# Do residuals mean the same in PCG and AMG?

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 August 27, 2012, 14:08 Do residuals mean the same in PCG and AMG? #1 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 436 Rep Power: 17 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

 August 31, 2012, 17:46 #2 Senior Member     Santiago Marquez Damian Join Date: Aug 2009 Location: Santa Fe, Santa Fe, Argentina Posts: 436 Rep Power: 17 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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