engine.c 71.2 KB
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/*******************************************************************************
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 * This file is part of SWIFT.
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 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
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 *
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 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published
 * by the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
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 *
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 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
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 *
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 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
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 *
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 ******************************************************************************/

/* Config parameters. */
#include "../config.h"

/* Some standard headers. */
#include <float.h>
#include <limits.h>
#include <sched.h>
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
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#include <stdbool.h>
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/* MPI headers. */
#ifdef WITH_MPI
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#include <mpi.h>
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/* METIS headers only used when MPI is also available. */
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#ifdef HAVE_METIS
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#include <metis.h>
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#endif
#endif

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#ifdef HAVE_LIBNUMA
#include <numa.h>
#endif

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/* This object's header. */
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#include "engine.h"
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/* Local headers. */
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#include "atomic.h"
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#include "cell.h"
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#include "cycle.h"
#include "debug.h"
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#include "error.h"
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#include "part.h"
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#include "timers.h"
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const char *engine_policy_names[12] = {
    "none",          "rand",   "steal",        "keep",
    "block",         "fix_dt", "cpu_tight",    "mpi",
    "numa_affinity", "hydro",  "self_gravity", "external_gravity"};
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/** The rank of the engine as a global variable (for messages). */
int engine_rank;

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/**
 * @brief Link a density/force task to a cell.
 *
 * @param e The #engine.
 * @param l The #link.
 * @param t The #task.
 *
 * @return The new #link pointer.
 */

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struct link *engine_addlink(struct engine *e, struct link *l, struct task *t) {
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  const int ind = atomic_inc(&e->nr_links);
  if (ind >= e->size_links) {
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    error("Link table overflow.");
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  }
  struct link *res = &e->links[ind];
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  res->next = l;
  res->t = t;
  return res;
}
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/**
 * @brief Generate the ghost and kick tasks for a hierarchy of cells.
 *
 * @param e The #engine.
 * @param c The #cell.
 * @param super The super #cell.
 */
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void engine_mkghosts(struct engine *e, struct cell *c, struct cell *super) {

  int k;
  struct scheduler *s = &e->sched;

  /* Am I the super-cell? */
  if (super == NULL && c->nr_tasks > 0) {

    /* Remember me. */
    super = c;

    /* Local tasks only... */
    if (c->nodeID == e->nodeID) {

      /* Generate the ghost task. */
      c->ghost = scheduler_addtask(s, task_type_ghost, task_subtype_none, 0, 0,
                                   c, NULL, 0);
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      /* Add the drift task. */
      c->drift = scheduler_addtask(s, task_type_drift, task_subtype_none, 0, 0,
                                   c, NULL, 0);
      /* Add the init task. */
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      c->init = scheduler_addtask(s, task_type_init, task_subtype_none, 0, 0, c,
                                  NULL, 0);
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      /* Add the kick task. */
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      c->kick = scheduler_addtask(s, task_type_kick, task_subtype_none, 0, 0, c,
                                  NULL, 0);
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    }
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  }
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  /* Set the super-cell. */
  c->super = super;

  /* Recurse. */
  if (c->split)
    for (k = 0; k < 8; k++)
      if (c->progeny[k] != NULL) engine_mkghosts(e, c->progeny[k], super);
}
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/**
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 * @brief Redistribute the particles amongst the nodes according
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 *      to their cell's node IDs.
 *
 * @param e The #engine.
 */

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void engine_redistribute(struct engine *e) {
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#ifdef WITH_MPI
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  int nr_nodes = e->nr_nodes, nodeID = e->nodeID;
  struct space *s = e->s;
  int my_cells = 0;
  int *cdim = s->cdim;
  struct cell *cells = s->cells;
  int nr_cells = s->nr_cells;

