xref: /petsc/src/vec/is/sf/impls/basic/allgatherv/sfallgatherv.c (revision 855db38d8495605a5e3dcabe2b88744316163d08)

#include <../src/vec/is/sf/impls/basic/allgatherv/sfallgatherv.h>

PETSC_INTERN PetscErrorCode PetscSFBcastAndOpBegin_Gatherv(PetscSF,MPI_Datatype,PetscMemType,const void*,PetscMemType,void*,MPI_Op);

/*===================================================================================*/
/*              Internal routines for PetscSFPack                                    */
/*===================================================================================*/
PETSC_INTERN PetscErrorCode PetscSFPackGet_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,const void *rootdata,PetscMemType leafmtype,const void *leafdata,PetscSFPack *mylink)
{
  PetscErrorCode         ierr;
  PetscSF_Allgatherv     *dat = (PetscSF_Allgatherv*)sf->data;
  PetscSFPack            link,*p;
  PetscBool              match;
  PetscInt               i,j;

  PetscFunctionBegin;
  ierr = PetscSFPackSetErrorOnUnsupportedOverlap(sf,unit,rootdata,leafdata);CHKERRQ(ierr);
  /* Look for types in cache */
  for (p=&dat->avail; (link=*p); p=&link->next) {
    ierr = MPIPetsc_Type_compare(unit,link->unit,&match);CHKERRQ(ierr);
    if (match) {
      *p = link->next; /* Remove from available list */
      goto found;
    }
  }

  ierr = PetscNew(&link);CHKERRQ(ierr);
  ierr = PetscSFPackSetUp_Host(sf,link,unit);CHKERRQ(ierr);

  link->rootbuflen = sf->nroots;
  link->leafbuflen = sf->nleaves;
  link->nrootreqs  = 1;
  link->nleafreqs  = 0;
  ierr = PetscMalloc1(4,&link->reqs);CHKERRQ(ierr); /* 4 = (nrootreqs+nleafreqs)*4 */
  for (i=0; i<4; i++) link->reqs[i] = MPI_REQUEST_NULL; /* Initialized to NULL so that we know which need to be freed in Destroy */

  for (i=0; i<2; i++) {
    for (j=0; j<2; j++) {
      link->rootreqs[i][j] = link->reqs + (2*i+j);
      link->leafreqs[i][j] = NULL; /* leaf requests are not needed. Make it NULL to segfault accident use */
    }
  }

  /* DO NOT allocate link->rootbuf[]/leafleaf[]. We use lazy allocation since these buffers are likely not needed */
found:
  link->rootmtype = rootmtype;
  link->leafmtype = leafmtype;
#if defined(PETSC_HAVE_CUDA)
  ierr = PetscSFPackSetUp_Device(sf,link,unit);CHKERRQ(ierr);
#endif
  link->rkey      = rootdata;
  link->lkey      = leafdata;
  link->next      = dat->inuse;
  dat->inuse      = link;

  *mylink         = link;
  PetscFunctionReturn(0);
}

/*===================================================================================*/
/*              Implementations of SF public APIs                                    */
/*===================================================================================*/

/* PetscSFGetGraph is non-collective. An implementation should not have collective calls */
PETSC_INTERN PetscErrorCode PetscSFGetGraph_Allgatherv(PetscSF sf,PetscInt *nroots,PetscInt *nleaves,const PetscInt **ilocal,const PetscSFNode **iremote)
{
  PetscErrorCode ierr;
  PetscInt       i,j,k;
  const PetscInt *range;
  PetscMPIInt    size;

  PetscFunctionBegin;
  ierr = MPI_Comm_size(PetscObjectComm((PetscObject)sf),&size);CHKERRQ(ierr);
  if (nroots)  *nroots  = sf->nroots;
  if (nleaves) *nleaves = sf->nleaves;
  if (ilocal)  *ilocal  = NULL; /* Contiguous leaves */
  if (iremote) {
    if (!sf->remote && sf->nleaves) { /* The && sf->nleaves makes sfgatherv able to inherit this routine */
      ierr = PetscLayoutGetRanges(sf->map,&range);CHKERRQ(ierr);
      ierr = PetscMalloc1(sf->nleaves,&sf->remote);CHKERRQ(ierr);
      sf->remote_alloc = sf->remote;
      for (i=0; i<size; i++) {
        for (j=range[i],k=0; j<range[i+1]; j++,k++) {
          sf->remote[j].rank  = i;
          sf->remote[j].index = k;
        }
      }
    }
    *iremote = sf->remote;
  }
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFSetUp_Allgatherv(PetscSF sf)
{
  PetscErrorCode     ierr;
  PetscSF_Allgatherv *dat = (PetscSF_Allgatherv*)sf->data;
  PetscMPIInt        size;
  PetscInt           i;
  const PetscInt     *range;

