impls/window/sfwindow.c

#include <petsc/private/sfimpl.h> /*I "petscsf.h" I*/

typedef struct _n_PetscSFDataLink *PetscSFDataLink;
typedef struct _n_PetscSFWinLink  *PetscSFWinLink;

typedef struct {
  PetscSFWindowSyncType   sync;   /* FENCE, LOCK, or ACTIVE synchronization */
  PetscSFDataLink         link;   /* List of MPI data types, lazily constructed for each data type */
  PetscSFWinLink          wins;   /* List of active windows */
  PetscSFWindowFlavorType flavor; /* Current PETSCSF_WINDOW_FLAVOR_ */
  PetscSF                 dynsf;
  MPI_Info                info;
} PetscSF_Window;

struct _n_PetscSFDataLink {
  MPI_Datatype    unit;
  MPI_Datatype   *mine;
  MPI_Datatype   *remote;
  PetscSFDataLink next;
};

struct _n_PetscSFWinLink {
  PetscBool               inuse;
  size_t                  bytes;
  void                   *addr;
  void                   *paddr;
  MPI_Win                 win;
  MPI_Request            *reqs;
  PetscSFWindowFlavorType flavor;
  MPI_Aint               *dyn_target_addr;
  PetscBool               epoch;
  PetscSFWinLink          next;
};

const char *const PetscSFWindowSyncTypes[]   = {"FENCE", "LOCK", "ACTIVE", "PetscSFWindowSyncType", "PETSCSF_WINDOW_SYNC_", NULL};
const char *const PetscSFWindowFlavorTypes[] = {"CREATE", "DYNAMIC", "ALLOCATE", "SHARED", "PetscSFWindowFlavorType", "PETSCSF_WINDOW_FLAVOR_", NULL};

/* Built-in MPI_Ops act elementwise inside MPI_Accumulate, but cannot be used with composite types inside collectives (MPI_Allreduce) */
static PetscErrorCode PetscSFWindowOpTranslate(MPI_Op *op) {
  PetscFunctionBegin;
  if (*op == MPIU_SUM) *op = MPI_SUM;
  else if (*op == MPIU_MAX) *op = MPI_MAX;
  else if (*op == MPIU_MIN) *op = MPI_MIN;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowGetDataTypes - gets composite local and remote data types for each rank

   Not Collective

   Input Parameters:
+  sf - star forest
-  unit - data type for each node

   Output Parameters:
+  localtypes - types describing part of local leaf buffer referencing each remote rank
-  remotetypes - types describing part of remote root buffer referenced for each remote rank

   Level: developer

.seealso: `PetscSFSetGraph()`, `PetscSFView()`
@*/
static PetscErrorCode PetscSFWindowGetDataTypes(PetscSF sf, MPI_Datatype unit, const MPI_Datatype **localtypes, const MPI_Datatype **remotetypes) {
  PetscSF_Window    *w = (PetscSF_Window *)sf->data;
  PetscSFDataLink    link;
  PetscInt           i, nranks;
  const PetscInt    *roffset, *rmine, *rremote;
  const PetscMPIInt *ranks;

  PetscFunctionBegin;
  /* Look for types in cache */
  for (link = w->link; link; link = link->next) {
    PetscBool match;
    PetscCall(MPIPetsc_Type_compare(unit, link->unit, &match));
    if (match) {
      *localtypes  = link->mine;
      *remotetypes = link->remote;
      PetscFunctionReturn(0);
    }
  }

  /* Create new composite types for each send rank */
  PetscCall(PetscSFGetRootRanks(sf, &nranks, &ranks, &roffset, &rmine, &rremote));
  PetscCall(PetscNew(&link));
  PetscCallMPI(MPI_Type_dup(unit, &link->unit));
  PetscCall(PetscMalloc2(nranks, &link->mine, nranks, &link->remote));
  for (i = 0; i < nranks; i++) {
    PetscInt     rcount = roffset[i + 1] - roffset[i];
    PetscMPIInt *rmine, *rremote;
#if !defined(PETSC_USE_64BIT_INDICES)
    rmine   = sf->rmine + sf->roffset[i];
    rremote = sf->rremote + sf->roffset[i];
#else
    PetscInt j;
    PetscCall(PetscMalloc2(rcount, &rmine, rcount, &rremote));
    for (j = 0; j < rcount; j++) {
      PetscCall(PetscMPIIntCast(sf->rmine[sf->roffset[i] + j], rmine + j));
      PetscCall(PetscMPIIntCast(sf->rremote[sf->roffset[i] + j], rremote + j));
    }
#endif

    PetscCallMPI(MPI_Type_create_indexed_block(rcount, 1, rmine, link->unit, &link->mine[i]));
    PetscCallMPI(MPI_Type_create_indexed_block(rcount, 1, rremote, link->unit, &link->remote[i]));
#if defined(PETSC_USE_64BIT_INDICES)
    PetscCall(PetscFree2(rmine, rremote));
#endif
    PetscCallMPI(MPI_Type_commit(&link->mine[i]));
    PetscCallMPI(MPI_Type_commit(&link->remote[i]));
  }
  link->next = w->link;
  w->link    = link;

  *localtypes  = link->mine;
  *remotetypes = link->remote;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowSetFlavorType - Set flavor type for MPI_Win creation

   Logically Collective

   Input Parameters:
+  sf - star forest for communication
-  flavor - flavor type

   Options Database Key:
.  -sf_window_flavor <flavor> - sets the flavor type CREATE, DYNAMIC, ALLOCATE or SHARED (see PetscSFWindowFlavorType)

   Level: advanced

   Notes: Windows reusage follow this rules:

     PETSCSF_WINDOW_FLAVOR_CREATE: creates a new window every time, uses MPI_Win_create