  /* Start by sorting the particles according to their nodes and
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     getting the counts. The counts array is indexed as
     count[from * nr_nodes + to]. */
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  int *counts, *dest;
  double ih[3], dim[3];
  ih[0] = s->ih[0];
  ih[1] = s->ih[1];
  ih[2] = s->ih[2];
  dim[0] = s->dim[0];
  dim[1] = s->dim[1];
  dim[2] = s->dim[2];
  if ((counts = (int *)malloc(sizeof(int) *nr_nodes *nr_nodes)) == NULL ||
      (dest = (int *)malloc(sizeof(int) * s->nr_parts)) == NULL)
    error("Failed to allocate count and dest buffers.");
  bzero(counts, sizeof(int) * nr_nodes * nr_nodes);
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  struct part *parts = s->parts;
  for (int k = 0; k < s->nr_parts; k++) {
    for (int j = 0; j < 3; j++) {
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      if (parts[k].x[j] < 0.0)
        parts[k].x[j] += dim[j];
      else if (parts[k].x[j] >= dim[j])
        parts[k].x[j] -= dim[j];
    }
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    const int cid = cell_getid(cdim, parts[k].x[0] * ih[0],
                               parts[k].x[1] * ih[1], parts[k].x[2] * ih[2]);
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    /* if (cid < 0 || cid >= s->nr_cells)
       error("Bad cell id %i for part %i at [%.3e,%.3e,%.3e].",
             cid, k, parts[k].x[0], parts[k].x[1], parts[k].x[2]); */
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    dest[k] = cells[cid].nodeID;
    counts[nodeID * nr_nodes + dest[k]] += 1;
  }
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  space_parts_sort(s, dest, s->nr_parts, 0, nr_nodes - 1);
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  /* Get all the counts from all the nodes. */
  if (MPI_Allreduce(MPI_IN_PLACE, counts, nr_nodes * nr_nodes, MPI_INT, MPI_SUM,
                    MPI_COMM_WORLD) != MPI_SUCCESS)
    error("Failed to allreduce particle transfer counts.");

  /* Get the new number of parts for this node, be generous in allocating. */
  int nr_parts = 0;
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  for (int k = 0; k < nr_nodes; k++) nr_parts += counts[k * nr_nodes + nodeID];
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  struct part *parts_new = NULL;
  struct xpart *xparts_new = NULL, *xparts = s->xparts;
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  if (posix_memalign((void **)&parts_new, part_align,
                     sizeof(struct part) * nr_parts * 1.2) != 0 ||
      posix_memalign((void **)&xparts_new, part_align,
                     sizeof(struct xpart) * nr_parts * 1.2) != 0)
    error("Failed to allocate new part data.");

  /* Emit the sends and recvs for the particle data. */
  MPI_Request *reqs;
  if ((reqs = (MPI_Request *)malloc(sizeof(MPI_Request) * 4 * nr_nodes)) ==
      NULL)
    error("Failed to allocate MPI request list.");
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  for (int k = 0; k < 4 * nr_nodes; k++) reqs[k] = MPI_REQUEST_NULL;
  for (int offset_send = 0, offset_recv = 0, k = 0; k < nr_nodes; k++) {
    int ind_send = nodeID * nr_nodes + k;
    int ind_recv = k * nr_nodes + nodeID;
    if (counts[ind_send] > 0) {
      if (k == nodeID) {
        memcpy(&parts_new[offset_recv], &s->parts[offset_send],
               sizeof(struct part) * counts[ind_recv]);
        memcpy(&xparts_new[offset_recv], &s->xparts[offset_send],
               sizeof(struct xpart) * counts[ind_recv]);
        offset_send += counts[ind_send];
        offset_recv += counts[ind_recv];
      } else {
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        if (MPI_Isend(&s->parts[offset_send], counts[ind_send],
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                      e->part_mpi_type, k, 2 * ind_send + 0, MPI_COMM_WORLD,
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                      &reqs[4 * k]) != MPI_SUCCESS)
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          error("Failed to isend parts to node %i.", k);
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        if (MPI_Isend(&s->xparts[offset_send], counts[ind_send],
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                      e->xpart_mpi_type, k, 2 * ind_send + 1, MPI_COMM_WORLD,
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                      &reqs[4 * k + 1]) != MPI_SUCCESS)
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          error("Failed to isend xparts to node %i.", k);
        offset_send += counts[ind_send];
      }
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    }
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    if (k != nodeID && counts[ind_recv] > 0) {
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      if (MPI_Irecv(&parts_new[offset_recv], counts[ind_recv], e->part_mpi_type,
                    k, 2 * ind_recv + 0, MPI_COMM_WORLD,
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                    &reqs[4 * k + 2]) != MPI_SUCCESS)
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        error("Failed to emit irecv of parts from node %i.", k);
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      if (MPI_Irecv(&xparts_new[offset_recv], counts[ind_recv],
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                    e->xpart_mpi_type, k, 2 * ind_recv + 1, MPI_COMM_WORLD,
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                    &reqs[4 * k + 3]) != MPI_SUCCESS)
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        error("Failed to emit irecv of parts from node %i.", k);
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      offset_recv += counts[ind_recv];
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    }
  }
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  /* Wait for all the sends and recvs to tumble in. */
  MPI_Status stats[4 * nr_nodes];
  int res;
  if ((res = MPI_Waitall(4 * nr_nodes, reqs, stats)) != MPI_SUCCESS) {
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    for (int k = 0; k < 4 * nr_nodes; k++) {
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      char buff[MPI_MAX_ERROR_STRING];
      int res;
      MPI_Error_string(stats[k].MPI_ERROR, buff, &res);
      message("request %i has error '%s'.", k, buff);
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    }
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    error("Failed during waitall for part data.");
  }
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  /* Verify that all parts are in the right place. */
  /* for ( k = 0 ; k < nr_parts ; k++ ) {
      cid = cell_getid( cdim , parts_new[k].x[0]*ih[0] , parts_new[k].x[1]*ih[1]
     , parts_new[k].x[2]*ih[2] );
      if ( cells[ cid ].nodeID != nodeID )
          error( "Received particle (%i) that does not belong here (nodeID=%i)."
     , k , cells[ cid ].nodeID );
      } */

