openmpi/ompi/mca/coll/base/coll_base_allgather.c

/*
 * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
 *                         University Research and Technology
 *                         Corporation.  All rights reserved.
 * Copyright (c) 2004-2015 The University of Tennessee and The University
 *                         of Tennessee Research Foundation.  All rights
 *                         reserved.
 * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
 *                         University of Stuttgart.  All rights reserved.
 * Copyright (c) 2004-2005 The Regents of the University of California.
 *                         All rights reserved.
 * Copyright (c) 2009      University of Houston. All rights reserved.
 * Copyright (c) 2013      Los Alamos National Security, LLC. All Rights
 *                         reserved.
 * Copyright (c) 2014      Research Organization for Information Science
 *                         and Technology (RIST). All rights reserved.
 * $COPYRIGHT$
 *
 * Additional copyrights may follow
 *
 * $HEADER$
 */

#include "ompi_config.h"

#include "mpi.h"
#include "opal/util/bit_ops.h"
#include "ompi/constants.h"
#include "ompi/datatype/ompi_datatype.h"
#include "ompi/communicator/communicator.h"
#include "ompi/mca/coll/coll.h"
#include "ompi/mca/coll/base/coll_tags.h"
#include "ompi/mca/coll/base/coll_base_functions.h"
#include "coll_base_topo.h"
#include "coll_base_util.h"

/*
 * ompi_coll_base_allgather_intra_bruck
 *
 * Function:     allgather using O(log(N)) steps.
 * Accepts:      Same arguments as MPI_Allgather
 * Returns:      MPI_SUCCESS or error code
 *
 * Description:  Variation to All-to-all algorithm described by Bruck et al.in
 *               "Efficient Algorithms for All-to-all Communications
 *                in Multiport Message-Passing Systems"
 * Memory requirements:  non-zero ranks require shift buffer to perform final
 *               step in the algorithm.
 *
 * Example on 6 nodes:
 *   Initialization: everyone has its own buffer at location 0 in rbuf
 *                   This means if user specified MPI_IN_PLACE for sendbuf
 *                   we must copy our block from recvbuf to begining!
 *    #     0      1      2      3      4      5
 *         [0]    [1]    [2]    [3]    [4]    [5]
 *   Step 0: send message to (rank - 2^0), receive message from (rank + 2^0)
 *    #     0      1      2      3      4      5
 *         [0]    [1]    [2]    [3]    [4]    [5]
 *         [1]    [2]    [3]    [4]    [5]    [0]
 *   Step 1: send message to (rank - 2^1), receive message from (rank + 2^1)
 *           message contains all blocks from location 0 to 2^1*block size
 *    #     0      1      2      3      4      5
 *         [0]    [1]    [2]    [3]    [4]    [5]
 *         [1]    [2]    [3]    [4]    [5]    [0]
 *         [2]    [3]    [4]    [5]    [0]    [1]
 *         [3]    [4]    [5]    [0]    [1]    [2]
 *   Step 2: send message to (rank - 2^2), receive message from (rank + 2^2)
 *           message size is "all remaining blocks"
 *    #     0      1      2      3      4      5
 *         [0]    [1]    [2]    [3]    [4]    [5]
 *         [1]    [2]    [3]    [4]    [5]    [0]
 *         [2]    [3]    [4]    [5]    [0]    [1]
 *         [3]    [4]    [5]    [0]    [1]    [2]
 *         [4]    [5]    [0]    [1]    [2]    [3]
 *         [5]    [0]    [1]    [2]    [3]    [4]
 *    Finalization: Do a local shift to get data in correct place
 *    #     0      1      2      3      4      5
 *         [0]    [0]    [0]    [0]    [0]    [0]
 *         [1]    [1]    [1]    [1]    [1]    [1]
 *         [2]    [2]    [2]    [2]    [2]    [2]
 *         [3]    [3]    [3]    [3]    [3]    [3]
 *         [4]    [4]    [4]    [4]    [4]    [4]
 *         [5]    [5]    [5]    [5]    [5]    [5]
 */
int ompi_coll_base_allgather_intra_bruck(void *sbuf, int scount,
                                          struct ompi_datatype_t *sdtype,
                                          void* rbuf, int rcount,
                                          struct ompi_datatype_t *rdtype,
                                          struct ompi_communicator_t *comm,
                                          mca_coll_base_module_t *module)
{
    int line = -1, rank, size, sendto, recvfrom, distance, blockcount, err = 0;
    ptrdiff_t slb, rlb, sext, rext;
    char *tmpsend = NULL, *tmprecv = NULL;

    size = ompi_comm_size(comm);
    rank = ompi_comm_rank(comm);

