[petsc-dev] Can I modify x on SampleShellPCApply callback ?

[Franck Houssen]

Can I modify x on SampleShellPCApply callback ? 

I am running debian (gcc-6.4), using petsc-dev synchronized on: 
>> git log 
commit f9d5775f43f69cbce5a7014a6ce3b24cc0e1214a (HEAD -> master, origin/master, origin/HEAD) 

when I run petsc/src/ksp/ksp/examples/tutorials/ex15.c, I get: 
>> mpirun -n 2 ./ex15.exe -user_defined_pc 
Norm of error 5.90715e-08 iterations 15 

OK, so far, so good. 

Now, I just modify x in SampleShellPCApply (reset to 0. at the end = doing nothing): 
>> git diff 
diff --git a/src/ksp/ksp/examples/tutorials/ex15.c b/src/ksp/ksp/examples/tutorials/ex15.c 
index badccd93b5..482e5ae6b5 100644 
--- a/src/ksp/ksp/examples/tutorials/ex15.c 
+++ b/src/ksp/ksp/examples/tutorials/ex15.c 
@@ -296,6 +296,7 @@ PetscErrorCode SampleShellPCApply(PC pc,Vec x,Vec y) 

ierr = PCShellGetContext(pc,(void**)&shell);CHKERRQ(ierr); 
ierr = VecPointwiseMult(y,x,shell->diag);CHKERRQ(ierr); 
+ ierr = VecSet(x, 0.);CHKERRQ(ierr); 

return 0; 
} 
>> mpirun -n 2 ./ex15.exe -user_defined_pc 
[0]PETSC ERROR: --------------------- Error Message -------------------------------------------------------------- 
[0]PETSC ERROR: Object is in wrong state 
[0]PETSC ERROR: Vec is locked read only, argument # 1 
[1]PETSC ERROR: #1 VecSet() line 547 in /home/fghoussen/Documents/INRIA/petsc/src/vec/vec/interface/rvector.c 
[1]PETSC ERROR: #2 SampleShellPCApply() line 299 in /home/fghoussen/Documents/INRIA/petsc/local/../src/ksp/ksp/examples/tutorials/ex15.c 

Why is that ? Is there a way to "unlock" x ? 

It seems, it's possible to modify y but not x: why ? 

>> git diff 
diff --git a/src/ksp/ksp/examples/tutorials/ex15.c b/src/ksp/ksp/examples/tutorials/ex15.c 
index badccd93b5..98ae001319 100644 
--- a/src/ksp/ksp/examples/tutorials/ex15.c 
+++ b/src/ksp/ksp/examples/tutorials/ex15.c 
@@ -294,6 +294,7 @@ PetscErrorCode SampleShellPCApply(PC pc,Vec x,Vec y) 
SampleShellPC *shell; 
PetscErrorCode ierr; 

+ ierr = VecSet(y, 0.);CHKERRQ(ierr); 
ierr = PCShellGetContext(pc,(void**)&shell);CHKERRQ(ierr); 
ierr = VecPointwiseMult(y,x,shell->diag);CHKERRQ(ierr); 
>> mpirun -n 2 ./ex15.exe -user_defined_pc 
Norm of error 5.90715e-08 iterations 15 

Does PETSc uses x even after the callback has been called ? 
My understanding is that at the end of the callback only y matters: y replaces x that has become no-use (y is the new residu): am I wrong ? 

If possible, I would like to reuse x (avoiding to re-create a vector) ? Is there a way to "unlock" x ? 

Franck 


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20170716/2ded3e0c/attachment.html>