  /* Set the new part data, free the old. */
  free(parts);
  free(xparts);
  s->parts = parts_new;
  s->xparts = xparts_new;
  s->nr_parts = nr_parts;
  s->size_parts = 1.2 * nr_parts;

  /* Be verbose about what just happened. */
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  for (int k = 0; k < nr_cells; k++)
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    if (cells[k].nodeID == nodeID) my_cells += 1;
  message("node %i now has %i parts in %i cells.", nodeID, nr_parts, my_cells);

  /* Clean up other stuff. */
  free(reqs);
  free(counts);
  free(dest);

#else
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  error("SWIFT was not compiled with MPI support.");
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#endif
}
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/**
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 * @brief Repartition the cells amongst the nodes.
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 *
 * @param e The #engine.
 */
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void engine_repartition(struct engine *e) {
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#if defined(WITH_MPI) && defined(HAVE_METIS)

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  int i, j, k, l, cid, cjd, ii, jj, kk, res;
  idx_t *inds, *nodeIDs;
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  idx_t *weights_v = NULL, *weights_e = NULL;
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  struct space *s = e->s;
  int nr_cells = s->nr_cells, my_cells = 0;
  struct cell *cells = s->cells;
  int ind[3], *cdim = s->cdim;
  struct task *t, *tasks = e->sched.tasks;
  struct cell *ci, *cj;
  int nr_nodes = e->nr_nodes, nodeID = e->nodeID;
  float wscale = 1e-3, vscale = 1e-3, wscale_buff;
  idx_t wtot = 0;
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  idx_t wmax = 1e9 / e->nr_nodes;
  idx_t wmin;
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  /* Clear the repartition flag. */
  e->forcerepart = 0;

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  /* Nothing to do if only using a single node. Also avoids METIS
   * bug that doesn't handle this case well. */
  if (nr_nodes == 1) return;
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  /* Allocate the inds and weights. */
  if ((inds = (idx_t *)malloc(sizeof(idx_t) * 26 *nr_cells)) == NULL ||
      (weights_v = (idx_t *)malloc(sizeof(idx_t) *nr_cells)) == NULL ||
      (weights_e = (idx_t *)malloc(sizeof(idx_t) * 26 *nr_cells)) == NULL ||
      (nodeIDs = (idx_t *)malloc(sizeof(idx_t) * nr_cells)) == NULL)
    error("Failed to allocate inds and weights arrays.");

  /* Fill the inds array. */
  for (cid = 0; cid < nr_cells; cid++) {
    ind[0] = cells[cid].loc[0] / s->cells[cid].h[0] + 0.5;
    ind[1] = cells[cid].loc[1] / s->cells[cid].h[1] + 0.5;
    ind[2] = cells[cid].loc[2] / s->cells[cid].h[2] + 0.5;
    l = 0;
    for (i = -1; i <= 1; i++) {
      ii = ind[0] + i;
      if (ii < 0)
        ii += cdim[0];
      else if (ii >= cdim[0])
        ii -= cdim[0];
      for (j = -1; j <= 1; j++) {
        jj = ind[1] + j;
        if (jj < 0)
          jj += cdim[1];
        else if (jj >= cdim[1])
          jj -= cdim[1];
        for (k = -1; k <= 1; k++) {
          kk = ind[2] + k;
          if (kk < 0)
            kk += cdim[2];
          else if (kk >= cdim[2])
            kk -= cdim[2];
          if (i || j || k) {
            inds[cid * 26 + l] = cell_getid(cdim, ii, jj, kk);
            l += 1;
          }
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        }
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      }
    }
  }

