[petsc-dev] Is DMGetDMKSPWrite() really not collective?

[Lawrence Mitchell]

> On 14 Jun 2019, at 18:44, Zhang, Junchao via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> Hello,
>    I am investigating petsc issue 306. One can produce the problem with src/snes/examples/tutorials/ex9.c and mpirun -n 3 ./ex9 -snes_grid_sequence 3 -snes_converged_reason -pc_type mg
>   The program can run to finish,  or crash, or hang.  The error appears either in PetscGatherMessageLengths or PetscCommBuildTwoSided().  From from my debugging, the following routine is suspicious.  It claims not collective, but it might call DMKSPCreate, which can indirectly call a collective MPI_Comm_dup().

I would have thought that DMKSPCreate could only call MPI_Comm_dup (via PetscCommDuplicate) if the incoming dm has a communicator which is /not/ a PETSc communicator. Given that all PETSc objects must (?) return a PETSc communicator when calling PetscObjectComm, this function is presumably incidentally not collective (although it is logically collective I would have thought), oh, and with PETSC_USE_DEBUG, there's a barrier even in the "return immediately with a PETSc comm" case.

FWIW, the call site looks to be collective (PCSetUp_MG).

Lawrence

>   Can someone familiar with the code explain it?  Thanks.
> 
>   /*@C
>    DMGetDMKSPWrite - get write access to private DMKSP context from a DM
> 
>    Not Collective
> 
>    Input Argument:
> .  dm - DM to be used with KSP
> 
>    Output Argument:
> .  kspdm - private DMKSP context
> 
>    Level: developer
> 
> .seealso: DMGetDMKSP()
> @*/
> PetscErrorCode DMGetDMKSPWrite(DM dm,DMKSP *kspdm)
> {
>   PetscErrorCode ierr;
>   DMKSP          kdm;
> 
>   PetscFunctionBegin;
>   PetscValidHeaderSpecific(dm,DM_CLASSID,1);
>   ierr = DMGetDMKSP(dm,&kdm);CHKERRQ(ierr);
>   if (!kdm->originaldm) kdm->originaldm = dm;
>   if (kdm->originaldm != dm) {  /* Copy on write */
>     DMKSP oldkdm = kdm;
>     ierr      = PetscInfo(dm,"Copying DMKSP due to write\n");CHKERRQ(ierr);
>     ierr      = DMKSPCreate(PetscObjectComm((PetscObject)dm),&kdm);CHKERRQ(ierr);
>     ierr      = DMKSPCopy(oldkdm,kdm);CHKERRQ(ierr);
>     ierr      = DMKSPDestroy((DMKSP*)&dm->dmksp);CHKERRQ(ierr);
>     dm->dmksp = (PetscObject)kdm;
>   }
>   *kspdm = kdm;
>   PetscFunctionReturn(0);
> }
> 
> The calling stack is
> ...
> #20 0x00007fcf3e3d23e2 in PetscCommDuplicate (comm_in=comm_in at entry=-2080374782, comm_out=comm_out at entry=0x557a18304db0, first_tag=first_tag at entry=0x557a18304de4) at /home/jczhang/petsc/src/sys/objects/tagm.c:162
> #21 0x00007fcf3e3d7730 in PetscHeaderCreate_Private (h=0x557a18304d70, classid=<optimized out>, class_name=class_name at entry=0x7fcf3f7f762a "DMKSP", descr=descr at entry=0x7fcf3f7f762a "DMKSP",  mansec=mansec at entry=0x7fcf3f7f762a "DMKSP", comm=comm at entry=-2080374782, destroy=0x7fcf3f350570 <DMKSPDestroy>, view=0x0)    at /home/jczhang/petsc/src/sys/objects/inherit.c:64
> #22 0x00007fcf3f3504c9 in DMKSPCreate (comm=-2080374782, kdm=kdm at entry=0x7ffc1d4d00f8)     at /home/jczhang/petsc/src/ksp/ksp/interface/dmksp.c:24
> #23 0x00007fcf3f35150f in DMGetDMKSPWrite (dm=0x557a18541a10, kspdm=kspdm at entry=0x7ffc1d4d01a8)     at /home/jczhang/petsc/src/ksp/ksp/interface/dmksp.c:155
> #24 0x00007fcf3f1bb20e in PCSetUp_MG (pc=<optimized out>) at /home/jczhang/petsc/src/ksp/pc/impls/mg/mg.c:682
> #25 0x00007fcf3f204bea in PCSetUp (pc=0x557a17dc1860) at /home/jczhang/petsc/src/ksp/pc/interface/precon.c:894
> #26 0x00007fcf3f32ba4b in KSPSetUp (ksp=0x557a17d73500) at /home/jczhang/petsc/src/ksp/ksp/interface/itfunc.c:377
> #27 0x00007fcf3f41e43e in SNESSolve_VINEWTONRSLS (snes=0x557a17bff210) at /home/jczhang/petsc/src/snes/impls/vi/rs/virs.c:502
> #28 0x00007fcf3f3fa191 in SNESSolve (snes=0x557a17bff210, b=0x0, x=<optimized out>)     at /home/jczhang/petsc/src/snes/interface/snes.c:4433
> #29 0x0000557a16432095 in main (argc=<optimized out>, argv=<optimized out>)     at /home/jczhang/petsc/src/snes/examples/tutorials/ex9.c:105
> 
> --Junchao Zhang