     PETSCSF_WINDOW_FLAVOR_DYNAMIC: uses MPI_Win_create_dynamic/MPI_Win_attach and tries to reuse windows by comparing the root array. Intended to be used on repeated applications of the same SF, e.g.
       for i=1 to K
         PetscSFOperationBegin(rootdata1,leafdata_whatever);
         PetscSFOperationEnd(rootdata1,leafdata_whatever);
         ...
         PetscSFOperationBegin(rootdataN,leafdata_whatever);
         PetscSFOperationEnd(rootdataN,leafdata_whatever);
       endfor
       The following pattern will instead raise an error
         PetscSFOperationBegin(rootdata1,leafdata_whatever);
         PetscSFOperationEnd(rootdata1,leafdata_whatever);
         PetscSFOperationBegin(rank ? rootdata1 : rootdata2,leafdata_whatever);
         PetscSFOperationEnd(rank ? rootdata1 : rootdata2,leafdata_whatever);

     PETSCSF_WINDOW_FLAVOR_ALLOCATE: uses MPI_Win_allocate, reuses any pre-existing window which fits the data and it is not in use

     PETSCSF_WINDOW_FLAVOR_SHARED: uses MPI_Win_allocate_shared, reusage policy as for PETSCSF_WINDOW_FLAVOR_ALLOCATE

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowGetFlavorType()`
@*/
PetscErrorCode PetscSFWindowSetFlavorType(PetscSF sf, PetscSFWindowFlavorType flavor) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidLogicalCollectiveEnum(sf, flavor, 2);
  PetscTryMethod(sf, "PetscSFWindowSetFlavorType_C", (PetscSF, PetscSFWindowFlavorType), (sf, flavor));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowSetFlavorType_Window(PetscSF sf, PetscSFWindowFlavorType flavor) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  w->flavor = flavor;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowGetFlavorType - Get flavor type for PetscSF communication

   Logically Collective

   Input Parameter:
.  sf - star forest for communication

   Output Parameter:
.  flavor - flavor type

   Level: advanced

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowSetFlavorType()`
@*/
PetscErrorCode PetscSFWindowGetFlavorType(PetscSF sf, PetscSFWindowFlavorType *flavor) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidPointer(flavor, 2);
  PetscUseMethod(sf, "PetscSFWindowGetFlavorType_C", (PetscSF, PetscSFWindowFlavorType *), (sf, flavor));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowGetFlavorType_Window(PetscSF sf, PetscSFWindowFlavorType *flavor) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  *flavor = w->flavor;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowSetSyncType - Set synchronization type for PetscSF communication

   Logically Collective

   Input Parameters:
+  sf - star forest for communication
-  sync - synchronization type

   Options Database Key:
.  -sf_window_sync <sync> - sets the synchronization type FENCE, LOCK, or ACTIVE (see PetscSFWindowSyncType)

   Level: advanced

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowGetSyncType()`
@*/
PetscErrorCode PetscSFWindowSetSyncType(PetscSF sf, PetscSFWindowSyncType sync) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidLogicalCollectiveEnum(sf, sync, 2);
  PetscTryMethod(sf, "PetscSFWindowSetSyncType_C", (PetscSF, PetscSFWindowSyncType), (sf, sync));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowSetSyncType_Window(PetscSF sf, PetscSFWindowSyncType sync) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  w->sync = sync;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowGetSyncType - Get synchronization type for PetscSF communication

   Logically Collective

   Input Parameter:
.  sf - star forest for communication

   Output Parameter:
.  sync - synchronization type

   Level: advanced

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowSetSyncType()`
@*/
PetscErrorCode PetscSFWindowGetSyncType(PetscSF sf, PetscSFWindowSyncType *sync) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidPointer(sync, 2);
  PetscUseMethod(sf, "PetscSFWindowGetSyncType_C", (PetscSF, PetscSFWindowSyncType *), (sf, sync));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowGetSyncType_Window(PetscSF sf, PetscSFWindowSyncType *sync) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  *sync = w->sync;
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowSetInfo - Set the MPI_Info handle that will be used for subsequent windows allocation

   Logically Collective

   Input Parameters:
+  sf - star forest for communication
-  info - MPI_Info handle

   Level: advanced

   Notes: the info handle is duplicated with a call to MPI_Info_dup unless info = MPI_INFO_NULL.

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowGetInfo()`
@*/
PetscErrorCode PetscSFWindowSetInfo(PetscSF sf, MPI_Info info) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscTryMethod(sf, "PetscSFWindowSetInfo_C", (PetscSF, MPI_Info), (sf, info));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowSetInfo_Window(PetscSF sf, MPI_Info info) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  if (w->info != MPI_INFO_NULL) PetscCallMPI(MPI_Info_free(&w->info));
  if (info != MPI_INFO_NULL) PetscCallMPI(MPI_Info_dup(info, &w->info));
  PetscFunctionReturn(0);
}

/*@C
   PetscSFWindowGetInfo - Get the MPI_Info handle used for windows allocation

   Logically Collective

   Input Parameter:
.  sf - star forest for communication

   Output Parameter:
.  info - MPI_Info handle

   Level: advanced

   Notes: if PetscSFWindowSetInfo() has not be called, this returns MPI_INFO_NULL

.seealso: `PetscSFSetFromOptions()`, `PetscSFWindowSetInfo()`
@*/
PetscErrorCode PetscSFWindowGetInfo(PetscSF sf, MPI_Info *info) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(sf, PETSCSF_CLASSID, 1);
  PetscValidPointer(info, 2);
  PetscUseMethod(sf, "PetscSFWindowGetInfo_C", (PetscSF, MPI_Info *), (sf, info));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFWindowGetInfo_Window(PetscSF sf, MPI_Info *info) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  *info = w->info;
  PetscFunctionReturn(0);
}

