve a reduced linear problem
> (reuction done with EpetraExt_SubCopy_CrsMatrix fix).
> Since the reduced matrix is symmetric positive definite,
> I am using AZ_gc for higher efficiency. However, I keep
> getting the following warning message:
> 
> AZ_check_options: WARNING: Preconditioned matrix may
> not be symmetric (due to overlap).
> 
> Even with clearly symmteric matrices like this one:
> 
> Number of Global Rows        = 3
> Number of Global Cols        = 3
> Number of Global Diagonals   = 3
> Number of Global Nonzeros    = 9
> Global Maximum Num Entries   = 3
> 
> Number of My Rows        = 3
> Number of My Cols        = 3
> Number of My Diagonals   = 3
> Number of My Nonzeros    = 9
> My Maximum Num Entries   = 3
> 
> P   Row   Col    Value     
> 0    3     3     0.805556 
> 0    3     4     0    
> 0    3     5     0    
> 0    4     3     0    
> 0    4     4     6.21571e-10    
> 0    4     5    -1.11883e-05    
> 0    5     3     0    
> 0    5     4    -1.11883e-05    
> 0    5     5     0.268519    
> 
> Why does AztecOO think that this matrix could not be
> symmetric? and how can I prevent it from thinking this
> way?
> 
> The other issue I have is with problem size. Once my
> problem exceeds certain size I get the following error:
> 
> Epetra ERROR -3, AztecOO.cpp, line 824
> 
> on a single processor, which indicates that a numerical
> loss of accuracy has occured. With more than a single
> processor, I lose convergence even smaller problem sizes,
> without even getting this error message.
> 
> Why am I losing convergence with bigger problem sizes?
> and how come the same problem size that will solve with
> single processor will not solve on more processors?
> 
> Thanks.
> 
> 
> -ammar
> 
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