[petsc-users] question about arbitrary eigenvector selection in SLEPC

Giacomo Mulas gmulas at oa-cagliari.inaf.it
Thu May 29 10:03:31 CDT 2014

Hi Jose, and list.

I am in the process of writing the function to use with

Inside it, I will need to take some given component (which one is included
in the info passed via ctx) of the eigenvector and square it. To do this,
since the eigenvector is not necessarily local, I will need to first do a
scatter to a local 1-component vector. So this would be like:

... some omitted machinery to cast the info from *ctx to more easily
accessible form...

   ierr = ISCreateStride(PETXC_COMM_WORLD,1,myindex,1,&is1_from);CHKERRQ(ierr);
   ierr = ISCreateStride(PETSC_COMM_WORLD,1,0,1,&is1_to);CHKERRQ(ierr);
   ierr = VecCreateSeq(PETSC_COMM_SELF, 1, &localx1);CHKERRQ(ierr);
   ierr = VecScatterCreate(xr,is1_from,localx1,is1_to,&scatter1); CHKERRQ(ierr);
   ierr = VecScatterBegin(scatter1,xr,localx1,INSERT_VALUES,
   ierr = VecScatterEnd(scatter1,xr,localx1,INSERT_VALUES,
   ierr = VecGetArray(localx1,&comp);
   *rr = comp*comp;
   ierr = VecRestoreArray(localx1, &comp);
   ierr = VecDestroy(localx1);
   ierr = VecScatterDestroy(&scatter1);
   ierr = ISDestroy(&is1_from);
   ierr = ISDestroy(&is1_to);
   *ri = 0;

... some internal housekeeping omitted

   return 0;

The questions are:

1) when the arbitrary function is called, is it called on all nodes
simultaneously, so that collective functions can be expected to work
properly, being called on all involved nodes at the same time? Should all
processes compute the *rr and *ri to be returned, and return the same value?
would it be more efficient to create a unit vector uv containing only one
nonzero component, and then use VecDot(xr, uv, &comp), instead of pulling
the component I need and squaring it as I did above?

2) since the stride, the 1-component vector, the scatter are presumably 
the same through all calls within one EPSSolve, can I take them out of 
the arbitrary function, and make them available to it through *ctx? For this
to work, the structure of xr, the eigenvector passed to the arbitrary
function, must be known outside of EPSSolve.

Thanks, bye


Giacomo Mulas <gmulas at oa-cagliari.inaf.it>

INAF - Osservatorio Astronomico di Cagliari
via della scienza 5 - 09047 Selargius (CA)

tel.   +39 070 71180244
mob. : +39 329  6603810

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                          (Freddy Mercury)

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