    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                 "coll:base:allgather_intra_bruck rank %d", rank));

    err = ompi_datatype_get_extent (sdtype, &slb, &sext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    err = ompi_datatype_get_extent (rdtype, &rlb, &rext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Initialization step:
       - if send buffer is not MPI_IN_PLACE, copy send buffer to block 0 of
       receive buffer, else
       - if rank r != 0, copy r^th block from receive buffer to block 0.
    */
    tmprecv = (char*) rbuf;
    if (MPI_IN_PLACE != sbuf) {
        tmpsend = (char*) sbuf;
        err = ompi_datatype_sndrcv(tmpsend, scount, sdtype, tmprecv, rcount, rdtype);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl;  }

    } else if (0 != rank) {  /* non root with MPI_IN_PLACE */
        tmpsend = ((char*)rbuf) + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
        err = ompi_datatype_copy_content_same_ddt(rdtype, rcount, tmprecv, tmpsend);
        if (err < 0) { line = __LINE__; goto err_hndl; }
    }

    /* Communication step:
       At every step i, rank r:
       - doubles the distance
       - sends message which starts at begining of rbuf and has size
       (blockcount * rcount) to rank (r - distance)
       - receives message of size blockcount * rcount from rank (r + distance)
       at location (rbuf + distance * rcount * rext)
       - blockcount doubles until last step when only the remaining data is
       exchanged.
    */
    blockcount = 1;
    tmpsend = (char*) rbuf;
    for (distance = 1; distance < size; distance<<=1) {

        recvfrom = (rank + distance) % size;
        sendto = (rank - distance + size) % size;

        tmprecv = tmpsend + (ptrdiff_t)distance * (ptrdiff_t)rcount * rext;

        if (distance <= (size >> 1)) {
            blockcount = distance;
        } else {
            blockcount = size - distance;
        }

        /* Sendreceive */
        err = ompi_coll_base_sendrecv(tmpsend, blockcount * rcount, rdtype,
                                       sendto, MCA_COLL_BASE_TAG_ALLGATHER,
                                       tmprecv, blockcount * rcount, rdtype,
                                       recvfrom, MCA_COLL_BASE_TAG_ALLGATHER,
                                       comm, MPI_STATUS_IGNORE, rank);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    }

    /* Finalization step:
       On all nodes except 0, data needs to be shifted locally:
       - create temporary shift buffer,
       see discussion in coll_basic_reduce.c about the size and begining
       of temporary buffer.
       - copy blocks [0 .. (size - rank - 1)] from rbuf to shift buffer
       - move blocks [(size - rank) .. size] from rbuf to begining of rbuf
       - copy blocks from shift buffer starting at block [rank] in rbuf.
    */
    if (0 != rank) {
        ptrdiff_t true_extent, true_lb;
        char *free_buf = NULL, *shift_buf = NULL;

        err = ompi_datatype_get_true_extent(rdtype, &true_lb, &true_extent);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

        free_buf = (char*) calloc(((true_extent +
                                    ((ptrdiff_t)(size - rank) * (ptrdiff_t)rcount - 1) * rext)),
                                  sizeof(char));
        if (NULL == free_buf) {
            line = __LINE__; err = OMPI_ERR_OUT_OF_RESOURCE; goto err_hndl;
        }
        shift_buf = free_buf - true_lb;

        /* 1. copy blocks [0 .. (size - rank - 1)] from rbuf to shift buffer */
        err = ompi_datatype_copy_content_same_ddt(rdtype, ((ptrdiff_t)(size - rank) * (ptrdiff_t)rcount),
                                                  shift_buf, rbuf);
        if (err < 0) { line = __LINE__; goto err_hndl;  }

        /* 2. move blocks [(size - rank) .. size] from rbuf to the begining of rbuf */
        tmpsend = (char*) rbuf + (ptrdiff_t)(size - rank) * (ptrdiff_t)rcount * rext;
        err = ompi_datatype_copy_content_same_ddt(rdtype, (ptrdiff_t)rank * (ptrdiff_t)rcount,
                                                  rbuf, tmpsend);
        if (err < 0) { line = __LINE__; goto err_hndl;  }

