[petsc-users] difference between local and global vectors

[TAY wee-beng]

On 19/8/2015 4:26 PM, Dave May wrote:
>
>
> On 19 August 2015 at 10:20, TAY wee-beng <zonexo at gmail.com 
> <mailto:zonexo at gmail.com>> wrote:
>
>
>     On 19/8/2015 1:17 PM, Dave May wrote:
>>
>>
>>     On 19 August 2015 at 03:38, TAY wee-beng <zonexo at gmail.com
>>     <mailto:zonexo at gmail.com>> 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?
Well, I mean when I don't need to solve the linear eqn. But of course, 
later on in the code, when I need to, I will require the global vector.

Thanks
>
> Cheers,
>   Dave
>

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