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-   -   Findcell in parallel (http://www.cfd-online.com/Forums/openfoam-programming-development/60958-findcell-parallel.html)

magnus October 13, 2006 06:35

Findcell in parallel
 
Hi,

I had some problems defining a momentum source. the first step was to locate the cells that should have nonzero source. If I make the call

label point=mesh.findCell(r) // r is vector located in the domain

if ( point >= 0)
{
Info << "cell found";
}

It works fine in serial, but when I decompose the domain and run in parallel the cell cannot be found. Did I miss something here? Do I need to explicit loop over the processors or what?

I guess this is trivial for most of you so remember; a short answer is infinately more appreciated than no answer http://www.cfd-online.com/OpenFOAM_D...part/happy.gif

Thanks!

gschaider October 13, 2006 06:48

Every processor looks for that
 
Every processor looks for that vector in it's own sub-Grid. Only the one that has the right cell enters the if-statemen. As I understand it (havn't looked at THAT source) the Info-stream is only output for the first processor, all others are mute. So if the point is NOT on the first processor nothing is output (allthough he was found).

My fix (but I'm not very good with that) would be:

label point=mesh.findCell(r)
{
label tmpPoint=myPoint;

reduce(tmpPoint,maxOp<label>());
// is point on ANY processor bigger than 0
if ( tmpPoint >= 0)
{
Info << "cell found";
}
}
if(point>=0) {
// do it only on the processor that HAS this point
}

eugene October 13, 2006 07:26

Bernhard is right, if you want
 
Bernhard is right, if you want all processors to be able to report the message, use

Pout << "cell found"

Instead of Info. This will prefix the statement with the originating processor number as well (say thanks to Mattijs).

magnus October 13, 2006 08:58

Thanks alot guys! That rea
 
Thanks alot guys!

That really made me alot clearer on whats going on. Now it works like a charm (Although it should have worked before also but the output was not what i believed it to be).

/M

ivan_cozza November 10, 2009 06:15

Hi Foamers,
it's possible to use findCell to find a cell on a processor that's different from the local one? If not, is there any workaround to do it?
Thanks, Ivan

fra76 November 14, 2009 06:31

If I understand your question, a possible solution is to use findCell on all the processors, then reduce the result so that each processor knows where the requested point is.

Hope this helps,
Francesco

ivan_cozza November 15, 2009 15:07

Quote:

Originally Posted by fra76 (Post 236222)
If I understand your question, a possible solution is to use findCell on all the processors, then reduce the result so that each processor knows where the requested point is.

Hope this helps,
Francesco

Hi Francesco,
unfortunately your suggestion can't work, as in my code each cell have to search points that can be on different processors, and findCell finds cells that belong to the local processor...

fra76 November 17, 2009 15:01

If it's only one point (I'm writing by heart, so please forgive errors):

Code:

labelList localCellId(Pstream::nProcs(),0);
localCellId[Pstream::myProcNo()] = mesh.findCell(YOUR_POINT);
reduce(localCellId, maxOp<labeList>());

After that, the list localCellId should be != -1 only in the entry that corresponds to the cell owner, with the value that is the owner's cell id.

If you have to look for a point in the mesh, I think that you have to spread this information among all the processors before searching. Each processor can only look for things in its domain, and then can communicate the result of the search operation to the others, if needed.

Francesco

FabienF September 11, 2012 05:58

related problem
 
Hi,
I have a somewhat related problem.
I want to perform a simple post-processing task (in parallel, using meshsearch).
Basically:
Quote:

- each processor loops over its interior field
-> current_pt=mesh.C()[cellI];
-> U_current[cellI]=U[cellI];
- if the value of field X (here mesh.C().component(2)) is above a threshold, it looks for an offset point:
-> sup_pt=current_pt+point(0.0 , 0.0, offset);
-> idx_sup = searchEngine.findCell(sup_pt)
-> U_sup[cellI]= Uint.interpolate(sup_pt,idx_sup)
- the new field Y gets the value Y[cellI]=U_curr[cellI]-U_sup[cellI]
Here is my implementation:

Quote:

meshSearch searchEngine(mesh);
interpolationCellPoint<vector> UInt(U);

vector U_sup=vector::zero;
vector U_curr=vector::zero;
point current_pt=point::zero;
point pt_sup=point::zero;

scalar offset=30.0;

forAll(mesh.C(), cellI)
{
current_pt=mesh.C()[cellI];
U_curr=U[cellI];

if ( current_pt[2]>offset )
{
pt_sup=current_pt+point(0.0,0.0,offset);

