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August 27, 2012, 14:08 |
Do residuals mean the same in PCG and AMG?
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#1 |
Senior Member
Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
Posts: 452
Rep Power: 23 |
Hi all, I'm doing a comparison between PCG and AMG. The problem is to solve with laplancianFoam and all Dirichlet BC's with both methods. I solved asking for the same absolute residuals of 1E-06 and then solved the problem using a direct solver in octave extracting the matrix and RHS with gdbOF.
The max absolute error is about 5E-5 with PCG and 2E-2 with AMG for a problem of 1000x1000 cells. This is the difference, same residuals in PCG and AMG lead to different quality solution. Next are the logs: PCG: diagonalPCG: Solving for T, Initial residual = 1, Final residual = 9.99042e-07, No Iterations 2231 ExecutionTime = 223.14 s ClockTime = 223 s AMG: GAMG: Solving for T, Initial residual = 1, Final residual = 5.69744e-07, No Iterations 12 ExecutionTime = 57.95 s ClockTime = 58 s So that: 1. Is the definition of residuals different between PCG and AMG? or 2. I'm setting something bad in AMG and it is not converging at all Any clues? Thx in advance.
__________________
Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC) - CONICET/UNL Tel: 54-342-4511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe - Argentina. http://www.cimec.org.ar |
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August 31, 2012, 17:46 |
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#2 |
Senior Member
Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
Posts: 452
Rep Power: 23 |
I found that you have to ask for lower residuals in AMG than PCG to obtain the similar max error in the solution.
Any clues?
__________________
Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC) - CONICET/UNL Tel: 54-342-4511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe - Argentina. http://www.cimec.org.ar |
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