        /* 3. copy blocks from shift buffer back to rbuf starting at block [rank]. */
        tmprecv = (char*) rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
        err = ompi_datatype_copy_content_same_ddt(rdtype, (ptrdiff_t)(size - rank) * (ptrdiff_t)rcount,
                                                  tmprecv, shift_buf);
        if (err < 0) { line = __LINE__; goto err_hndl;  }

        free(free_buf);
    }

    return OMPI_SUCCESS;

 err_hndl:
    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,  "%s:%4d\tError occurred %d, rank %2d",
                 __FILE__, line, err, rank));
    return err;
}

/*
 * ompi_coll_base_allgather_intra_recursivedoubling
 *
 * Function:     allgather using O(log(N)) steps.
 * Accepts:      Same arguments as MPI_Allgather
 * Returns:      MPI_SUCCESS or error code
 *
 * Description:  Recursive doubling algorithm for MPI_Allgather implementation.
 *               This algorithm is used in MPICH-2 for small- and medium-sized
 *               messages on power-of-two processes.
 *
 * Limitation:   Current implementation only works on power-of-two number of
 *               processes.
 *               In case this algorithm is invoked on non-power-of-two
 *               processes, Bruck algorithm will be invoked.
 *
 * Memory requirements:
 *               No additional memory requirements beyond user-supplied buffers.
 *
 * Example on 4 nodes:
 *   Initialization: everyone has its own buffer at location rank in rbuf
 *    #     0      1      2      3
 *         [0]    [ ]    [ ]    [ ]
 *         [ ]    [1]    [ ]    [ ]
 *         [ ]    [ ]    [2]    [ ]
 *         [ ]    [ ]    [ ]    [3]
 *   Step 0: exchange data with (rank ^ 2^0)
 *    #     0      1      2      3
 *         [0]    [0]    [ ]    [ ]
 *         [1]    [1]    [ ]    [ ]
 *         [ ]    [ ]    [2]    [2]
 *         [ ]    [ ]    [3]    [3]
 *   Step 1: exchange data with (rank ^ 2^1) (if you can)
 *    #     0      1      2      3
 *         [0]    [0]    [0]    [0]
 *         [1]    [1]    [1]    [1]
 *         [2]    [2]    [2]    [2]
 *         [3]    [3]    [3]    [3]
 *
 *  TODO: Modify the algorithm to work with any number of nodes.
 *        We can modify code to use identical implementation like MPICH-2:
 *        - using recursive-halving algorithm, at the end of each step,
 *          determine if there are nodes who did not exchange their data in that
 *          step, and send them appropriate messages.
 */
int
ompi_coll_base_allgather_intra_recursivedoubling(void *sbuf, int scount,
                                                  struct ompi_datatype_t *sdtype,
                                                  void* rbuf, int rcount,
                                                  struct ompi_datatype_t *rdtype,
                                                  struct ompi_communicator_t *comm,
                                                  mca_coll_base_module_t *module)
{
    int line = -1, rank, size, pow2size, err;
    int remote, distance, sendblocklocation;
    ptrdiff_t slb, rlb, sext, rext;
    char *tmpsend = NULL, *tmprecv = NULL;

    size = ompi_comm_size(comm);
    rank = ompi_comm_rank(comm);

    pow2size = opal_next_poweroftwo (size);
    pow2size >>=1;

    /* Current implementation only handles power-of-two number of processes.
       If the function was called on non-power-of-two number of processes,
       print warning and call bruck allgather algorithm with same parameters.
    */
    if (pow2size != size) {
        OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                     "coll:base:allgather_intra_recursivedoubling WARNING: non-pow-2 size %d, switching to bruck algorithm",
                     size));

        return ompi_coll_base_allgather_intra_bruck(sbuf, scount, sdtype,
                                                     rbuf, rcount, rdtype,
                                                     comm, module);
    }

    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                 "coll:base:allgather_intra_recursivedoubling rank %d, size %d",
                 rank, size));

    err = ompi_datatype_get_extent (sdtype, &slb, &sext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    err = ompi_datatype_get_extent (rdtype, &rlb, &rext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Initialization step:
       - if send buffer is not MPI_IN_PLACE, copy send buffer to block 0 of
       receive buffer
    */
    if (MPI_IN_PLACE != sbuf) {
        tmpsend = (char*) sbuf;
        tmprecv = (char*) rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
        err = ompi_datatype_sndrcv(tmpsend, scount, sdtype, tmprecv, rcount, rdtype);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl;  }

    }

    /* Communication step:
       At every step i, rank r:
       - exchanges message with rank remote = (r ^ 2^i).

