[Trilinos-Users] AztecOO and Problem Size

Ammar T. Al-Sayegh alsayegh at purdue.edu
Sat Nov 19 16:27:28 MST 2005

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

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?



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