  PetscFunctionBegin;
  ierr = MPI_Comm_size(PetscObjectComm((PetscObject)sf),&size);CHKERRQ(ierr);
  if (sf->nleaves) { /* This if (sf->nleaves) test makes sfgatherv able to inherit this routine */
    ierr = PetscMalloc1(size,&dat->recvcounts);CHKERRQ(ierr);
    ierr = PetscMalloc1(size,&dat->displs);CHKERRQ(ierr);
    ierr = PetscLayoutGetRanges(sf->map,&range);CHKERRQ(ierr);

    for (i=0; i<size; i++) {
      ierr = PetscMPIIntCast(range[i],&dat->displs[i]);CHKERRQ(ierr);
      ierr = PetscMPIIntCast(range[i+1]-range[i],&dat->recvcounts[i]);CHKERRQ(ierr);
    }
  }
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFReset_Allgatherv(PetscSF sf)
{
  PetscErrorCode         ierr;
  PetscSF_Allgatherv     *dat = (PetscSF_Allgatherv*)sf->data;

  PetscFunctionBegin;
  ierr = PetscFree(dat->iranks);CHKERRQ(ierr);
  ierr = PetscFree(dat->ioffset);CHKERRQ(ierr);
  ierr = PetscFree(dat->irootloc);CHKERRQ(ierr);
  ierr = PetscFree(dat->recvcounts);CHKERRQ(ierr);
  ierr = PetscFree(dat->displs);CHKERRQ(ierr);
  if (dat->inuse) SETERRQ(PetscObjectComm((PetscObject)sf),PETSC_ERR_ARG_WRONGSTATE,"Outstanding operation has not been completed");
  ierr = PetscSFPackDestroyAvailable(&dat->avail);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFDestroy_Allgatherv(PetscSF sf)
{
  PetscErrorCode ierr;

  PetscFunctionBegin;
  ierr = PetscSFReset_Allgatherv(sf);CHKERRQ(ierr);
  ierr = PetscFree(sf->data);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*
  Prepare the rootbuf, leafbuf etc used by MPI in PetscSFBcastAndOpBegin.

Input Arguments:
+ sf    - the start forest
. link  - the link PetscSFBcastAndOp is currently using
- op    - the reduction op

Output Arguments:
+rootmtype_mpi  - memtype of rootbuf_mpi
.rootbuf_mpi    - root buffer used by MPI in the following MPI call
.leafmtype_mpi  - memtype of leafbuf_mpi
-leafbuf_mpi    - leaf buffer used by MPI in the following MPI call

Notes:
  This function was created because things became complex when rootdata or leafdata is on device, but the user does not want to use GPU-aware MPI.
  We have to copy data from device to host before doing MPI. This function encapsulates all varieties and is reused by Allgatherv & Allgahter.
*/
PETSC_INTERN PetscErrorCode PetscSFBcastPrepareMPIBuffers_Allgatherv(PetscSF sf,PetscSFPack link,MPI_Op op,PetscMemType *rootmtype_mpi,const void **rootbuf_mpi,PetscMemType *leafmtype_mpi, void **leafbuf_mpi)
{
  PetscErrorCode         ierr;