/*
   PetscSFGetWindow - Get a window for use with a given data type

   Collective on PetscSF

   Input Parameters:
+  sf - star forest
.  unit - data type
.  array - array to be sent
.  sync - type of synchronization PetscSFWindowSyncType
.  epoch - PETSC_TRUE to acquire the window and start an epoch, PETSC_FALSE to just acquire the window
.  fenceassert - assert parameter for call to MPI_Win_fence(), if sync == PETSCSF_WINDOW_SYNC_FENCE
.  postassert - assert parameter for call to MPI_Win_post(), if sync == PETSCSF_WINDOW_SYNC_ACTIVE
-  startassert - assert parameter for call to MPI_Win_start(), if sync == PETSCSF_WINDOW_SYNC_ACTIVE

   Output Parameters:
+  target_disp - target_disp argument for RMA calls (significative for PETSCSF_WINDOW_FLAVOR_DYNAMIC only)
+  reqs - array of requests (significative for sync == PETSCSF_WINDOW_SYNC_LOCK only)
-  win - window

   Level: developer
.seealso: `PetscSFGetRootRanks()`, `PetscSFWindowGetDataTypes()`
*/
static PetscErrorCode PetscSFGetWindow(PetscSF sf, MPI_Datatype unit, void *array, PetscSFWindowSyncType sync, PetscBool epoch, PetscMPIInt fenceassert, PetscMPIInt postassert, PetscMPIInt startassert, const MPI_Aint **target_disp, MPI_Request **reqs, MPI_Win *win) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  MPI_Aint        lb, lb_true, bytes, bytes_true;
  PetscSFWinLink  link;
#if defined(PETSC_HAVE_MPI_FEATURE_DYNAMIC_WINDOW)
  MPI_Aint winaddr;
  PetscInt nranks;
#endif
  PetscBool reuse = PETSC_FALSE, update = PETSC_FALSE;
  PetscBool dummy[2];
  MPI_Aint  wsize;

  PetscFunctionBegin;
  PetscCallMPI(MPI_Type_get_extent(unit, &lb, &bytes));
  PetscCallMPI(MPI_Type_get_true_extent(unit, &lb_true, &bytes_true));
  PetscCheck(lb == 0 && lb_true == 0, PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "No support for unit type with nonzero lower bound, write petsc-maint@mcs.anl.gov if you want this feature");
  PetscCheck(bytes == bytes_true, PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "No support for unit type with modified extent, write petsc-maint@mcs.anl.gov if you want this feature");
  if (w->flavor != PETSCSF_WINDOW_FLAVOR_CREATE) reuse = PETSC_TRUE;
  for (link = w->wins; reuse && link; link = link->next) {
    PetscBool winok = PETSC_FALSE;
    if (w->flavor != link->flavor) continue;
    switch (w->flavor) {
    case PETSCSF_WINDOW_FLAVOR_DYNAMIC: /* check available matching array, error if in use (we additionally check that the matching condition is the same across processes) */
      if (array == link->addr) {
        if (PetscDefined(USE_DEBUG)) {
          dummy[0] = PETSC_TRUE;
          dummy[1] = PETSC_TRUE;
          PetscCallMPI(MPI_Allreduce(MPI_IN_PLACE, dummy, 1, MPIU_BOOL, MPI_LAND, PetscObjectComm((PetscObject)sf)));
          PetscCallMPI(MPI_Allreduce(MPI_IN_PLACE, dummy + 1, 1, MPIU_BOOL, MPI_LOR, PetscObjectComm((PetscObject)sf)));
          PetscCheck(dummy[0] == dummy[1], PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "PETSCSF_WINDOW_FLAVOR_DYNAMIC requires root pointers to be consistently used across the comm. Use PETSCSF_WINDOW_FLAVOR_CREATE or PETSCSF_WINDOW_FLAVOR_ALLOCATE instead");
        }
        PetscCheck(!link->inuse, PetscObjectComm((PetscObject)sf), PETSC_ERR_PLIB, "Window in use");
        PetscCheck(!epoch || !link->epoch, PetscObjectComm((PetscObject)sf), PETSC_ERR_PLIB, "Window epoch not finished");
        winok       = PETSC_TRUE;
        link->paddr = array;
      } else if (PetscDefined(USE_DEBUG)) {
        dummy[0] = PETSC_FALSE;
        dummy[1] = PETSC_FALSE;
        PetscCallMPI(MPI_Allreduce(MPI_IN_PLACE, dummy, 1, MPIU_BOOL, MPI_LAND, PetscObjectComm((PetscObject)sf)));
        PetscCallMPI(MPI_Allreduce(MPI_IN_PLACE, dummy + 1, 1, MPIU_BOOL, MPI_LOR, PetscObjectComm((PetscObject)sf)));
        PetscCheck(dummy[0] == dummy[1], PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "PETSCSF_WINDOW_FLAVOR_DYNAMIC requires root pointers to be consistently used across the comm. Use PETSCSF_WINDOW_FLAVOR_CREATE or PETSCSF_WINDOW_FLAVOR_ALLOCATE instead");
      }
      break;
    case PETSCSF_WINDOW_FLAVOR_ALLOCATE: /* check available by matching size, allocate if in use */
    case PETSCSF_WINDOW_FLAVOR_SHARED:
      if (!link->inuse && bytes == (MPI_Aint)link->bytes) {
        update      = PETSC_TRUE;
        link->paddr = array;
        winok       = PETSC_TRUE;
      }
      break;
    default: SETERRQ(PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "No support for flavor %s", PetscSFWindowFlavorTypes[w->flavor]);
    }
    if (winok) {
      *win = link->win;
      PetscCall(PetscInfo(sf, "Reusing window %" PETSC_MPI_WIN_FMT " of flavor %d for comm %" PETSC_MPI_COMM_FMT "\n", link->win, link->flavor, PetscObjectComm((PetscObject)sf)));
      goto found;
    }
  }

  wsize = (MPI_Aint)bytes * sf->nroots;
  PetscCall(PetscNew(&link));
  link->bytes           = bytes;
  link->next            = w->wins;
  link->flavor          = w->flavor;
  link->dyn_target_addr = NULL;
  link->reqs            = NULL;
  w->wins               = link;
  if (sync == PETSCSF_WINDOW_SYNC_LOCK) {
    PetscInt i;