    */
    sendblocklocation = rank;
    for (distance = 0x1; distance < size; distance<<=1) {
        remote = rank ^ distance;

        if (rank < remote) {
            tmpsend = (char*)rbuf + (ptrdiff_t)sendblocklocation * (ptrdiff_t)rcount * rext;
            tmprecv = (char*)rbuf + (ptrdiff_t)(sendblocklocation + distance) * (ptrdiff_t)rcount * rext;
        } else {
            tmpsend = (char*)rbuf + (ptrdiff_t)sendblocklocation * (ptrdiff_t)rcount * rext;
            tmprecv = (char*)rbuf + (ptrdiff_t)(sendblocklocation - distance) * (ptrdiff_t)rcount * rext;
            sendblocklocation -= distance;
        }

        /* Sendreceive */
        err = ompi_coll_base_sendrecv(tmpsend, (ptrdiff_t)distance * (ptrdiff_t)rcount, rdtype,
                                       remote, MCA_COLL_BASE_TAG_ALLGATHER,
                                       tmprecv, (ptrdiff_t)distance * (ptrdiff_t)rcount, rdtype,
                                       remote, MCA_COLL_BASE_TAG_ALLGATHER,
                                       comm, MPI_STATUS_IGNORE, rank);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    }

    return OMPI_SUCCESS;

 err_hndl:
    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,  "%s:%4d\tError occurred %d, rank %2d",
                 __FILE__, line, err, rank));
    return err;
}



/*
 * ompi_coll_base_allgather_intra_ring
 *
 * Function:     allgather using O(N) steps.
 * Accepts:      Same arguments as MPI_Allgather
 * Returns:      MPI_SUCCESS or error code
 *
 * Description:  Ring algorithm for all gather.
 *               At every step i, rank r receives message from rank (r - 1)
 *               containing data from rank (r - i - 1) and sends message to rank
 *               (r + 1) containing data from rank (r - i), with wrap arounds.
 * Memory requirements:
 *               No additional memory requirements.
 *
 */
int ompi_coll_base_allgather_intra_ring(void *sbuf, int scount,
                                         struct ompi_datatype_t *sdtype,
                                         void* rbuf, int rcount,
                                         struct ompi_datatype_t *rdtype,
                                         struct ompi_communicator_t *comm,
                                         mca_coll_base_module_t *module)
{
    int line = -1, rank, size, err, sendto, recvfrom, i, recvdatafrom, senddatafrom;
    ptrdiff_t slb, rlb, sext, rext;
    char *tmpsend = NULL, *tmprecv = NULL;

    size = ompi_comm_size(comm);
    rank = ompi_comm_rank(comm);

    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                 "coll:base:allgather_intra_ring rank %d", rank));

    err = ompi_datatype_get_extent (sdtype, &slb, &sext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    err = ompi_datatype_get_extent (rdtype, &rlb, &rext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Initialization step:
       - if send buffer is not MPI_IN_PLACE, copy send buffer to appropriate block
       of receive buffer
    */
    tmprecv = (char*) rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
    if (MPI_IN_PLACE != sbuf) {
        tmpsend = (char*) sbuf;
        err = ompi_datatype_sndrcv(tmpsend, scount, sdtype, tmprecv, rcount, rdtype);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl;  }
    }

    /* Communication step:
       At every step i: 0 .. (P-1), rank r:
       - receives message from [(r - 1 + size) % size] containing data from rank
       [(r - i - 1 + size) % size]
       - sends message to rank [(r + 1) % size] containing data from rank
       [(r - i + size) % size]
       - sends message which starts at begining of rbuf and has size
    */
    sendto = (rank + 1) % size;
    recvfrom  = (rank - 1 + size) % size;

    for (i = 0; i < size - 1; i++) {
        recvdatafrom = (rank - i - 1 + size) % size;
        senddatafrom = (rank - i + size) % size;

        tmprecv = (char*)rbuf + (ptrdiff_t)recvdatafrom * (ptrdiff_t)rcount * rext;
        tmpsend = (char*)rbuf + (ptrdiff_t)senddatafrom * (ptrdiff_t)rcount * rext;