  /* Init the weights arrays. */
  bzero(weights_e, sizeof(idx_t) * 26 * nr_cells);
  bzero(weights_v, sizeof(idx_t) * nr_cells);

  /* Loop over the tasks... */
  for (j = 0; j < e->sched.nr_tasks; j++) {

    /* Get a pointer to the kth task. */
    t = &tasks[j];

    /* Skip un-interesting tasks. */
    if (t->type != task_type_self && t->type != task_type_pair &&
        t->type != task_type_sub && t->type != task_type_ghost &&
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        t->type != task_type_drift && t->type != task_type_kick &&
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        t->type != task_type_init)
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      continue;

    /* Get the task weight. */
    idx_t w = (t->toc - t->tic) * wscale;
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    if (w < 0) error("Bad task weight (%" SCIDX ").", w);
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    /* Do we need to re-scale? */
    wtot += w;
    while (wtot > wmax) {
      wscale /= 2;
      wtot /= 2;
      w /= 2;
      for (k = 0; k < 26 * nr_cells; k++) weights_e[k] *= 0.5;
      for (k = 0; k < nr_cells; k++) weights_v[k] *= 0.5;
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    }
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    /* Get the top-level cells involved. */
    for (ci = t->ci; ci->parent != NULL; ci = ci->parent)
      ;
    if (t->cj != NULL)
      for (cj = t->cj; cj->parent != NULL; cj = cj->parent)
        ;
    else
      cj = NULL;

    /* Get the cell IDs. */
    cid = ci - cells;

    /* Different weights for different tasks. */
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    if (t->type == task_type_ghost || t->type == task_type_drift ||
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        t->type == task_type_kick) {
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      /* Particle updates add only to vertex weight. */
      weights_v[cid] += w;

    }

    /* Self interaction? */
    else if ((t->type == task_type_self && ci->nodeID == nodeID) ||
             (t->type == task_type_sub && cj == NULL && ci->nodeID == nodeID)) {

      /* Self interactions add only to vertex weight. */
      weights_v[cid] += w;

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    }
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    /* Pair? */
    else if (t->type == task_type_pair ||
             (t->type == task_type_sub && cj != NULL)) {

      /* In-cell pair? */
      if (ci == cj) {

        /* Add weight to vertex for ci. */
        weights_v[cid] += w;

      }

      /* Distinct cells with local ci? */
      else if (ci->nodeID == nodeID) {

        /* Index of the jth cell. */
        cjd = cj - cells;

        /* Add half of weight to each cell. */
        if (ci->nodeID == nodeID) weights_v[cid] += 0.5 * w;
        if (cj->nodeID == nodeID) weights_v[cjd] += 0.5 * w;

        /* Add Weight to edge. */
        for (k = 26 * cid; inds[k] != cjd; k++)
          ;
        weights_e[k] += w;
        for (k = 26 * cjd; inds[k] != cid; k++)
          ;
        weights_e[k] += w;
      }
    }
  }

  /* Get the minimum scaling and re-scale if necessary. */
  if ((res = MPI_Allreduce(&wscale, &wscale_buff, 1, MPI_FLOAT, MPI_MIN,
                           MPI_COMM_WORLD)) != MPI_SUCCESS) {
    char buff[MPI_MAX_ERROR_STRING];
    MPI_Error_string(res, buff, &i);
    error("Failed to allreduce the weight scales (%s).", buff);
  }
  if (wscale_buff != wscale) {
    float scale = wscale_buff / wscale;
    for (k = 0; k < 26 * nr_cells; k++) weights_e[k] *= scale;
    for (k = 0; k < nr_cells; k++) weights_v[k] *= scale;
  }

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/* Merge the weights arrays across all nodes. */
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#if IDXTYPEWIDTH == 32
  if ((res = MPI_Reduce((nodeID == 0) ? MPI_IN_PLACE : weights_v, weights_v,
                        nr_cells, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD)) !=
      MPI_SUCCESS) {
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#else
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  if ((res = MPI_Reduce((nodeID == 0) ? MPI_IN_PLACE : weights_v, weights_v,
                        nr_cells, MPI_LONG_LONG_INT, MPI_SUM, 0,
                        MPI_COMM_WORLD)) != MPI_SUCCESS) {
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#endif
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    char buff[MPI_MAX_ERROR_STRING];
    MPI_Error_string(res, buff, &i);
    error("Failed to allreduce vertex weights (%s).", buff);
  }
#if IDXTYPEWIDTH == 32
  if (MPI_Reduce((nodeID == 0) ? MPI_IN_PLACE : weights_e, weights_e,
                 26 * nr_cells, MPI_INT, MPI_SUM, 0,
                 MPI_COMM_WORLD) != MPI_SUCCESS)
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#else
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  if (MPI_Reduce((nodeID == 0) ? MPI_IN_PLACE : weights_e, weights_e,
                 26 * nr_cells, MPI_LONG_LONG_INT, MPI_SUM, 0,
                 MPI_COMM_WORLD) != MPI_SUCCESS)
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#endif
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    error("Failed to allreduce edge weights.");

