Stall in some multi-node parallel calculations

General Discussion
76

Now, although I have not modified macro_ddm.idp further, I can’t reproduce the error about intersection… The output is the following with the -ns flag:

-- FreeFem++ v4.9 (Fri Jun 18 14:45:02 CEST 2021 - git v4.9)
 Load: lg_fem lg_mesh lg_mesh3 eigenvalue parallelempi 
 load: init metis (v  5 )
 sizestack + 1024 =10824  ( 9800 )

  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
2 true neighbor is 1 (1)
2 true neighbor is 3 (2)
2: 
2	
	  1	  3	
190	426	
rank 2 sending/receiving 190 to 1
 --- global mesh of 4800 elements (prior to refinement) partitioned with metis  --metisA: 4-way Edge-Cut:       3, Balance:  1.01 Nodal=0/Dual 1
 (in 4.319191e-03)
0 true neighbor is 1 (1)
 --- partition of unity built (in 2.465439e-01)
0: 
1	
	  1	
480	
rank 0 sending/receiving 480 to 1
1 true neighbor is 0 (1)
1 false neighbor bis is 2 (1)
1 true neighbor is 3 (2)
1: 
2	
	  0	  3	
480	964	
rank 1 sending/receiving 480 to 0
rank 1 sending/receiving 964 to 3
3 true neighbor is 1 (1)
3 true neighbor is 2 (2)
3: 
2	
	  1	  2	
964	426	
rank 3 sending/receiving 964 to 1
rank 3 sending/receiving 426 to 2
rank 0 received from 1 (1) with tag 0 and count 480
rank 2 sending/receiving 426 to 3
rank 1 received from 0 (0) with tag 0 and count 480
rank 2 received from 3 (3) with tag 0 and count 426
rank 1 received from 3 (3) with tag 0 and count 964
rank 3 received from 2 (2) with tag 0 and count 426
rank 3 received from 1 (1) with tag 0 and count 964

and without the -ns flag:

(...)
   41 :  sizestack + 1024 =10824  ( 9800 )

  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
  -- Square mesh : nb vertices  =1681 ,  nb triangles = 3200 ,  nb boundary edges 160
 --- global mesh of 4800 elements (prior to refinement) partitioned with metis  --metisA: 4-way Edge-Cut:       3, Balance:  1.01 Nodal=0/Dual 1
 (in 4.315138e-03)
0 true neighbor is 1 (1)
 --- partition of unity built (in 2.509263e-01)
0: 
1	
	  1	
480	
rank 0 sending/receiving 480 to 1
1 true neighbor is 0 (1)
1 false neighbor bis is 2 (1)
1 true neighbor is 3 (2)
1: 
2	
	  0	  3	
480	964	
rank 1 sending/receiving 480 to 0
rank 1 sending/receiving 964 to 3
rank 0 received from 1 (1) with tag 0 and count 480
2 true neighbor is 1 (1)
2 true neighbor is 3 (2)
2: 
2	
	  1	  3	
190	426	
rank 2 sending/receiving 190 to 1
rank 2 sending/receiving 426 to 3
rank 1 received from 0 (0) with tag 0 and count 480
rank 1 received from 3 (3) with tag 0 and count 964
3 true neighbor is 1 (1)
3 true neighbor is 2 (2)
3: 
2	
	  1	  2	
964	426	
rank 3 sending/receiving 964 to 1
rank 3 sending/receiving 426 to 2
rank 2 received from 3 (3) with tag 0 and count 426
rank 3 received from 2 (2) with tag 0 and count 426
rank 3 received from 1 (1) with tag 0 and count 964