        /* Sendreceive */
        err = ompi_coll_base_sendrecv(tmpsend, rcount, rdtype, sendto,
                                       MCA_COLL_BASE_TAG_ALLGATHER,
                                       tmprecv, rcount, rdtype, recvfrom,
                                       MCA_COLL_BASE_TAG_ALLGATHER,
                                       comm, MPI_STATUS_IGNORE, rank);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    }

    return OMPI_SUCCESS;

 err_hndl:
    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,  "%s:%4d\tError occurred %d, rank %2d",
                 __FILE__, line, err, rank));
    return err;
}

/*
 * ompi_coll_base_allgather_intra_neighborexchange
 *
 * Function:     allgather using N/2 steps (O(N))
 * Accepts:      Same arguments as MPI_Allgather
 * Returns:      MPI_SUCCESS or error code
 *
 * Description:  Neighbor Exchange algorithm for allgather.
 *               Described by Chen et.al. in
 *               "Performance Evaluation of Allgather Algorithms on
 *                Terascale Linux Cluster with Fast Ethernet",
 *               Proceedings of the Eighth International Conference on
 *               High-Performance Computing inn Asia-Pacific Region
 *               (HPCASIA'05), 2005
 *
 *               Rank r exchanges message with one of its neighbors and
 *               forwards the data further in the next step.
 *
 *               No additional memory requirements.
 *
 * Limitations:  Algorithm works only on even number of processes.
 *               For odd number of processes we switch to ring algorithm.
 *
 * Example on 6 nodes:
 *  Initial state
 *    #     0      1      2      3      4      5
 *         [0]    [ ]    [ ]    [ ]    [ ]    [ ]
 *         [ ]    [1]    [ ]    [ ]    [ ]    [ ]
 *         [ ]    [ ]    [2]    [ ]    [ ]    [ ]
 *         [ ]    [ ]    [ ]    [3]    [ ]    [ ]
 *         [ ]    [ ]    [ ]    [ ]    [4]    [ ]
 *         [ ]    [ ]    [ ]    [ ]    [ ]    [5]
 *   Step 0:
 *    #     0      1      2      3      4      5
 *         [0]    [0]    [ ]    [ ]    [ ]    [ ]
 *         [1]    [1]    [ ]    [ ]    [ ]    [ ]
 *         [ ]    [ ]    [2]    [2]    [ ]    [ ]
 *         [ ]    [ ]    [3]    [3]    [ ]    [ ]
 *         [ ]    [ ]    [ ]    [ ]    [4]    [4]
 *         [ ]    [ ]    [ ]    [ ]    [5]    [5]
 *   Step 1:
 *    #     0      1      2      3      4      5
 *         [0]    [0]    [0]    [ ]    [ ]    [0]
 *         [1]    [1]    [1]    [ ]    [ ]    [1]
 *         [ ]    [2]    [2]    [2]    [2]    [ ]
 *         [ ]    [3]    [3]    [3]    [3]    [ ]
 *         [4]    [ ]    [ ]    [4]    [4]    [4]
 *         [5]    [ ]    [ ]    [5]    [5]    [5]
 *   Step 2:
 *    #     0      1      2      3      4      5
 *         [0]    [0]    [0]    [0]    [0]    [0]
 *         [1]    [1]    [1]    [1]    [1]    [1]
 *         [2]    [2]    [2]    [2]    [2]    [2]
 *         [3]    [3]    [3]    [3]    [3]    [3]
 *         [4]    [4]    [4]    [4]    [4]    [4]
 *         [5]    [5]    [5]    [5]    [5]    [5]
 */
int
ompi_coll_base_allgather_intra_neighborexchange(void *sbuf, int scount,
                                                 struct ompi_datatype_t *sdtype,
                                                 void* rbuf, int rcount,
                                                 struct ompi_datatype_t *rdtype,
                                                 struct ompi_communicator_t *comm,
                                                 mca_coll_base_module_t *module)
{
    int line = -1, rank, size, i, even_rank, err;
    int neighbor[2], offset_at_step[2], recv_data_from[2], send_data_from;
    ptrdiff_t slb, rlb, sext, rext;
    char *tmpsend = NULL, *tmprecv = NULL;

    size = ompi_comm_size(comm);
    rank = ompi_comm_rank(comm);

    if (size % 2) {
        OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                     "coll:base:allgather_intra_neighborexchange WARNING: odd size %d, switching to ring algorithm",
                     size));
        return ompi_coll_base_allgather_intra_ring(sbuf, scount, sdtype,
                                                    rbuf, rcount, rdtype,
                                                    comm, module);
    }