  /* As of here, only one node needs to compute the partition. */
  if (nodeID == 0) {

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    /* Final rescale of all weights to avoid a large range. Large ranges have
     * been seen to cause an incomplete graph. */
    wmin = wmax;
    wmax = 0.0;
    for (k = 0; k < 26 * nr_cells; k++) {
      wmax = weights_e[k] > wmax ? weights_e[k] : wmax;
      wmin = weights_e[k] < wmin ? weights_e[k] : wmin;
    }
    if ((wmax - wmin) > engine_maxmetisweight) {
      wscale = engine_maxmetisweight / (wmax - wmin);
      for (k = 0; k < 26 * nr_cells; k++) {
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        weights_e[k] = (weights_e[k] - wmin) * wscale + 1;
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      }
      for (k = 0; k < nr_cells; k++) {
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        weights_v[k] = (weights_v[k] - wmin) * wscale + 1;
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      }
    }
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    /* Check that the edge weights are fully symmetric. */
    /* for ( cid = 0 ; cid < nr_cells ; cid++ )
        for ( k = 0 ; k < 26 ; k++ ) {
            cjd = inds[ cid*26 + k ];
            for ( j = 26*cjd ; inds[j] != cid ; j++ );
            if ( weights_e[ cid*26+k ] != weights_e[ j ] )
                error( "Unsymmetric edge weights detected (%i vs %i)." ,
       weights_e[ cid*26+k ] , weights_e[ j ] );
            } */
    /* int w_min = weights_e[0], w_max = weights_e[0], w_tot = weights_e[0];
    for ( k = 1 ; k < 26*nr_cells ; k++ ) {
        w_tot += weights_e[k];
        if ( weights_e[k] < w_min )
            w_min = weights_e[k];
        else if ( weights_e[k] > w_max )
            w_max = weights_e[k];
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        }
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    message( "edge weights in [ %i , %i ], tot=%i." , w_min , w_max , w_tot );
    w_min = weights_e[0], w_max = weights_e[0]; w_tot = weights_v[0];
    for ( k = 1 ; k < nr_cells ; k++ ) {
        w_tot += weights_v[k];
        if ( weights_v[k] < w_min )
            w_min = weights_v[k];
        else if ( weights_v[k] > w_max )
            w_max = weights_v[k];
        }
    message( "vertex weights in [ %i , %i ], tot=%i." , w_min , w_max , w_tot );
    */

    /* Make sure there are no zero weights. */
    for (k = 0; k < 26 * nr_cells; k++)
      if (weights_e[k] == 0) weights_e[k] = 1;
    for (k = 0; k < nr_cells; k++)
      if ((weights_v[k] *= vscale) == 0) weights_v[k] = 1;

    /* Allocate and fill the connection array. */
    idx_t *offsets;
    if ((offsets = (idx_t *)malloc(sizeof(idx_t) * (nr_cells + 1))) == NULL)
      error("Failed to allocate offsets buffer.");
    offsets[0] = 0;
    for (k = 0; k < nr_cells; k++) offsets[k + 1] = offsets[k] + 26;

    /* Set the METIS options. +1 to keep the GCC sanitizer happy. */
    idx_t options[METIS_NOPTIONS + 1];
    METIS_SetDefaultOptions(options);
    options[METIS_OPTION_OBJTYPE] = METIS_OBJTYPE_CUT;
    options[METIS_OPTION_NUMBERING] = 0;
    options[METIS_OPTION_CONTIG] = 1;
    options[METIS_OPTION_NCUTS] = 10;
    options[METIS_OPTION_NITER] = 20;
    // options[ METIS_OPTION_UFACTOR ] = 1;

    /* Set the initial partition, although this is probably ignored. */
    for (k = 0; k < nr_cells; k++) nodeIDs[k] = cells[k].nodeID;