  PetscFunctionBegin;
  /* If rootdata is on device but no gpu-aware mpi, we need to copy rootdata to rootbuf on host before bcast; otherwise we directly bcast from leafdata */
  if (link->rootmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) {
    if (!link->rootbuf[PETSC_MEMTYPE_HOST]) {ierr = PetscMallocWithMemType(PETSC_MEMTYPE_HOST,link->rootbuflen*link->unitbytes,(void**)&link->rootbuf[PETSC_MEMTYPE_HOST]);CHKERRQ(ierr);}
    ierr           = PetscMemcpyWithMemType(PETSC_MEMTYPE_HOST,PETSC_MEMTYPE_DEVICE,link->rootbuf[PETSC_MEMTYPE_HOST],link->rkey,link->rootbuflen*link->unitbytes);CHKERRQ(ierr);
    *rootbuf_mpi   = link->rootbuf[PETSC_MEMTYPE_HOST];
    *rootmtype_mpi = PETSC_MEMTYPE_HOST;
  } else {
    *rootbuf_mpi   = link->rkey;
    *rootmtype_mpi = link->rootmtype;
  }

  if (link->leafmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) {  /* If leafdata is on device but no gpu-aware mpi, we need a leafbuf on host to receive bcast'ed data */
    if (!link->leafbuf[PETSC_MEMTYPE_HOST]) {ierr = PetscMallocWithMemType(PETSC_MEMTYPE_HOST,link->leafbuflen*link->unitbytes,(void**)&link->leafbuf[PETSC_MEMTYPE_HOST]);CHKERRQ(ierr);}
    *leafbuf_mpi   = link->leafbuf[PETSC_MEMTYPE_HOST];
    *leafmtype_mpi = PETSC_MEMTYPE_HOST;
  } else if (op == MPIU_REPLACE) { /* If op is MPIU_REPLACE, we can directly bcast to leafdata. No intermediate buffer is needed. */
    *leafbuf_mpi   = (char *)link->lkey;
    *leafmtype_mpi = link->leafmtype;
  } else { /* Otherwise, op is a reduction. Have to allocate a buffer aside leafdata to apply the op. The buffer is either on host or device, depending on where leafdata is. */
    if (!link->leafbuf[link->leafmtype]) {ierr = PetscMallocWithMemType(link->leafmtype,link->leafbuflen*link->unitbytes,(void**)&link->leafbuf[link->leafmtype]);CHKERRQ(ierr);}
    *leafbuf_mpi   = link->leafbuf[link->leafmtype];
    *leafmtype_mpi = link->leafmtype;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFBcastAndOpBegin_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,const void *rootdata,PetscMemType leafmtype,void *leafdata,MPI_Op op)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;
  PetscMPIInt            sendcount;
  MPI_Comm               comm;
  const void             *rootbuf_mpi; /* buffer used by MPI */
  void                   *leafbuf_mpi;
  PetscMemType           rootmtype_mpi,leafmtype_mpi; /* Seen by MPI */
  PetscSF_Allgatherv     *dat = (PetscSF_Allgatherv*)sf->data;

  PetscFunctionBegin;
  ierr = PetscSFPackGet_Allgatherv(sf,unit,rootmtype,rootdata,leafmtype,leafdata,&link);CHKERRQ(ierr);
  ierr = PetscObjectGetComm((PetscObject)sf,&comm);CHKERRQ(ierr);
  ierr = PetscMPIIntCast(sf->nroots,&sendcount);CHKERRQ(ierr);
  ierr = PetscSFBcastPrepareMPIBuffers_Allgatherv(sf,link,op,&rootmtype_mpi,&rootbuf_mpi,&leafmtype_mpi,&leafbuf_mpi);CHKERRQ(ierr);
  ierr = MPIU_Iallgatherv(rootbuf_mpi,sendcount,unit,leafbuf_mpi,dat->recvcounts,dat->displs,unit,comm,link->rootreqs[PETSCSF_ROOT2LEAF_BCAST][rootmtype_mpi]);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFBcastAndOpEnd_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,const void *rootdata,PetscMemType leafmtype,void *leafdata,MPI_Op op)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;