    PetscCall(PetscMalloc1(sf->nranks, &link->reqs));
    for (i = 0; i < sf->nranks; i++) link->reqs[i] = MPI_REQUEST_NULL;
  }
  switch (w->flavor) {
  case PETSCSF_WINDOW_FLAVOR_CREATE:
    PetscCallMPI(MPI_Win_create(array, wsize, (PetscMPIInt)bytes, w->info, PetscObjectComm((PetscObject)sf), &link->win));
    link->addr  = array;
    link->paddr = array;
    break;
#if defined(PETSC_HAVE_MPI_FEATURE_DYNAMIC_WINDOW)
  case PETSCSF_WINDOW_FLAVOR_DYNAMIC: PetscCallMPI(MPI_Win_create_dynamic(w->info, PetscObjectComm((PetscObject)sf), &link->win));
#if defined(PETSC_HAVE_OMPI_MAJOR_VERSION) /* some OpenMPI versions do not support MPI_Win_attach(win,NULL,0); */
    PetscCallMPI(MPI_Win_attach(link->win, wsize ? array : (void *)dummy, wsize));
#else
    PetscCallMPI(MPI_Win_attach(link->win, array, wsize));
#endif
    link->addr  = array;
    link->paddr = array;
    PetscCheck(w->dynsf, PetscObjectComm((PetscObject)sf), PETSC_ERR_ORDER, "Must call PetscSFSetUp()");
    PetscCall(PetscSFSetUp(w->dynsf));
    PetscCall(PetscSFGetRootRanks(w->dynsf, &nranks, NULL, NULL, NULL, NULL));
    PetscCall(PetscMalloc1(nranks, &link->dyn_target_addr));
    PetscCallMPI(MPI_Get_address(array, &winaddr));
    PetscCall(PetscSFBcastBegin(w->dynsf, MPI_AINT, &winaddr, link->dyn_target_addr, MPI_REPLACE));
    PetscCall(PetscSFBcastEnd(w->dynsf, MPI_AINT, &winaddr, link->dyn_target_addr, MPI_REPLACE));
    break;
  case PETSCSF_WINDOW_FLAVOR_ALLOCATE:
    PetscCallMPI(MPI_Win_allocate(wsize, (PetscMPIInt)bytes, w->info, PetscObjectComm((PetscObject)sf), &link->addr, &link->win));
    update      = PETSC_TRUE;
    link->paddr = array;
    break;
#endif
#if defined(PETSC_HAVE_MPI_PROCESS_SHARED_MEMORY)
  case PETSCSF_WINDOW_FLAVOR_SHARED:
    PetscCallMPI(MPI_Win_allocate_shared(wsize, (PetscMPIInt)bytes, w->info, PetscObjectComm((PetscObject)sf), &link->addr, &link->win));
    update      = PETSC_TRUE;
    link->paddr = array;
    break;
#endif
  default: SETERRQ(PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "No support for flavor %s", PetscSFWindowFlavorTypes[w->flavor]);
  }
  PetscCall(PetscInfo(sf, "New window %" PETSC_MPI_WIN_FMT " of flavor %d for comm %" PETSC_MPI_COMM_FMT "\n", link->win, link->flavor, PetscObjectComm((PetscObject)sf)));
  *win = link->win;

found:

  if (target_disp) *target_disp = link->dyn_target_addr;
  if (reqs) *reqs = link->reqs;
  if (update) { /* locks are needed for the "separate" memory model only, the fence guaranties memory-synchronization */
    PetscMPIInt rank;

    PetscCallMPI(MPI_Comm_rank(PetscObjectComm((PetscObject)sf), &rank));
    if (sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_lock(MPI_LOCK_EXCLUSIVE, rank, MPI_MODE_NOCHECK, *win));
    PetscCall(PetscMemcpy(link->addr, array, sf->nroots * bytes));
    if (sync == PETSCSF_WINDOW_SYNC_LOCK) {
      PetscCallMPI(MPI_Win_unlock(rank, *win));
      PetscCallMPI(MPI_Win_fence(0, *win));
    }
  }
  link->inuse = PETSC_TRUE;
  link->epoch = epoch;
  if (epoch) {
    switch (sync) {
    case PETSCSF_WINDOW_SYNC_FENCE: PetscCallMPI(MPI_Win_fence(fenceassert, *win)); break;
    case PETSCSF_WINDOW_SYNC_LOCK: /* Handled outside */ break;
    case PETSCSF_WINDOW_SYNC_ACTIVE: {
      MPI_Group   ingroup, outgroup;
      PetscMPIInt isize, osize;

      /* OpenMPI 4.0.2 with btl=vader does not like calling
         - MPI_Win_complete when ogroup is empty
         - MPI_Win_wait when igroup is empty
         So, we do not even issue the corresponding start and post calls
         The MPI standard (Sec. 11.5.2 of MPI 3.1) only requires that
         start(outgroup) has a matching post(ingroup)
         and this is guaranteed by PetscSF
      */
      PetscCall(PetscSFGetGroups(sf, &ingroup, &outgroup));
      PetscCallMPI(MPI_Group_size(ingroup, &isize));
      PetscCallMPI(MPI_Group_size(outgroup, &osize));
      if (isize) PetscCallMPI(MPI_Win_post(ingroup, postassert, *win));
      if (osize) PetscCallMPI(MPI_Win_start(outgroup, startassert, *win));
    } break;
    default: SETERRQ(PetscObjectComm((PetscObject)sf), PETSC_ERR_PLIB, "Unknown synchronization type");
    }
  }
  PetscFunctionReturn(0);
}