    /* Call METIS. */
    idx_t one = 1, idx_nr_cells = nr_cells, idx_nr_nodes = nr_nodes;
    idx_t objval;
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    /* Dump graph in METIS format */
    /*dumpMETISGraph("metis_graph", idx_nr_cells, one, offsets, inds,
                   weights_v, NULL, weights_e);*/

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    if (METIS_PartGraphRecursive(&idx_nr_cells, &one, offsets, inds, weights_v,
                                 NULL, weights_e, &idx_nr_nodes, NULL, NULL,
                                 options, &objval, nodeIDs) != METIS_OK)
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      error("Call to METIS_PartGraphRecursive failed.");
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    /* Dump the 3d array of cell IDs. */
    /* printf( "engine_repartition: nodeIDs = reshape( [" );
    for ( i = 0 ; i < cdim[0]*cdim[1]*cdim[2] ; i++ )
        printf( "%i " , (int)nodeIDs[ i ] );
    printf("] ,%i,%i,%i);\n",cdim[0],cdim[1],cdim[2]); */
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    /* Check that the nodeIDs are ok. */
    for (k = 0; k < nr_cells; k++)
      if (nodeIDs[k] < 0 || nodeIDs[k] >= nr_nodes)
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        error("Got bad nodeID %" PRIDX " for cell %i.", nodeIDs[k], k);
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    /* Check that the partition is complete and all nodes have some work. */
    int present[nr_nodes];
    int failed = 0;
    for (i = 0; i < nr_nodes; i++) present[i] = 0;
    for (i = 0; i < nr_cells; i++) present[nodeIDs[i]]++;
    for (i = 0; i < nr_nodes; i++) {
      if (!present[i]) {
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        failed = 1;
        message("Node %d is not present after repartition", i);
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      }
    }

    /* If partition failed continue with the current one, but make this
     * clear. */
    if (failed) {
      message(
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          "WARNING: METIS repartition has failed, continuing with "
          "the current partition, load balance will not be optimal");
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      for (k = 0; k < nr_cells; k++) nodeIDs[k] = cells[k].nodeID;
    }
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  }

/* Broadcast the result of the partition. */
#if IDXTYPEWIDTH == 32
  if (MPI_Bcast(nodeIDs, nr_cells, MPI_INT, 0, MPI_COMM_WORLD) != MPI_SUCCESS)
    error("Failed to bcast the node IDs.");
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#else
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  if (MPI_Bcast(nodeIDs, nr_cells, MPI_LONG_LONG_INT, 0, MPI_COMM_WORLD) !=
      MPI_SUCCESS)
    error("Failed to bcast the node IDs.");
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#endif
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  /* Set the cell nodeIDs and clear any non-local parts. */
  for (k = 0; k < nr_cells; k++) {
    cells[k].nodeID = nodeIDs[k];
    if (nodeIDs[k] == nodeID) my_cells += 1;
  }

  /* Clean up. */
  free(inds);
  free(weights_v);
  free(weights_e);
  free(nodeIDs);

  /* Now comes the tricky part: Exchange particles between all nodes.
     This is done in two steps, first allreducing a matrix of
     how many particles go from where to where, then re-allocating
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     the parts array, and emitting the sends and receives.
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     Finally, the space, tasks, and proxies need to be rebuilt. */

  /* Redistribute the particles between the nodes. */
  engine_redistribute(e);

  /* Make the proxies. */
  engine_makeproxies(e);

  /* Tell the engine it should re-build whenever possible */
  e->forcerebuild = 1;

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#else
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  error("SWIFT was not compiled with MPI and METIS support.");
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#endif
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}
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/**
 * @brief Add up/down gravity tasks to a cell hierarchy.
 *
 * @param e The #engine.
 * @param c The #cell
 * @param up The upward gravity #task.
 * @param down The downward gravity #task.
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 */
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void engine_addtasks_grav(struct engine *e, struct cell *c, struct task *up,
                          struct task *down) {
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  /* Link the tasks to this cell. */
  c->grav_up = up;
  c->grav_down = down;

  /* Recurse? */
  if (c->split)
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL)
        engine_addtasks_grav(e, c->progeny[k], up, down);
}
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/**
 * @brief Add send tasks to a hierarchy of cells.
 *
 * @param e The #engine.
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 * @param ci The sending #cell.
 * @param cj The receiving #cell
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 */