Pout << " pt :x "<< current_pt << endl;
Pout << " pt sup: "<< pt_sup << endl;

List<label> idx_sup(Pstream::nProcs(),0);
List<scalar> dist_sup(Pstream::nProcs(),GREAT);
List<vector> Usup(Pstream::nProcs(),vector::zero);

idx_sup[Pstream::myProcNo()] = searchEngine.findNearestCell(pt_sup,cellI);
reduce(idx_sup,maxOp<List<label> >());

searcherSup=1000000.0;

for (int i=0; i<Pstream::nProcs() ;i++)
{
dist_sup[i]=mag(mesh.C()[idx_sup[i]]-pt_sup);
Pout << " pt sup found: "<< i << " "<< mesh.C()[idx_sup[i]] << " " << dist_sup[i] <<endl;
if (dist_sup[i]<searcherSup)
{
searcherSup=dist_sup[i];
found_sup=i;
}
}
Pout << " pt sup kept: "<< mesh.C()[idx_sup[found_sup]] << endl << endl;

if (Pstream::myProcNo()==found_sup)
{
Usup[Pstream::myProcNo()]=UInt.interpolate(pt_sup,idx_sup[found_sup]);
}

reduce(Usup,maxOp<List<vector> >());

U_sup = Usup[found_sup];
Y[cellI] = U_curr-U_sup;

Info << " U : "<< U_curr << endl;
Info << " U sup: "<< U_sup << endl;
}
}
Pout << "Finished! << endl;
It seems that the meshSearch finds the right points, but the problem is that processor2 (due to my decomposition and the offset value) finishes its task first, and the other processors seem to get stuck has soon as this happens.

(By the way, it is a simplified version of my problem. I am not trying to find offset points outside my mesh, as this sample code would suggest)

Do you have an idea why? I have been struggling for quiet a while!!!

Thanks,

Fabien

carowjp March 5, 2013 21:40

findCell in parallel and reduce
 
Fabien & Foamers,

Did you solve this problem?

I suspect it may be related to what I am seeing when I loop over cells in a decomposed mesh. Unfortunately, I don't have an answer only another question...

Code:

forAll(alpha.internalField(),cellI)
{
              vector centerR = ptsR[cellI];
              labelList targetCellId(Pstream::nProcs(),0);

              targetCellId[Pstream::myProcNo()] = mesh.findCell(centerR);

        reduce(targetCellId, maxOp<List<label> >());
        Pout << "targetCellId: " << targetCellId << endl;
}

Without reduce it returns:
Code:

[3] targetCellId: 4(0 0 0 -1)
[1] targetCellId: 4(0 -1 0 0)
[2] targetCellId: 4(0 0 -1 0)
[1] targetCellId: 4(0 288 0 0)
[0] targetCellId: 4{0}
[0] targetCellId: 4(1 0 0 0)
[3] targetCellId: 4(0 0 0 -1)
[2] targetCellId: 4(0 0 -1 0)
[2] targetCellId: 4(0 0 -1 0)
[0] targetCellId: 4(34 0 0 0)
[1] targetCellId: 4(0 289 0 0)
[3] targetCellId: 4(0 0 0 -1)
[0] targetCellId: 4(35 0 0 0)
[1] targetCellId: 4(0 290 0 0)

With reduce it returns:
Code:

[0] targetCellId: 4{0}
[1] targetCellId: 4{0}
[2] targetCellId: 4{0}
[3] targetCellId: 4{0}
[0] targetCellId: 4(1 288 0 0)
[1] targetCellId: 4(1 288 0 0)
[2] targetCellId: 4(1 288 0 0)
[3] targetCellId: 4(1 288 0 0)
[3] targetCellId: 4(34 289 0 0)
[2] targetCellId: 4(34 289 0 0)
[0] targetCellId: 4(34 289 0 0)
[1] targetCellId: 4(34 289 0 0)
[3] targetCellId: 4(35 290 0 0)
[2] targetCellId: 4(35 290 0 0)
[0] targetCellId: 4(35 290 0 0)
[1] targetCellId: 4(35 290 0 0)

I am expecting reduce to return one cell ID and the rest zeros. However, you can see that reduce seems to be combining the results of multiple finds from different CPU findCell requests! Each point only exists on one of the four decomposed mesh domains. It seems the CPUs can race ahead of each other and reduce then gives a muddled answer.

How can I match each CPU's unique findCell request with a unique found cell ID?

thanks and regards,

James


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