/*
   PetscSFFindWindow - Finds a window that is already in use

   Not Collective

   Input Parameters:
+  sf - star forest
.  unit - data type
-  array - array with which the window is associated

   Output Parameters:
+  win - window
-  reqs - outstanding requests associated to the window

   Level: developer

.seealso: `PetscSFGetWindow()`, `PetscSFRestoreWindow()`
*/
static PetscErrorCode PetscSFFindWindow(PetscSF sf, MPI_Datatype unit, const void *array, MPI_Win *win, MPI_Request **reqs) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  PetscSFWinLink  link;

  PetscFunctionBegin;
  *win = MPI_WIN_NULL;
  for (link = w->wins; link; link = link->next) {
    if (array == link->paddr) {
      PetscCall(PetscInfo(sf, "Window %" PETSC_MPI_WIN_FMT " of flavor %d for comm %" PETSC_MPI_COMM_FMT "\n", link->win, link->flavor, PetscObjectComm((PetscObject)sf)));
      *win  = link->win;
      *reqs = link->reqs;
      PetscFunctionReturn(0);
    }
  }
  SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "Requested window not in use");
}

/*
   PetscSFRestoreWindow - Restores a window obtained with PetscSFGetWindow()

   Collective

   Input Parameters:
+  sf - star forest
.  unit - data type
.  array - array associated with window
.  sync - type of synchronization PetscSFWindowSyncType
.  epoch - close an epoch, must match argument to PetscSFGetWindow()
.  update - if we have to update the local window array
-  win - window

   Level: developer

.seealso: `PetscSFFindWindow()`
*/
static PetscErrorCode PetscSFRestoreWindow(PetscSF sf, MPI_Datatype unit, void *array, PetscSFWindowSyncType sync, PetscBool epoch, PetscMPIInt fenceassert, PetscBool update, MPI_Win *win) {
  PetscSF_Window         *w = (PetscSF_Window *)sf->data;
  PetscSFWinLink         *p, link;
  PetscBool               reuse = PETSC_FALSE;
  PetscSFWindowFlavorType flavor;
  void                   *laddr;
  size_t                  bytes;

  PetscFunctionBegin;
  for (p = &w->wins; *p; p = &(*p)->next) {
    link = *p;
    if (*win == link->win) {
      PetscCheck(array == link->paddr, PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "Matched window, but not array");
      if (epoch != link->epoch) {
        PetscCheck(!epoch, PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "No epoch to end");
        SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "Restoring window without ending epoch");
      }
      laddr  = link->addr;
      flavor = link->flavor;
      bytes  = link->bytes;
      if (flavor != PETSCSF_WINDOW_FLAVOR_CREATE) reuse = PETSC_TRUE;
      else {
        *p     = link->next;
        update = PETSC_FALSE;
      } /* remove from list */
      goto found;
    }
  }
  SETERRQ(PETSC_COMM_SELF, PETSC_ERR_ARG_INCOMP, "Requested window not in use");

found:
  PetscCall(PetscInfo(sf, "Window %" PETSC_MPI_WIN_FMT " of flavor %d for comm %" PETSC_MPI_COMM_FMT "\n", link->win, link->flavor, PetscObjectComm((PetscObject)sf)));
  if (epoch) {
    switch (sync) {
    case PETSCSF_WINDOW_SYNC_FENCE: PetscCallMPI(MPI_Win_fence(fenceassert, *win)); break;
    case PETSCSF_WINDOW_SYNC_LOCK: /* Handled outside */ break;
    case PETSCSF_WINDOW_SYNC_ACTIVE: {
      MPI_Group   ingroup, outgroup;
      PetscMPIInt isize, osize;

      /* OpenMPI 4.0.2 with btl=wader does not like calling
         - MPI_Win_complete when ogroup is empty
         - MPI_Win_wait when igroup is empty
         The MPI standard (Sec. 11.5.2 of MPI 3.1) only requires that
         - each process who issues a call to MPI_Win_start issues a call to MPI_Win_Complete
         - each process who issues a call to MPI_Win_post issues a call to MPI_Win_Wait
      */
      PetscCall(PetscSFGetGroups(sf, &ingroup, &outgroup));
      PetscCallMPI(MPI_Group_size(ingroup, &isize));
      PetscCallMPI(MPI_Group_size(outgroup, &osize));
      if (osize) PetscCallMPI(MPI_Win_complete(*win));
      if (isize) PetscCallMPI(MPI_Win_wait(*win));
    } break;
    default: SETERRQ(PetscObjectComm((PetscObject)sf), PETSC_ERR_PLIB, "Unknown synchronization type");
    }
  }
  if (update) {
    if (sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_fence(MPI_MODE_NOPUT | MPI_MODE_NOSUCCEED, *win));
    PetscCall(PetscMemcpy(array, laddr, sf->nroots * bytes));
  }
  link->epoch = PETSC_FALSE;
  link->inuse = PETSC_FALSE;
  link->paddr = NULL;
  if (!reuse) {
    PetscCall(PetscFree(link->dyn_target_addr));
    PetscCall(PetscFree(link->reqs));
    PetscCallMPI(MPI_Win_free(&link->win));
    PetscCall(PetscFree(link));
    *win = MPI_WIN_NULL;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFSetUp_Window(PetscSF sf) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  MPI_Group       ingroup, outgroup;

  PetscFunctionBegin;
  PetscCall(PetscSFSetUpRanks(sf, MPI_GROUP_EMPTY));
  if (!w->dynsf) {
    PetscInt     i;
    PetscSFNode *remotes;