    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                 "coll:base:allgather_intra_neighborexchange rank %d", rank));

    err = ompi_datatype_get_extent (sdtype, &slb, &sext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    err = ompi_datatype_get_extent (rdtype, &rlb, &rext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Initialization step:
       - if send buffer is not MPI_IN_PLACE, copy send buffer to appropriate block
       of receive buffer
    */
    tmprecv = (char*) rbuf + (ptrdiff_t)rank *(ptrdiff_t) rcount * rext;
    if (MPI_IN_PLACE != sbuf) {
        tmpsend = (char*) sbuf;
        err = ompi_datatype_sndrcv(tmpsend, scount, sdtype, tmprecv, rcount, rdtype);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl;  }
    }

    /* Determine neighbors, order in which blocks will arrive, etc. */
    even_rank = !(rank % 2);
    if (even_rank) {
        neighbor[0] = (rank + 1) % size;
        neighbor[1] = (rank - 1 + size) % size;
        recv_data_from[0] = rank;
        recv_data_from[1] = rank;
        offset_at_step[0] = (+2);
        offset_at_step[1] = (-2);
    } else {
        neighbor[0] = (rank - 1 + size) % size;
        neighbor[1] = (rank + 1) % size;
        recv_data_from[0] = neighbor[0];
        recv_data_from[1] = neighbor[0];
        offset_at_step[0] = (-2);
        offset_at_step[1] = (+2);
    }

    /* Communication loop:
       - First step is special: exchange a single block with neighbor[0].
       - Rest of the steps:
       update recv_data_from according to offset, and
       exchange two blocks with appropriate neighbor.
       the send location becomes previous receve location.
    */
    tmprecv = (char*)rbuf + (ptrdiff_t)neighbor[0] * (ptrdiff_t)rcount * rext;
    tmpsend = (char*)rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
    /* Sendreceive */
    err = ompi_coll_base_sendrecv(tmpsend, rcount, rdtype, neighbor[0],
                                   MCA_COLL_BASE_TAG_ALLGATHER,
                                   tmprecv, rcount, rdtype, neighbor[0],
                                   MCA_COLL_BASE_TAG_ALLGATHER,
                                   comm, MPI_STATUS_IGNORE, rank);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Determine initial sending location */
    if (even_rank) {
        send_data_from = rank;
    } else {
        send_data_from = recv_data_from[0];
    }

    for (i = 1; i < (size / 2); i++) {
        const int i_parity = i % 2;
        recv_data_from[i_parity] =
            (recv_data_from[i_parity] + offset_at_step[i_parity] + size) % size;

        tmprecv = (char*)rbuf + (ptrdiff_t)recv_data_from[i_parity] * (ptrdiff_t)rcount * rext;
        tmpsend = (char*)rbuf + (ptrdiff_t)send_data_from * rcount * rext;

        /* Sendreceive */
        err = ompi_coll_base_sendrecv(tmpsend, (ptrdiff_t)2 * (ptrdiff_t)rcount, rdtype,
                                       neighbor[i_parity],
                                       MCA_COLL_BASE_TAG_ALLGATHER,
                                       tmprecv, (ptrdiff_t)2 * (ptrdiff_t)rcount, rdtype,
                                       neighbor[i_parity],
                                       MCA_COLL_BASE_TAG_ALLGATHER,
                                       comm, MPI_STATUS_IGNORE, rank);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

        send_data_from = recv_data_from[i_parity];
    }

    return OMPI_SUCCESS;

 err_hndl:
    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,  "%s:%4d\tError occurred %d, rank %2d",
                 __FILE__, line, err, rank));
    return err;
}


int ompi_coll_base_allgather_intra_two_procs(void *sbuf, int scount,
                                              struct ompi_datatype_t *sdtype,
                                              void* rbuf, int rcount,
                                              struct ompi_datatype_t *rdtype,
                                              struct ompi_communicator_t *comm,
                                              mca_coll_base_module_t *module)
{
    int line = -1, err, rank, remote;
    char *tmpsend = NULL, *tmprecv = NULL;
    ptrdiff_t sext, rext, lb;

    rank = ompi_comm_rank(comm);

