Extend preallocation

[Adrian Tate]

----- Original Message -----
From: owner-petsc-users at mcs.anl.gov <owner-petsc-users at mcs.anl.gov>
To: petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Sent: Tue Sep 23 21:38:31 2008
Subject: Extend preallocation

Dear Petsc users,

I would like to compute the sparse projection matrix Q=Id - Pt*A*P.

Pt*A*P is very well computed by MatMatMult ; then I would like to use
MatShift in order to complete the computation.
Unfortunately, Pt*A*P has lots of zeros on its diagonal, thus MatShift is
very slow due to inapropriate preallocation.

Since the initial preallocation of Pt*A*P is very good, I would to know if
there is a method allowing to get the allocation data structure and to
extend it in order to preallocate the diagonal terms.

Best regards,

Nicolas



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