    PetscCall(PetscMalloc1(sf->nranks, &remotes));
    for (i = 0; i < sf->nranks; i++) {
      remotes[i].rank  = sf->ranks[i];
      remotes[i].index = 0;
    }
    PetscCall(PetscSFDuplicate(sf, PETSCSF_DUPLICATE_RANKS, &w->dynsf));
    PetscCall(PetscSFWindowSetFlavorType(w->dynsf, PETSCSF_WINDOW_FLAVOR_CREATE)); /* break recursion */
    PetscCall(PetscSFSetGraph(w->dynsf, 1, sf->nranks, NULL, PETSC_OWN_POINTER, remotes, PETSC_OWN_POINTER));
    PetscCall(PetscLogObjectParent((PetscObject)sf, (PetscObject)w->dynsf));
  }
  switch (w->sync) {
  case PETSCSF_WINDOW_SYNC_ACTIVE: PetscCall(PetscSFGetGroups(sf, &ingroup, &outgroup));
  default: break;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFSetFromOptions_Window(PetscSF sf, PetscOptionItems *PetscOptionsObject) {
  PetscSF_Window         *w      = (PetscSF_Window *)sf->data;
  PetscSFWindowFlavorType flavor = w->flavor;

  PetscFunctionBegin;
  PetscOptionsHeadBegin(PetscOptionsObject, "PetscSF Window options");
  PetscCall(PetscOptionsEnum("-sf_window_sync", "synchronization type to use for PetscSF Window communication", "PetscSFWindowSetSyncType", PetscSFWindowSyncTypes, (PetscEnum)w->sync, (PetscEnum *)&w->sync, NULL));
  PetscCall(PetscOptionsEnum("-sf_window_flavor", "flavor to use for PetscSF Window creation", "PetscSFWindowSetFlavorType", PetscSFWindowFlavorTypes, (PetscEnum)flavor, (PetscEnum *)&flavor, NULL));
  PetscCall(PetscSFWindowSetFlavorType(sf, flavor));
  PetscOptionsHeadEnd();
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFReset_Window(PetscSF sf) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  PetscSFDataLink link, next;
  PetscSFWinLink  wlink, wnext;
  PetscInt        i;

  PetscFunctionBegin;
  for (link = w->link; link; link = next) {
    next = link->next;
    PetscCallMPI(MPI_Type_free(&link->unit));
    for (i = 0; i < sf->nranks; i++) {
      PetscCallMPI(MPI_Type_free(&link->mine[i]));
      PetscCallMPI(MPI_Type_free(&link->remote[i]));
    }
    PetscCall(PetscFree2(link->mine, link->remote));
    PetscCall(PetscFree(link));
  }
  w->link = NULL;
  for (wlink = w->wins; wlink; wlink = wnext) {
    wnext = wlink->next;
    PetscCheck(!wlink->inuse, PetscObjectComm((PetscObject)sf), PETSC_ERR_ARG_WRONGSTATE, "Window still in use with address %p", (void *)wlink->addr);
    PetscCall(PetscFree(wlink->dyn_target_addr));
    PetscCall(PetscFree(wlink->reqs));
    PetscCallMPI(MPI_Win_free(&wlink->win));
    PetscCall(PetscFree(wlink));
  }
  w->wins = NULL;
  PetscCall(PetscSFDestroy(&w->dynsf));
  if (w->info != MPI_INFO_NULL) PetscCallMPI(MPI_Info_free(&w->info));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFDestroy_Window(PetscSF sf) {
  PetscFunctionBegin;
  PetscCall(PetscSFReset_Window(sf));
  PetscCall(PetscFree(sf->data));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetSyncType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetSyncType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetFlavorType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetFlavorType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetInfo_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetInfo_C", NULL));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFView_Window(PetscSF sf, PetscViewer viewer) {
  PetscSF_Window   *w = (PetscSF_Window *)sf->data;
  PetscBool         iascii;
  PetscViewerFormat format;

  PetscFunctionBegin;
  PetscCall(PetscViewerGetFormat(viewer, &format));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  if (iascii) {
    PetscCall(PetscViewerASCIIPrintf(viewer, "  current flavor=%s synchronization=%s MultiSF sort=%s\n", PetscSFWindowFlavorTypes[w->flavor], PetscSFWindowSyncTypes[w->sync], sf->rankorder ? "rank-order" : "unordered"));
    if (format == PETSC_VIEWER_ASCII_INFO_DETAIL) {
      if (w->info != MPI_INFO_NULL) {
        PetscMPIInt k, nkeys;
        char        key[MPI_MAX_INFO_KEY], value[MPI_MAX_INFO_VAL];

        PetscCallMPI(MPI_Info_get_nkeys(w->info, &nkeys));
        PetscCall(PetscViewerASCIIPrintf(viewer, "    current info with %d keys. Ordered key-value pairs follow:\n", nkeys));
        for (k = 0; k < nkeys; k++) {
          PetscMPIInt flag;

          PetscCallMPI(MPI_Info_get_nthkey(w->info, k, key));
          PetscCallMPI(MPI_Info_get(w->info, key, MPI_MAX_INFO_VAL, value, &flag));
          PetscCheck(flag, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Missing key %s", key);
          PetscCall(PetscViewerASCIIPrintf(viewer, "      %s = %s\n", key, value));
        }
      } else {
        PetscCall(PetscViewerASCIIPrintf(viewer, "    current info=MPI_INFO_NULL\n"));
      }
    }
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFDuplicate_Window(PetscSF sf, PetscSFDuplicateOption opt, PetscSF newsf) {
  PetscSF_Window       *w = (PetscSF_Window *)sf->data;
  PetscSFWindowSyncType synctype;