  PetscFunctionBegin;
  ierr = PetscSFPackGetInUse(sf,unit,rootdata,leafdata,PETSC_OWN_POINTER,&link);CHKERRQ(ierr);
  ierr = PetscSFPackWaitall(link,PETSCSF_ROOT2LEAF_BCAST);CHKERRQ(ierr);
  if (op != MPIU_REPLACE) {
    /* Have a leaf buffer aside leafdata to do Op */
    ierr = PetscSFUnpackAndOpLeafData(sf,link,NULL,leafdata,op,PETSC_FALSE);CHKERRQ(ierr);
  } else if (leafmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) {
    /* Just need to copy data in leafbuf on host to leafdata on device */
    ierr = PetscMemcpyWithMemType(PETSC_MEMTYPE_DEVICE,PETSC_MEMTYPE_HOST,leafdata,link->leafbuf[PETSC_MEMTYPE_HOST],link->leafbuflen*link->unitbytes);CHKERRQ(ierr);
  }
  ierr = PetscSFPackReclaim(sf,&link);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/*
  Prepare the rootbuf, leafbuf etc used by MPI in PetscSFReduceBegin.

Input Arguments:
+ sf    - the start forest
. link  - the link PetscSFReduceBegin is currently using
- op    - the reduction op

Output Arguments:
+rootmtype_mpi  - memtype of rootbuf_mpi
.rootbuf_mpi    - root buffer used by MPI in the following MPI call
.leafmtype_mpi  - memtype of leafbuf_mpi
-leafbuf_mpi    - leaf buffer used by MPI in the following MPI call

Notes: This function is called assuming op != MPIU_REPLACE.
*/
PETSC_INTERN PetscErrorCode PetscSFReducePrepareMPIBuffers_Allgatherv(PetscSF sf,PetscSFPack link,MPI_Op op,PetscMemType *rootmtype_mpi,void **rootbuf_mpi,PetscMemType *leafmtype_mpi,const void **leafbuf_mpi)
{
  PetscErrorCode         ierr;
  PetscMPIInt            rank,count;
  MPI_Comm               comm;
  const void             *leafdata_mpi;

  PetscFunctionBegin;
  ierr = PetscObjectGetComm((PetscObject)sf,&comm);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);

  /* Step 1: Reduce leafdata on all ranks to leafbuf on rank 0 */
  if (link->leafmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) { /* Need to copy leafdata to leafbuf on every rank */
    if (!link->leafbuf[PETSC_MEMTYPE_HOST]) {ierr = PetscMallocWithMemType(PETSC_MEMTYPE_HOST,link->leafbuflen*link->unitbytes,(void**)&link->leafbuf[PETSC_MEMTYPE_HOST]);CHKERRQ(ierr);}
    ierr = PetscMemcpyWithMemType(PETSC_MEMTYPE_HOST,PETSC_MEMTYPE_DEVICE,link->leafbuf[PETSC_MEMTYPE_HOST],link->lkey,link->leafbuflen*link->unitbytes);CHKERRQ(ierr);
    leafdata_mpi   = !rank ? MPI_IN_PLACE : link->leafbuf[PETSC_MEMTYPE_HOST];
    *leafmtype_mpi = PETSC_MEMTYPE_HOST;
  } else { /* Only need to allocate a leafbuf on rank 0. Then directly reduce leafdata to the leafbuf */
    if (!rank && !link->leafbuf[link->leafmtype]) {ierr = PetscMallocWithMemType(link->leafmtype,link->leafbuflen*link->unitbytes,(void**)&link->leafbuf[link->leafmtype]);CHKERRQ(ierr);}
    leafdata_mpi   = link->lkey;
    *leafmtype_mpi = link->leafmtype;
  }
  *leafbuf_mpi = (const char*)link->leafbuf[*leafmtype_mpi];
  ierr = PetscMPIIntCast(sf->nleaves*link->bs,&count);CHKERRQ(ierr);
  ierr = MPI_Reduce(leafdata_mpi,(void*)(*leafbuf_mpi),count,link->basicunit,op,0,comm);CHKERRQ(ierr); /* Must do reduce with MPI builltin datatype basicunit */