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void engine_addtasks_send(struct engine *e, struct cell *ci, struct cell *cj) {
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#ifdef WITH_MPI
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  int k;
  struct link *l = NULL;
  struct scheduler *s = &e->sched;
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  /* Check if any of the density tasks are for the target node. */
  for (l = ci->density; l != NULL; l = l->next)
    if (l->t->ci->nodeID == cj->nodeID ||
        (l->t->cj != NULL && l->t->cj->nodeID == cj->nodeID))
      break;
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  /* If so, attach send tasks. */
  if (l != NULL) {
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    /* Create the tasks. */
    struct task *t_xv =
        scheduler_addtask(&e->sched, task_type_send, task_subtype_none,
                          2 * ci->tag, 0, ci, cj, 0);
    struct task *t_rho =
        scheduler_addtask(&e->sched, task_type_send, task_subtype_none,
                          2 * ci->tag + 1, 0, ci, cj, 0);
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    /* The send_rho task depends on the cell's ghost task. */
    scheduler_addunlock(s, ci->super->ghost, t_rho);
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    /* The send_rho task should unlock the super-cell's kick task. */
    scheduler_addunlock(s, t_rho, ci->super->kick);
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    /* The send_xv task should unlock the super-cell's ghost task. */
    scheduler_addunlock(s, t_xv, ci->super->ghost);
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  }
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  /* Recurse? */
  else if (ci->split)
    for (k = 0; k < 8; k++)
      if (ci->progeny[k] != NULL) engine_addtasks_send(e, ci->progeny[k], cj);
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#else
  error("SWIFT was not compiled with MPI support.");
#endif
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}
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/**
 * @brief Add recv tasks to a hierarchy of cells.
 *
 * @param e The #engine.
 * @param c The #cell.
 * @param t_xv The recv_xv #task, if it has already been created.
 * @param t_rho The recv_rho #task, if it has already been created.
 */

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void engine_addtasks_recv(struct engine *e, struct cell *c, struct task *t_xv,
                          struct task *t_rho) {
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#ifdef WITH_MPI
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  int k;
  struct scheduler *s = &e->sched;
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  /* Do we need to construct a recv task? */
  if (t_xv == NULL && c->nr_density > 0) {
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    /* Create the tasks. */
    t_xv = c->recv_xv =
        scheduler_addtask(&e->sched, task_type_recv, task_subtype_none,
                          2 * c->tag, 0, c, NULL, 0);
    t_rho = c->recv_rho =
        scheduler_addtask(&e->sched, task_type_recv, task_subtype_none,
                          2 * c->tag + 1, 0, c, NULL, 0);
  }
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  /* Add dependencies. */
  for (struct link *l = c->density; l != NULL; l = l->next) {
    scheduler_addunlock(s, t_xv, l->t);
    scheduler_addunlock(s, l->t, t_rho);
  }
  for (struct link *l = c->force; l != NULL; l = l->next)
    scheduler_addunlock(s, t_rho, l->t);
  if (c->sorts != NULL) scheduler_addunlock(s, t_xv, c->sorts);

  /* Recurse? */
  if (c->split)
    for (k = 0; k < 8; k++)
      if (c->progeny[k] != NULL)
        engine_addtasks_recv(e, c->progeny[k], t_xv, t_rho);
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#else
  error("SWIFT was not compiled with MPI support.");
#endif
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}
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/**
 * @brief Exchange cell structures with other nodes.
 *
 * @param e The #engine.
 */
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void engine_exchange_cells(struct engine *e) {
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#ifdef WITH_MPI

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  int j, k, pid, count = 0;
  struct pcell *pcells;
  struct space *s = e->s;
  struct cell *cells = s->cells;
  int nr_cells = s->nr_cells;
  int nr_proxies = e->nr_proxies;
  int offset[nr_cells];
  MPI_Request reqs_in[engine_maxproxies];
  MPI_Request reqs_out[engine_maxproxies];
  MPI_Status status;

  /* Run through the cells and get the size of the ones that will be sent off.
   */
  for (k = 0; k < nr_cells; k++) {
    offset[k] = count;
    if (cells[k].sendto)
      count += (cells[k].pcell_size = cell_getsize(&cells[k]));
  }
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  /* Allocate the pcells. */
  if ((pcells = (struct pcell *)malloc(sizeof(struct pcell) * count)) == NULL)
    error("Failed to allocate pcell buffer.");

  /* Pack the cells. */
  cell_next_tag = 0;
  for (k = 0; k < nr_cells; k++)
    if (cells[k].sendto) {
      cell_pack(&cells[k], &pcells[offset[k]]);
      cells[k].pcell = &pcells[offset[k]];
    }