  PetscFunctionBegin;
  synctype = w->sync;
  /* HACK: Must use FENCE or LOCK when called from PetscSFGetGroups() because ACTIVE here would cause recursion. */
  if (!sf->setupcalled) synctype = PETSCSF_WINDOW_SYNC_LOCK;
  PetscCall(PetscSFWindowSetSyncType(newsf, synctype));
  PetscCall(PetscSFWindowSetFlavorType(newsf, w->flavor));
  PetscCall(PetscSFWindowSetInfo(newsf, w->info));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFBcastBegin_Window(PetscSF sf, MPI_Datatype unit, PetscMemType rootmtype, const void *rootdata, PetscMemType leafmtype, void *leafdata, MPI_Op op) {
  PetscSF_Window     *w = (PetscSF_Window *)sf->data;
  PetscInt            i, nranks;
  const PetscMPIInt  *ranks;
  const MPI_Aint     *target_disp;
  const MPI_Datatype *mine, *remote;
  MPI_Request        *reqs;
  MPI_Win             win;

  PetscFunctionBegin;
  PetscCheck(op == MPI_REPLACE, PetscObjectComm((PetscObject)sf), PETSC_ERR_SUP, "PetscSFBcastBegin_Window with op!=MPI_REPLACE has not been implemented");
  PetscCall(PetscSFGetRootRanks(sf, &nranks, &ranks, NULL, NULL, NULL));
  PetscCall(PetscSFWindowGetDataTypes(sf, unit, &mine, &remote));
  PetscCall(PetscSFGetWindow(sf, unit, (void *)rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOPUT | MPI_MODE_NOPRECEDE, MPI_MODE_NOPUT, 0, &target_disp, &reqs, &win));
  for (i = 0; i < nranks; i++) {
    MPI_Aint tdp = target_disp ? target_disp[i] : 0;

    if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) {
      PetscCallMPI(MPI_Win_lock(MPI_LOCK_SHARED, ranks[i], MPI_MODE_NOCHECK, win));
#if defined(PETSC_HAVE_MPI_RGET)
      PetscCallMPI(MPI_Rget(leafdata, 1, mine[i], ranks[i], tdp, 1, remote[i], win, &reqs[i]));
#else
      PetscCallMPI(MPI_Get(leafdata, 1, mine[i], ranks[i], tdp, 1, remote[i], win));
#endif
    } else {
      PetscCallMPI(MPI_Get(leafdata, 1, mine[i], ranks[i], tdp, 1, remote[i], win));
    }
  }
  PetscFunctionReturn(0);
}

PetscErrorCode PetscSFBcastEnd_Window(PetscSF sf, MPI_Datatype unit, const void *rootdata, void *leafdata, MPI_Op op) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  MPI_Win         win;
  MPI_Request    *reqs = NULL;

  PetscFunctionBegin;
  PetscCall(PetscSFFindWindow(sf, unit, rootdata, &win, &reqs));
  if (reqs) PetscCallMPI(MPI_Waitall(sf->nranks, reqs, MPI_STATUSES_IGNORE));
  if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) {
    PetscInt           i, nranks;
    const PetscMPIInt *ranks;

    PetscCall(PetscSFGetRootRanks(sf, &nranks, &ranks, NULL, NULL, NULL));
    for (i = 0; i < nranks; i++) PetscCallMPI(MPI_Win_unlock(ranks[i], win));
  }
  PetscCall(PetscSFRestoreWindow(sf, unit, (void *)rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOSTORE | MPI_MODE_NOSUCCEED, PETSC_FALSE, &win));
  PetscFunctionReturn(0);
}

PetscErrorCode PetscSFReduceBegin_Window(PetscSF sf, MPI_Datatype unit, PetscMemType leafmtype, const void *leafdata, PetscMemType rootmtype, void *rootdata, MPI_Op op) {
  PetscSF_Window     *w = (PetscSF_Window *)sf->data;
  PetscInt            i, nranks;
  const PetscMPIInt  *ranks;
  const MPI_Aint     *target_disp;
  const MPI_Datatype *mine, *remote;
  MPI_Win             win;

  PetscFunctionBegin;
  PetscCall(PetscSFGetRootRanks(sf, &nranks, &ranks, NULL, NULL, NULL));
  PetscCall(PetscSFWindowGetDataTypes(sf, unit, &mine, &remote));
  PetscCall(PetscSFWindowOpTranslate(&op));
  PetscCall(PetscSFGetWindow(sf, unit, rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOPRECEDE, 0, 0, &target_disp, NULL, &win));
  for (i = 0; i < nranks; i++) {
    MPI_Aint tdp = target_disp ? target_disp[i] : 0;

    if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_lock(MPI_LOCK_SHARED, ranks[i], MPI_MODE_NOCHECK, win));
    PetscCallMPI(MPI_Accumulate((void *)leafdata, 1, mine[i], ranks[i], tdp, 1, remote[i], op, win));
    if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_unlock(ranks[i], win));
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFReduceEnd_Window(PetscSF sf, MPI_Datatype unit, const void *leafdata, void *rootdata, MPI_Op op) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
  MPI_Win         win;
  MPI_Request    *reqs = NULL;

  PetscFunctionBegin;
  PetscCall(PetscSFFindWindow(sf, unit, rootdata, &win, &reqs));
  if (reqs) PetscCallMPI(MPI_Waitall(sf->nranks, reqs, MPI_STATUSES_IGNORE));
  PetscCall(PetscSFRestoreWindow(sf, unit, rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOSUCCEED, PETSC_TRUE, &win));
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFFetchAndOpBegin_Window(PetscSF sf, MPI_Datatype unit, PetscMemType rootmtype, void *rootdata, PetscMemType leafmtype, const void *leafdata, void *leafupdate, MPI_Op op) {
  PetscInt            i, nranks;
  const PetscMPIInt  *ranks;
  const MPI_Datatype *mine, *remote;
  const MPI_Aint     *target_disp;
  MPI_Win             win;
  PetscSF_Window     *w = (PetscSF_Window *)sf->data;
#if !defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
  PetscSFWindowFlavorType oldf;
#endif

