# Forum: open-discussion

Monitor Forum | Start New Thread
| |

RE: Very ill-conditioned symmetric positive definite systems [ Reply ] By: Xavier Lacoste on 2013-07-19 14:38 | [forum:125950] |

Hello Guo, We dont' have yet computation of the condition numbers of the matrix, I'll have a look at it. XL. |

RE: Very ill-conditioned symmetric positive definite systems [ Reply ] By: Guo Luo on 2013-07-17 23:52 | [forum:124892] |

Hi Xavier, Thanks a lot! Do you think it is possible to compute the error bound also (the last line of section 2.3 in MUMPS manual)? That is the quantity that I am really interested in. Thanks again! Regards, Guo |

RE: Very ill-conditioned symmetric positive definite systems [ Reply ] By: Xavier Lacoste on 2013-07-17 07:29 | [forum:124650] |

Hi Guo, I looked inside MUMPS manual http://graal.ens-lyon.fr/MUMPS/doc/userguide_4.10.0.pdf (2.3) They are using for each equation, |b - Ax |_i / (|b| + |A||x|)_i except if b-Ax is non zero and the denominator is too small. In that case, it computes : ( | b- Ax |_i/((|A||x|)_i + |A_i|_inf|x|_inf) And then they return the maximum value of this on all rows. We can had it I think, I'll try it. Xavier. |

RE: Very ill-conditioned symmetric positive definite systems [ Reply ] By: Guo Luo on 2013-07-16 22:46 | [forum:124636] error_analysis_mumps4.10.0.pdf (250) downloads |

Hi XL, Thanks a lot for your quick response. I have been using MUMPS before which can perform classical error analysis based on the residuals. The diagnostics that MUMPS computes can be found in the attached file. The reference [12] cited in the file is the following: [12] M. Arioli, J. Demmel, and I. S. Duff. Solving sparse linear systems with sparse backward error. SIAM Journal on Matrix Analysis and Applications, 10:165–190, 1989. It would be great if similar diagnostics can be added to pastix. I am not sure if I am asking for too much. If you prefer I can read the paper myself and then tell you the explicit formulas used by MUMPS to compute all these diagnostics. Thanks again for your time. And have a nice day. Guo |

RE: Very ill-conditioned symmetric positive definite systems [ Reply ] By: Xavier Lacoste on 2013-07-16 06:30 | [forum:123987] |

Hello, (1) When DPARM_EPSILON_MAGN_CTRL is set to a positive number, the threshold for pivoting is set to norm1(A)*sqrt(epsilon), else it is set to -epsilon. In your case you get 0. This mean that no static pivoting is performed. In all other cases, when a value smaller than epsilon is detected on the diagonal, it is replaced by epsilon. (2) Computing ||Ax-b||/||b|| is the only test on accuracy we are performing. If you want us to add a new one, we could do it. Thanks for you feed back, XL. |

Very ill-conditioned symmetric positive definite systems [ Reply ] By: Guo Luo on 2013-07-13 01:50 | [forum:123768] |

Dear Pastix developers, I am using pastix 5.2.1 to solve large ill-conditioned SPD systems resulting from a Galerkin discretization of a 2D Poisson equation. I didn't have a precise estimate of the condition number of my system, but believe it could be as large as 10^9 or even higher. The system was solved using the parameters: IPARM_FACTORIZATION = API_FACT_LLT; DPARM_EPSILON_REFINEMENT = 1e-13; DPARM_EPSILON_MAGN_CTRL = 0; and all others being default. The solution completes successfully, but I am a bit concerned with the actual accuracy of the solution. I am wondering (1) what pastix precisely does when DPARM_EPSILON_MAGN_CTRL is set to 0? (2) whether pastix provides any backward error estimates, besides the residual DPARM_RELATIVE_ERROR? Thank you very much. Guo |