  /* Step 2: Prepare the root buffer (we'll scatter the reduction result to it in a moment) */
  if (link->rootmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) *rootmtype_mpi = PETSC_MEMTYPE_HOST;
  else *rootmtype_mpi = link->rootmtype;

  if (!link->rootbuf[*rootmtype_mpi]) {ierr = PetscMallocWithMemType(*rootmtype_mpi,link->rootbuflen*link->unitbytes,(void**)&link->rootbuf[*rootmtype_mpi]);CHKERRQ(ierr);}
  *rootbuf_mpi = link->rootbuf[*rootmtype_mpi];
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFReduceBegin_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType leafmtype,const void *leafdata,PetscMemType rootmtype,void *rootdata,MPI_Op op)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;
  PetscSF_Allgatherv     *dat = (PetscSF_Allgatherv*)sf->data;
  PetscInt               rstart;
  PetscMPIInt            rank,recvcount;
  MPI_Comm               comm;
  const void             *leafbuf_mpi;
  void                   *rootbuf_mpi;
  PetscMemType           leafmtype_mpi,rootmtype_mpi; /* Seen by MPI */

  PetscFunctionBegin;
  ierr = PetscSFPackGet_Allgatherv(sf,unit,rootmtype,rootdata,leafmtype,leafdata,&link);CHKERRQ(ierr);
  ierr = PetscObjectGetComm((PetscObject)sf,&comm);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);

  if (op == MPIU_REPLACE) {
    /* REPLACE is only meaningful when all processes have the same leafdata to reduce. Therefore copy from local leafdata is fine */
    ierr = PetscLayoutGetRange(sf->map,&rstart,NULL);CHKERRQ(ierr);
    ierr = PetscMemcpyWithMemType(rootmtype,leafmtype,rootdata,(const char*)leafdata+(size_t)rstart*link->unitbytes,(size_t)sf->nroots*link->unitbytes);CHKERRQ(ierr);
  } else {
    ierr = PetscMPIIntCast(sf->nroots,&recvcount);CHKERRQ(ierr);
    ierr = PetscSFReducePrepareMPIBuffers_Allgatherv(sf,link,op,&rootmtype_mpi,&rootbuf_mpi,&leafmtype_mpi,&leafbuf_mpi);CHKERRQ(ierr);
    ierr = MPIU_Iscatterv(leafbuf_mpi,dat->recvcounts,dat->displs,unit,rootbuf_mpi,recvcount,unit,0,comm,link->rootreqs[PETSCSF_LEAF2ROOT_REDUCE][rootmtype_mpi]);CHKERRQ(ierr);
  }
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFReduceEnd_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType leafmtype,const void *leafdata,PetscMemType rootmtype,void *rootdata,MPI_Op op)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;

  PetscFunctionBegin;
  ierr = PetscSFPackGetInUse(sf,unit,rootdata,leafdata,PETSC_OWN_POINTER,&link);CHKERRQ(ierr);
  ierr = PetscSFPackWaitall(link,PETSCSF_LEAF2ROOT_REDUCE);CHKERRQ(ierr);
  if (op != MPIU_REPLACE) {
    ierr = PetscSFUnpackAndOpRootData(sf,link,NULL,rootdata,op,PETSC_FALSE);CHKERRQ(ierr);
  } else if (rootmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) {
    ierr = PetscMemcpyWithMemType(PETSC_MEMTYPE_DEVICE,PETSC_MEMTYPE_HOST,rootdata,link->rootbuf[PETSC_MEMTYPE_HOST],link->rootbuflen*link->unitbytes);CHKERRQ(ierr);
  }
  ierr = PetscSFPackReclaim(sf,&link);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

static PetscErrorCode PetscSFBcastToZero_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,const void *rootdata,PetscMemType leafmtype,void *leafdata)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;
  PetscMPIInt            rank;

  PetscFunctionBegin;
  ierr = PetscSFBcastAndOpBegin_Gatherv(sf,unit,rootmtype,rootdata,leafmtype,leafdata,MPIU_REPLACE);CHKERRQ(ierr);
  ierr = PetscSFPackGetInUse(sf,unit,rootdata,leafdata,PETSC_OWN_POINTER,&link);CHKERRQ(ierr);
  ierr = PetscSFPackWaitall(link,PETSCSF_ROOT2LEAF_BCAST);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(PetscObjectComm((PetscObject)sf),&rank);CHKERRQ(ierr);
  if (!rank && leafmtype == PETSC_MEMTYPE_DEVICE && !use_gpu_aware_mpi) {
    ierr = PetscMemcpyWithMemType(PETSC_MEMTYPE_DEVICE,PETSC_MEMTYPE_HOST,leafdata,link->leafbuf[PETSC_MEMTYPE_HOST],link->leafbuflen*link->unitbytes);CHKERRQ(ierr);
  }
  ierr = PetscSFPackReclaim(sf,&link);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/* This routine is very tricky (I believe it is rarely used with this kind of graph so just provide a simple but not-optimal implementation).

