Thanks, Matthew! I will look at numerical stability more carefully. Khai
On Thu, Dec 8, 2016 at 6:25 PM, Matthew Knepley <[email protected]> wrote: > On Thu, Dec 8, 2016 at 5:10 PM, Khai Pham <[email protected]> wrote: > >> Hi Hong, >> >> I more concern about the difference between A1 and A2 is in the order of >> O(1.e-8) as I run the same code twice. Should it be in the order of machine >> epsilon? >> > > If you have 17 orders of magnitude difference between elements, then its > easy to cancellation when doing subtraction and have > differences on that order due to permuted operations. > > Matt > > >> Khai >> >> On Thu, Dec 8, 2016 at 4:54 PM, Hong <[email protected]> wrote: >> >>> Khai : >>>> >>>> Thanks for your response. The output is not the solution. They are the >>>> component of the matrix in matlab format. I would expect the difference in >>>> the order of machine precesion. The difference in solutions in two runs is >>>> in the order of 1e-5. >>>> >>> >>> If your matrices A1 and A2 have difference of O(1.e-8), then the >>> computed solution may differ by >>> Condition_number(A) * machine_epsion. Do you know cond(A)? >>> >>> Please alway send your request to petsc-maint. >>> >>> Hong >>> >>>> >>>> >>>> On Dec 8, 2016 4:01 PM, "Hong" <[email protected]> wrote: >>>> >>>> Khai : >>>> Your solution components have values ranging from 1.e+9 to 1.e-8, and >>>> the values only differ in the order of 1.e-8, which are within >>>> computational error tolerance. >>>> I would consider all solutions same within the approximation tolerance. >>>> >>>> Hong >>>> >>>> Hello, >>>>> >>>>> I have problem with matrix assembly for linear solver KSP. I run the >>>>> the same problem with 4 processors multiple time. Using flag >>>>> -mat_view ::ascii_matlab to view the matrix. The matrix outputs are not >>>>> the >>>>> same during each run. Please see the attached file for the comparison >>>>> between two runs. I checked all the input for each processor as it calls >>>>> to >>>>> MatSetValues ( indices of the matrix and values ) and it's consistent all >>>>> the time. I also checked the allocation information (with flag -info) and >>>>> it looks fine. Could you give me an advice how to deal with this issue? >>>>> Thanks ! >>>>> >>>>> Best, >>>>> Khai >>>>> >>>> >>>> >>>> >>> >> > > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener >
