# Issue with ghost elements **URL:** \ **Category:** General Discussion\ **Created:** [December 30, 2020, 1:22pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715 "2020-12-30T13:22:47Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Mik](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mik](https://community.freefem.org/u/Mik)\ **Post date:** [December 30, 2020, 1:22pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/1 "2020-12-30T13:22:47Z")
Dear All, I am having an issue with the transfer of my variable from one iteration to the next while implementing a number of processes (n)\>1. My problem has the addition of material and moving heat source with each iteration. From the youtube videos of freefem, I got to know about some highly efficient functionality such as ‘’’ **restrict & macro Th1N2O() myn2o**// ‘’’ . However, what I lack currently in my code is a way to handle the **ghost nodes in my global solution**. With n=1, I am getting good results, whereas, with n=4 as soon my heat source is coming near the ghost nodes, its giving some faulty results. Is there some macros or function that can be implemented to handle this? I am writing some important sections of my code for your understanding. Thank you in advance for your help. Sincerely, > ThGlobal= readmesh3(“Global.mesh”); // Global Mesh > > Material addition for loop { > mesh3 Th1 = trunc(ThGlobal… > mesh3 backupTh=Th1; > > fespace Vhbackup(backupTh,P1); > int[int] subIDx1=restrict(Vhbackup,Vhglobal,n2o); > Vhbackup Tbackup; > Tbackup=Tglobal; > > macro dimension()3// EOM > include “macro\_ddm.idp” > int[int] myn2o; > macro Th1N2O() myn2o// > buildDmesh(Th1) > fespace Vh(Th1,P1); > > int[int] subIDx2=restrict(Vh,Vhbackup,myn2o); > Vh T, TT; > T=Tbackup; > > load “PETSc” > Mat A; > createMat(Th1,A,P1) > T = A^-1\*b; > Tbackup=T; > Tglobal=Tbackup; > } ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [December 30, 2020, 4:02pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/2 "2020-12-30T16:02:21Z")
Could you please copy/paste a fully runnable example? I think I get what you want to do, but it will much faster for me to help you out if I can actually run your code. ---
**Author:** ![Mik](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mik](https://community.freefem.org/u/Mik)\ **Post date:** [December 30, 2020, 4:55pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/3 "2020-12-30T16:55:16Z")
Thank you for your reply. I hereby attach the link for my .edp and mesh file. > **[test - Google Drive](https://drive.google.com/drive/folders/1HqET3tPE17oOCXi7qfBu1XJyHxLP01dx)** Let me know if the files are not properly shared; Regards, ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [January 2, 2021, 2:02pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/4 "2021-01-02T14:02:17Z")
Right, you are missing the partition of unity. You can check [this](https://youtu.be/-Aw2O46V2bo?t=5713) tutorial which explains this. If you don’t succeed at applying this to your code, let me know and I’ll do it myself. Thanks. ---
**Author:** ![Mik](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mik](https://community.freefem.org/u/Mik)\ **Post date:** [January 4, 2021, 1:23pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/5 "2021-01-04T13:23:03Z")
Dear Prj, Thank you for your reply. I have added the partition of unity to my problem and thanks to the partition of unity, ghost nodes are not giving me any issue. However, I believe I am having a problem when I am storing the data (in this case T ) from my previous iteration. In my case, my _T from the previous iteration acts as an initial condition for my next iteration and there is also an addition of elements at each iteration._ When I compare my result from the iter=0 to the initial condition at iter=1, I am getting a completely different T. Normally, both of them should be equal except for the elements that I added to encounter the material addition. When I run for the number of processors (n) 1, the initial condition of my current iteration (Tcurrent.vtu) is the same as my results from the previous iteration (Tbackup.vtu). However, for n other than 1, giving faulty result. I am certain that it is something trivial issue, I must be doing something foolish. Would you be able to see this issue and tell me where I am doing wrong? Here is the link to my .edp and .mesh file. > **[test - Google Drive](https://drive.google.com/drive/folders/1HqET3tPE17oOCXi7qfBu1XJyHxLP01dx?usp=sharing)** Regards ---
**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\ **Post date:** [January 4, 2021, 1:35pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/6 "2021-01-04T13:35:07Z")
You are missing a reduction operation, which I believe is covered in the video tutorial I sent earlier. ```auto T[].*=A.D; Tbackup[](subIDx2)=T[]; ``` should become something like ```auto T[].*=A.D; real[int] reduction(Tbackup[].n); reduction(subIDx2)=T[]; mpiAllReduce(reduction,Tbackup[],mpiCommWorld,mpiSUM); ``` Otherwise, the processes don’t share the same values (when `mpisize > 1`) and you get garbage for the next iteration. ---
**Author:** ![Mik](https://avatars.discourse-cdn.com/v4/letter/m/958977/32.png) [@Mik](https://community.freefem.org/u/Mik)\ **Post date:** [January 4, 2021, 2:26pm UTC](https://community.freefem.org/t/issue-with-ghost-elements/715/7 "2021-01-04T14:26:40Z")
**Thanks a lot for your quick reply.** Indeed, I missed the reduction operator. **Thank you for your help, everything is working perfectly.** Have a good day ahead. Sincere Regards.