    OPAL_OUTPUT((ompi_coll_base_framework.framework_output,
                 "ompi_coll_base_allgather_intra_two_procs rank %d", rank));

    err = ompi_datatype_get_extent (sdtype, &lb, &sext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    err = ompi_datatype_get_extent (rdtype, &lb, &rext);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Exchange data:
       - compute source and destinations
       - send receive data
    */
    remote  = rank ^ 0x1;

    tmpsend = (char*)sbuf;
    if (MPI_IN_PLACE == sbuf) {
        tmpsend = (char*)rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext;
        scount = rcount;
        sdtype = rdtype;
    }
    tmprecv = (char*)rbuf + (ptrdiff_t)remote * (ptrdiff_t)rcount * rext;

    err = ompi_coll_base_sendrecv(tmpsend, scount, sdtype, remote,
                                   MCA_COLL_BASE_TAG_ALLGATHER,
                                   tmprecv, rcount, rdtype, remote,
                                   MCA_COLL_BASE_TAG_ALLGATHER,
                                   comm, MPI_STATUS_IGNORE, rank);
    if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl; }

    /* Place your data in correct location if necessary */
    if (MPI_IN_PLACE != sbuf) {
        err = ompi_datatype_sndrcv((char*)sbuf, scount, sdtype,
                                   (char*)rbuf + (ptrdiff_t)rank * (ptrdiff_t)rcount * rext, rcount, rdtype);
        if (MPI_SUCCESS != err) { line = __LINE__; goto err_hndl;  }
    }

    return MPI_SUCCESS;

 err_hndl:
    OPAL_OUTPUT((ompi_coll_base_framework.framework_output, "%s:%4d\tError occurred %d, rank %2d",
                 __FILE__, line, err, rank));
    return err;
}


/*
 * Linear functions are copied from the BASIC coll module
 * they do not segment the message and are simple implementations
 * but for some small number of nodes and/or small data sizes they
 * are just as fast as base/tree based segmenting operations
 * and as such may be selected by the decision functions
 * These are copied into this module due to the way we select modules
 * in V1. i.e. in V2 we will handle this differently and so will not
 * have to duplicate code.
 * JPG following the examples from other coll_base implementations. Dec06.
 */

/* copied function (with appropriate renaming) starts here */

/*
 *    allgather_intra_basic_linear
 *
 *    Function:    - allgather using other MPI collections
 *    Accepts:    - same as MPI_Allgather()
 *    Returns:    - MPI_SUCCESS or error code
 */
int
ompi_coll_base_allgather_intra_basic_linear(void *sbuf, int scount,
                                             struct ompi_datatype_t *sdtype,
                                             void *rbuf,
                                             int rcount,
                                             struct ompi_datatype_t *rdtype,
                                             struct ompi_communicator_t *comm,
                                             mca_coll_base_module_t *module)
{
    int err;
    ptrdiff_t lb, extent;

    /* Handle MPI_IN_PLACE (see explanantion in reduce.c for how to
       allocate temp buffer) -- note that rank 0 can use IN_PLACE
       natively, and we can just alias the right position in rbuf
       as sbuf and avoid using a temporary buffer if gather is
       implemented correctly */
    if (MPI_IN_PLACE == sbuf && 0 != ompi_comm_rank(comm)) {
        ompi_datatype_get_extent(rdtype, &lb, &extent);
        sbuf = ((char*) rbuf) + (ompi_comm_rank(comm) * extent * rcount);
        sdtype = rdtype;
        scount = rcount;
    }

    /* Gather and broadcast. */

    err = comm->c_coll.coll_gather(sbuf, scount, sdtype,
                                   rbuf, rcount, rdtype,
                                   0, comm, comm->c_coll.coll_gather_module);
    if (MPI_SUCCESS == err) {
        size_t length = (ptrdiff_t)rcount * ompi_comm_size(comm);
        if( length < (size_t)INT_MAX ) {
            err = comm->c_coll.coll_bcast(rbuf, (ptrdiff_t)rcount * ompi_comm_size(comm), rdtype,
                                          0, comm, comm->c_coll.coll_bcast_module);
        } else {
            ompi_datatype_t* temptype;
            ompi_datatype_create_contiguous(ompi_comm_size(comm), rdtype, &temptype);
            ompi_datatype_commit(&temptype);
            err = comm->c_coll.coll_bcast(rbuf, rcount, temptype,
                                          0, comm, comm->c_coll.coll_bcast_module);
            ompi_datatype_destroy(&temptype);
        }
    }

    /* All done */

    return err;
}

/* copied function (with appropriate renaming) ends here */