  PetscFunctionBegin;
  PetscCall(PetscSFGetRootRanks(sf, &nranks, &ranks, NULL, NULL, NULL));
  PetscCall(PetscSFWindowGetDataTypes(sf, unit, &mine, &remote));
  PetscCall(PetscSFWindowOpTranslate(&op));
#if !defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
  /* FetchAndOp without MPI_Get_Accumulate requires locking.
     we create a new window every time to not interfere with user-defined MPI_Info which may have used "no_locks"="true" */
  oldf      = w->flavor;
  w->flavor = PETSCSF_WINDOW_FLAVOR_CREATE;
  PetscCall(PetscSFGetWindow(sf, unit, rootdata, PETSCSF_WINDOW_SYNC_LOCK, PETSC_FALSE, 0, 0, 0, &target_disp, NULL, &win));
#else
  PetscCall(PetscSFGetWindow(sf, unit, rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOPRECEDE, 0, 0, &target_disp, NULL, &win));
#endif
  for (i = 0; i < nranks; i++) {
    MPI_Aint tdp = target_disp ? target_disp[i] : 0;

#if !defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
    PetscCallMPI(MPI_Win_lock(MPI_LOCK_EXCLUSIVE, ranks[i], 0, win));
    PetscCallMPI(MPI_Get(leafupdate, 1, mine[i], ranks[i], tdp, 1, remote[i], win));
    PetscCallMPI(MPI_Accumulate((void *)leafdata, 1, mine[i], ranks[i], tdp, 1, remote[i], op, win));
    PetscCallMPI(MPI_Win_unlock(ranks[i], win));
#else
    if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_lock(MPI_LOCK_SHARED, ranks[i], 0, win));
    PetscCallMPI(MPI_Get_accumulate((void *)leafdata, 1, mine[i], leafupdate, 1, mine[i], ranks[i], tdp, 1, remote[i], op, win));
    if (w->sync == PETSCSF_WINDOW_SYNC_LOCK) PetscCallMPI(MPI_Win_unlock(ranks[i], win));
#endif
  }
#if !defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
  w->flavor = oldf;
#endif
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFFetchAndOpEnd_Window(PetscSF sf, MPI_Datatype unit, void *rootdata, const void *leafdata, void *leafupdate, MPI_Op op) {
  MPI_Win win;
#if defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
  PetscSF_Window *w = (PetscSF_Window *)sf->data;
#endif
  MPI_Request *reqs = NULL;

  PetscFunctionBegin;
  PetscCall(PetscSFFindWindow(sf, unit, rootdata, &win, &reqs));
  if (reqs) PetscCallMPI(MPI_Waitall(sf->nranks, reqs, MPI_STATUSES_IGNORE));
#if defined(PETSC_HAVE_MPI_GET_ACCUMULATE)
  PetscCall(PetscSFRestoreWindow(sf, unit, rootdata, w->sync, PETSC_TRUE, MPI_MODE_NOSUCCEED, PETSC_TRUE, &win));
#else
  PetscCall(PetscSFRestoreWindow(sf, unit, rootdata, PETSCSF_WINDOW_SYNC_LOCK, PETSC_FALSE, 0, PETSC_TRUE, &win));
#endif
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFCreate_Window(PetscSF sf) {
  PetscSF_Window *w = (PetscSF_Window *)sf->data;

  PetscFunctionBegin;
  sf->ops->SetUp           = PetscSFSetUp_Window;
  sf->ops->SetFromOptions  = PetscSFSetFromOptions_Window;
  sf->ops->Reset           = PetscSFReset_Window;
  sf->ops->Destroy         = PetscSFDestroy_Window;
  sf->ops->View            = PetscSFView_Window;
  sf->ops->Duplicate       = PetscSFDuplicate_Window;
  sf->ops->BcastBegin      = PetscSFBcastBegin_Window;
  sf->ops->BcastEnd        = PetscSFBcastEnd_Window;
  sf->ops->ReduceBegin     = PetscSFReduceBegin_Window;
  sf->ops->ReduceEnd       = PetscSFReduceEnd_Window;
  sf->ops->FetchAndOpBegin = PetscSFFetchAndOpBegin_Window;
  sf->ops->FetchAndOpEnd   = PetscSFFetchAndOpEnd_Window;

  PetscCall(PetscNewLog(sf, &w));
  sf->data  = (void *)w;
  w->sync   = PETSCSF_WINDOW_SYNC_FENCE;
  w->flavor = PETSCSF_WINDOW_FLAVOR_CREATE;
  w->info   = MPI_INFO_NULL;

  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetSyncType_C", PetscSFWindowSetSyncType_Window));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetSyncType_C", PetscSFWindowGetSyncType_Window));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetFlavorType_C", PetscSFWindowSetFlavorType_Window));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetFlavorType_C", PetscSFWindowGetFlavorType_Window));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowSetInfo_C", PetscSFWindowSetInfo_Window));
  PetscCall(PetscObjectComposeFunction((PetscObject)sf, "PetscSFWindowGetInfo_C", PetscSFWindowGetInfo_Window));

#if defined(OMPI_MAJOR_VERSION) && (OMPI_MAJOR_VERSION < 1 || (OMPI_MAJOR_VERSION == 1 && OMPI_MINOR_VERSION <= 6))
  {
    PetscBool ackbug = PETSC_FALSE;
    PetscCall(PetscOptionsGetBool(NULL, NULL, "-acknowledge_ompi_onesided_bug", &ackbug, NULL));
    if (ackbug) {
      PetscCall(PetscInfo(sf, "Acknowledged Open MPI bug, proceeding anyway. Expect memory corruption.\n"));
    } else SETERRQ(PetscObjectComm((PetscObject)sf), PETSC_ERR_LIB, "Open MPI is known to be buggy (https://svn.open-mpi.org/trac/ompi/ticket/1905 and 2656), use -acknowledge_ompi_onesided_bug to proceed");
  }
#endif
  PetscFunctionReturn(0);
}