On 19 August 2015 at 10:20, TAY wee-beng <[email protected]
<mailto:[email protected]>> wrote:
On 19/8/2015 1:17 PM, Dave May wrote:
On 19 August 2015 at 03:38, TAY wee-beng <[email protected]
<mailto:[email protected]>> wrote:
Hi,
I am using DA. For e.g.
DM da_u
call
DMDACreate3d(MPI_COMM_WORLD,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE,DMDA_STENCIL_STAR,size_x,size_y,&
size_z,1,PETSC_DECIDE,PETSC_DECIDE,1,stencil_width,lx,PETSC_NULL_INTEGER,PETSC_NULL_INTEGER,da_u,ierr)
call DMCreateGlobalVector(da_u,u_global,ierr)
call DMCreateLocalVector(da_u,u_local,ierr)
To update the ghost values, I use:
call DMLocalToLocalBegin(da_u,u_local,INSERT_VALUES,u_local,ierr)
call DMLocalToLocalEnd(da_u,u_local,INSERT_VALUES,u_local,ierr)
This is incorrect.
The manpage for DMLocalToLocal clearly says "Maps from a local
vector (including ghost points that contain irrelevant values) to
another local vector where the ghost points in the second are set
correctly."
To update ghost values from a global vector (e.g. to perform the
scatter) you need to use DMGlobalToLocalBegin() ,
DMGlobalToLocalEnd().
I must apologize (and should have read my own email :D)
- I misunderstood what DMLocalToLocalBegin/End does.
Indeed it will give produce the correct / updated ghost values.
Hi Dave,
Thanks for the clarification although I'm still confused. Supposed
I have a 1D vector da_u, It has size 8, so it's like
da_u_array(8), with stencil width 1
So for 2 procs,
there will be 2 da_u_array - da_u_array(1:5) and da_u_array(4:8)
After performing some operations on each procs's da_u_array, I
need to update 1st procs's da_u_array(5) and 2nd procs's
da_u_array(4) from the 2nd and 1st procs respectively. I simply call:
call DMLocalToLocalBegin(da_u,u_local,INSERT_VALUES,u_local,ierr)
call DMLocalToLocalEnd(da_u,u_local,INSERT_VALUES,u_local,ierr)
and it seems to be enough. I check the ghost values and they have
been updated.
Yeah, this is correct.
Sorry about my mistake in the previous email regarding what
DMLocalToLocal actually does.
So if I am not using the linear solvers, I do not need the global
vector,is that so?
I guess in the end it is application specific whether you need a
global vector or not.
I would have thought you always would want a global vector.
What is your application where you don't require a global vector?
