[petsc-users] Questions abt DM and DMDALocalToLocalBegin / End

[Jed Brown]

TAY wee-beng <zonexo at gmail.com> writes:

> On 7/4/2014 10:40 AM, Jed Brown wrote:
>> but might not be
>> negligible if called once for each degree of freedom.
> Sorry I don't understand this sentence.
>
> So you mean if I have 3 dof u,v,w, it will not be cheap if I access and 
> restore it many times. Is that so?

It means that you should not do this:

do k=1,zm
  do j=1,ym
    do i=1,zm
      call DMDAVecGetArrayF90(... x,ierr)
      x(i,j,k) = something
      call DMDAVecRestoreArrayF90(... x,ierr)
    end
  end
end

Lift the call out of the loop.

call DMDAVecGetArrayF90(... x,ierr)
do k=1,zm
  do j=1,ym
    do i=1,zm
      x(i,j,k) = something
    end
  end
end
call DMDAVecRestoreArrayF90(... x,ierr)
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