   Suppose we have three ranks. Rank 0 has a root with value 1. Rank 0,1,2 has a leaf with value 2,3,4 respectively. The leaves are connected
   to the root on rank 0. Suppose op=MPI_SUM and rank 0,1,2 gets root state in their rank order. By definition of this routine, rank 0 sees 1
   in root, fetches it into its leafupate, then updates root to 1 + 2 = 3; rank 1 sees 3 in root, fetches it into its leafupate, then updates
   root to 3 + 3 = 6; rank 2 sees 6 in root, fetches it into its leafupdate, then updates root to 6 + 4 = 10.  At the end, leafupdate on rank
   0,1,2 is 1,3,6 respectively. root is 10.

   One optimized implementation could be: starting from the initial state:
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4

   Shift leaves rightwards to leafupdate. Rank 0 gathers the root value and puts it in leafupdate. We have:
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  1       2         3

   Then, do MPI_Scan on leafupdate and get:
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  1       3         6

   Rank 2 sums its leaf and leafupdate, scatters the result to the root, and gets
             rank-0   rank-1    rank-2
        Root     10
        Leaf     2       3         4
     Leafupdate  1       3         6

   We use a simpler implementation. From the same initial state, we copy leafdata to leafupdate
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  2       3         4

   Do MPI_Exscan on leafupdate,
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  2       2         5

   BcastAndOp from root to leafupdate,
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  3       3         6

   Copy root to leafupdate on rank-0
             rank-0   rank-1    rank-2
        Root     1
        Leaf     2       3         4
     Leafupdate  1       3         6

   Reduce from leaf to root,
             rank-0   rank-1    rank-2
        Root     10
        Leaf     2       3         4
     Leafupdate  1       3         6
*/
PETSC_INTERN PetscErrorCode PetscSFFetchAndOpBegin_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,void *rootdata,PetscMemType leafmtype,const void *leafdata,void *leafupdate,MPI_Op op)
{
  PetscErrorCode         ierr;
  PetscSFPack            link;
  MPI_Comm               comm;
  PetscMPIInt            count;

  PetscFunctionBegin;
  ierr = PetscObjectGetComm((PetscObject)sf,&comm);CHKERRQ(ierr);
  if (!use_gpu_aware_mpi && (rootmtype == PETSC_MEMTYPE_DEVICE || leafmtype == PETSC_MEMTYPE_DEVICE)) SETERRQ(comm,PETSC_ERR_SUP,"No support for FetchAndOp"); /* No known uses */
  /* Copy leafdata to leafupdate */
  ierr = PetscSFPackGet_Allgatherv(sf,unit,rootmtype,rootdata,leafmtype,leafdata,&link);CHKERRQ(ierr);
  ierr = PetscMemcpyWithMemType(leafmtype,leafmtype,leafupdate,leafdata,sf->nleaves*link->unitbytes);CHKERRQ(ierr);
  ierr = PetscSFPackGetInUse(sf,unit,rootdata,leafdata,PETSC_OWN_POINTER,&link);CHKERRQ(ierr);

  /* Exscan on leafupdate and then BcastAndOp rootdata to leafupdate */
  ierr = PetscMPIIntCast(sf->nleaves,&count);CHKERRQ(ierr);
  if (op == MPIU_REPLACE) {
    PetscMPIInt size,rank,prev,next;
    ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);
    ierr = MPI_Comm_size(comm,&size);CHKERRQ(ierr);
    prev = rank ?            rank-1 : MPI_PROC_NULL;
    next = (rank < size-1) ? rank+1 : MPI_PROC_NULL;
    ierr = MPI_Sendrecv_replace(leafupdate,count,unit,next,link->tag,prev,link->tag,comm,MPI_STATUSES_IGNORE);CHKERRQ(ierr);
  } else {ierr = MPI_Exscan(MPI_IN_PLACE,leafupdate,count,unit,op,comm);CHKERRQ(ierr);}
  ierr = PetscSFPackReclaim(sf,&link);CHKERRQ(ierr);
  ierr = PetscSFBcastAndOpBegin(sf,unit,rootdata,leafupdate,op);CHKERRQ(ierr);
  ierr = PetscSFBcastAndOpEnd(sf,unit,rootdata,leafupdate,op);CHKERRQ(ierr);

