Is the matrix "A" the same for all groups (and the only thing different for each group is b)?
Barry On Mon, 20 Aug 2007, Alejandro Garzon wrote: > Hi, I wrote a code for solving a time dependent pde. In each time step > I have to solve a linear system. I first wrote a prototype in matlab > and then a C version using Petsc. To my surprise for some input > parameters the matlab version runs faster than the Petsc code (on the > same single processor). I did > the performance comparison by timing 20 groups of 40 iterations. The > times for Petsc and matlab are shown bellow > > group Petsc matlab > > 1 0.244981 1.110283 > 2 0.244995 1.112919 > 3 0.241305 1.113608 > 4 0.244542 1.114669 > 5 7.534417 1.112450 > 6 0.242212 1.115867 > 7 0.246327 1.111135 > 8 0.241105 1.113442 > 9 0.244468 1.111215 > 10 0.241113 1.111334 > 11 0.244541 1.112467 > 12 0.241400 1.113525 > 13 0.245020 1.114077 > 14 0.241303 1.113409 > 15 0.244380 1.116238 > 16 0.241372 1.109931 > 17 0.244108 1.100667 > 18 0.240419 1.096030 > 19 0.244337 1.096293 > 20 17.139999 1.097120 > --------- -------- > total 29.0523 22.1967 > time > > As can be seen in the table, although in most of the groups the time > spent by the Petsc code is lower than that of matlab > (0.24 compared to 1.1) there are two groups (5 and 20) in the Petsc column > that > take a long time and make the total for the Petsc code bigger than > that for matlab. > > In Petsc and matlab the method used is bicg and the relative residual > is the same: 1e-8. The preconditioners are different, though. In petsc > I used the default preconditioner and in matlab I used incomplete LU > decomposition with drop tolerance. The code that solves the linear > system in matlab is > > [L,U] = luinc(A,droptol); <---- this is done only once before the > first iteration, droptol = 1e-12 > > x = bicg(A,b,relres,maxsteps,L,U); > > I know incomplete LU decomposition as a preconditioner is available in > Petsc but in order to use it one must provide two arguments in > addition to the drop tolerance and I didn't know what values to give > to them. > > My question is this: what options should be chosen so that the > preconditioner and method use by Petsc are the same as those shown in > the two lines of matlab code above? (could you contact the authors of the > matlab functions?) A comparison of the performace of Petsc and matlab > makes sense only if they are using the exact same methods. > > > Thanks. > > -- > Alejandro > > > > >