  /* Bcast roots to rank 0's leafupdate */
  ierr = PetscSFBcastToZero_Private(sf,unit,rootdata,leafupdate);CHKERRQ(ierr); /* Using this line makes Allgather SFs able to inherit this routine */

  /* Reduce leafdata to rootdata */
  ierr = PetscSFReduceBegin(sf,unit,leafdata,rootdata,op);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFFetchAndOpEnd_Allgatherv(PetscSF sf,MPI_Datatype unit,PetscMemType rootmtype,void *rootdata,PetscMemType leafmtype,const void *leafdata,void *leafupdate,MPI_Op op)
{
  PetscErrorCode         ierr;

  PetscFunctionBegin;
  ierr = PetscSFReduceEnd(sf,unit,leafdata,rootdata,op);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

/* Get root ranks accessing my leaves */
PETSC_INTERN PetscErrorCode PetscSFGetRootRanks_Allgatherv(PetscSF sf,PetscInt *nranks,const PetscMPIInt **ranks,const PetscInt **roffset,const PetscInt **rmine,const PetscInt **rremote)
{
  PetscErrorCode ierr;
  PetscInt       i,j,k,size;
  const PetscInt *range;

  PetscFunctionBegin;
  /* Lazily construct these large arrays if users really need them for this type of SF. Very likely, they do not */
  if (sf->nranks && !sf->ranks) { /* On rank!=0, sf->nranks=0. The sf->nranks test makes this routine also works for sfgatherv */
    size = sf->nranks;
    ierr = PetscLayoutGetRanges(sf->map,&range);CHKERRQ(ierr);
    ierr = PetscMalloc4(size,&sf->ranks,size+1,&sf->roffset,sf->nleaves,&sf->rmine,sf->nleaves,&sf->rremote);CHKERRQ(ierr);
    for (i=0; i<size; i++) sf->ranks[i] = i;
    ierr = PetscArraycpy(sf->roffset,range,size+1);CHKERRQ(ierr);
    for (i=0; i<sf->nleaves; i++) sf->rmine[i] = i; /*rmine are never NULL even for contiguous leaves */
    for (i=0; i<size; i++) {
      for (j=range[i],k=0; j<range[i+1]; j++,k++) sf->rremote[j] = k;
    }
  }

  if (nranks)  *nranks  = sf->nranks;
  if (ranks)   *ranks   = sf->ranks;
  if (roffset) *roffset = sf->roffset;
  if (rmine)   *rmine   = sf->rmine;
  if (rremote) *rremote = sf->rremote;
  PetscFunctionReturn(0);
}

/* Get leaf ranks accessing my roots */
PETSC_INTERN PetscErrorCode PetscSFGetLeafRanks_Allgatherv(PetscSF sf,PetscInt *niranks,const PetscMPIInt **iranks,const PetscInt **ioffset,const PetscInt **irootloc)
{
  PetscErrorCode     ierr;
  PetscSF_Allgatherv *dat = (PetscSF_Allgatherv*)sf->data;
  MPI_Comm           comm;
  PetscMPIInt        size,rank;
  PetscInt           i,j;

  PetscFunctionBegin;
  /* Lazily construct these large arrays if users really need them for this type of SF. Very likely, they do not */
  ierr = PetscObjectGetComm((PetscObject)sf,&comm);CHKERRQ(ierr);
  ierr = MPI_Comm_size(comm,&size);CHKERRQ(ierr);
  ierr = MPI_Comm_rank(comm,&rank);CHKERRQ(ierr);
  if (niranks) *niranks = size;

  /* PetscSF_Basic has distinguished incoming ranks. Here we do not need that. But we must put self as the first and
     sort other ranks. See comments in PetscSFSetUp_Basic about MatGetBrowsOfAoCols_MPIAIJ on why.
   */
  if (iranks) {
    if (!dat->iranks) {
      ierr = PetscMalloc1(size,&dat->iranks);CHKERRQ(ierr);
      dat->iranks[0] = rank;
      for (i=0,j=1; i<size; i++) {if (i == rank) continue; dat->iranks[j++] = i;}
    }
    *iranks = dat->iranks; /* dat->iranks was init'ed to NULL by PetscNewLog */
  }

  if (ioffset) {
    if (!dat->ioffset) {
      ierr = PetscMalloc1(size+1,&dat->ioffset);CHKERRQ(ierr);
      for (i=0; i<=size; i++) dat->ioffset[i] = i*sf->nroots;
    }
    *ioffset = dat->ioffset;
  }

  if (irootloc) {
    if (!dat->irootloc) {
      ierr = PetscMalloc1(sf->nleaves,&dat->irootloc);CHKERRQ(ierr);
      for (i=0; i<size; i++) {
        for (j=0; j<sf->nroots; j++) dat->irootloc[i*sf->nroots+j] = j;
      }
    }
    *irootloc = dat->irootloc;
  }
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFCreateLocalSF_Allgatherv(PetscSF sf,PetscSF *out)
{
  PetscInt       i,nroots,nleaves,rstart,*ilocal;
  PetscSFNode    *iremote;
  PetscSF        lsf;
  PetscErrorCode ierr;

  PetscFunctionBegin;
  nleaves = sf->nleaves ? sf->nroots : 0; /* sf->nleaves can be zero with SFGather(v) */
  nroots  = nleaves;
  ierr    = PetscMalloc1(nleaves,&ilocal);CHKERRQ(ierr);
  ierr    = PetscMalloc1(nleaves,&iremote);CHKERRQ(ierr);
  ierr    = PetscLayoutGetRange(sf->map,&rstart,NULL);CHKERRQ(ierr);

  for (i=0; i<nleaves; i++) {
    ilocal[i]        = rstart + i; /* lsf does not change leave indices */
    iremote[i].rank  = 0;          /* rank in PETSC_COMM_SELF */
    iremote[i].index = i;          /* root index */
  }

  ierr = PetscSFCreate(PETSC_COMM_SELF,&lsf);CHKERRQ(ierr);
  ierr = PetscSFSetGraph(lsf,nroots,nleaves,ilocal,PETSC_OWN_POINTER,iremote,PETSC_OWN_POINTER);CHKERRQ(ierr);
  ierr = PetscSFSetUp(lsf);CHKERRQ(ierr);
  *out = lsf;
  PetscFunctionReturn(0);
}

PETSC_INTERN PetscErrorCode PetscSFCreate_Allgatherv(PetscSF sf)
{
  PetscErrorCode     ierr;
  PetscSF_Allgatherv *dat = (PetscSF_Allgatherv*)sf->data;

  PetscFunctionBegin;
  sf->ops->SetUp           = PetscSFSetUp_Allgatherv;
  sf->ops->Reset           = PetscSFReset_Allgatherv;
  sf->ops->Destroy         = PetscSFDestroy_Allgatherv;
  sf->ops->GetRootRanks    = PetscSFGetRootRanks_Allgatherv;
  sf->ops->GetLeafRanks    = PetscSFGetLeafRanks_Allgatherv;
  sf->ops->GetGraph        = PetscSFGetGraph_Allgatherv;
  sf->ops->BcastAndOpBegin = PetscSFBcastAndOpBegin_Allgatherv;
  sf->ops->BcastAndOpEnd   = PetscSFBcastAndOpEnd_Allgatherv;
  sf->ops->ReduceBegin     = PetscSFReduceBegin_Allgatherv;
  sf->ops->ReduceEnd       = PetscSFReduceEnd_Allgatherv;
  sf->ops->FetchAndOpBegin = PetscSFFetchAndOpBegin_Allgatherv;
  sf->ops->FetchAndOpEnd   = PetscSFFetchAndOpEnd_Allgatherv;
  sf->ops->CreateLocalSF   = PetscSFCreateLocalSF_Allgatherv;
  sf->ops->BcastToZero     = PetscSFBcastToZero_Allgatherv;

  ierr = PetscNewLog(sf,&dat);CHKERRQ(ierr);
  sf->data = (void*)dat;
  PetscFunctionReturn(0);
}