  /* Launch the proxies. */
  for (k = 0; k < nr_proxies; k++) {
    proxy_cells_exch1(&e->proxies[k]);
    reqs_in[k] = e->proxies[k].req_cells_count_in;
    reqs_out[k] = e->proxies[k].req_cells_count_out;
  }

  /* Wait for each count to come in and start the recv. */
  for (k = 0; k < nr_proxies; k++) {
    if (MPI_Waitany(nr_proxies, reqs_in, &pid, &status) != MPI_SUCCESS ||
        pid == MPI_UNDEFINED)
      error("MPI_Waitany failed.");
    // message( "request from proxy %i has arrived." , pid );
    proxy_cells_exch2(&e->proxies[pid]);
  }

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  /* Wait for all the sends to have finished too. */
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  if (MPI_Waitall(nr_proxies, reqs_out, MPI_STATUSES_IGNORE) != MPI_SUCCESS)
    error("MPI_Waitall on sends failed.");

  /* Set the requests for the cells. */
  for (k = 0; k < nr_proxies; k++) {
    reqs_in[k] = e->proxies[k].req_cells_in;
    reqs_out[k] = e->proxies[k].req_cells_out;
  }

  /* Wait for each pcell array to come in from the proxies. */
  for (k = 0; k < nr_proxies; k++) {
    if (MPI_Waitany(nr_proxies, reqs_in, &pid, &status) != MPI_SUCCESS ||
        pid == MPI_UNDEFINED)
      error("MPI_Waitany failed.");
    // message( "cell data from proxy %i has arrived." , pid );
    for (count = 0, j = 0; j < e->proxies[pid].nr_cells_in; j++)
      count += cell_unpack(&e->proxies[pid].pcells_in[count],
                           e->proxies[pid].cells_in[j], e->s);
  }

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  /* Wait for all the sends to have finished too. */
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  if (MPI_Waitall(nr_proxies, reqs_out, MPI_STATUSES_IGNORE) != MPI_SUCCESS)
    error("MPI_Waitall on sends failed.");

  /* Count the number of particles we need to import and re-allocate
     the buffer if needed. */
  for (count = 0, k = 0; k < nr_proxies; k++)
    for (j = 0; j < e->proxies[k].nr_cells_in; j++)
      count += e->proxies[k].cells_in[j]->count;
  if (count > s->size_parts_foreign) {
    if (s->parts_foreign != NULL) free(s->parts_foreign);
    s->size_parts_foreign = 1.1 * count;
    if (posix_memalign((void **)&s->parts_foreign, part_align,
                       sizeof(struct part) * s->size_parts_foreign) != 0)
      error("Failed to allocate foreign part data.");
  }
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  /* Unpack the cells and link to the particle data. */
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  struct part *parts = s->parts_foreign;
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  for (k = 0; k < nr_proxies; k++) {
    for (count = 0, j = 0; j < e->proxies[k].nr_cells_in; j++) {
      count += cell_link(e->proxies[k].cells_in[j], parts);
      parts = &parts[e->proxies[k].cells_in[j]->count];
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    }
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  }
  s->nr_parts_foreign = parts - s->parts_foreign;

  /* Is the parts buffer large enough? */
  if (s->nr_parts_foreign > s->size_parts_foreign)
    error("Foreign parts buffer too small.");

  /* Free the pcell buffer. */
  free(pcells);
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#else
  error("SWIFT was not compiled with MPI support.");
#endif
}
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/**
 * @brief Exchange straying parts with other nodes.
 *
 * @param e The #engine.
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 * @param offset The index in the parts array as of which the foreign parts
 *reside.
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 * @param ind The ID of the foreign #cell.
 * @param N The number of stray parts.
 *
 * @return The number of arrived parts copied to parts and xparts.
 */
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int engine_exchange_strays(struct engine *e, int offset, int *ind, int N) {
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#ifdef WITH_MPI

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  int k, pid, count = 0, nr_in = 0, nr_out = 0;
  MPI_Request reqs_in[2 * engine_maxproxies];
  MPI_Request reqs_out[2 * engine_maxproxies];
  MPI_Status status;
  struct proxy *p;
  struct space *s = e->s;

  /* Re-set the proxies. */
  for (k = 0; k < e->nr_proxies; k++) e->proxies[k].nr_parts_out = 0;

  /* Put the parts into the corresponding proxies. */
  for (k = 0; k < N; k++) {
    int node_id = e->s->cells[ind[k]].nodeID;
    if (node_id < 0 || node_id >= e->nr_nodes)
      error("Bad node ID %i.", node_id);
    pid = e->proxy_ind[node_id];
    if (pid < 0)
      error(
          "Do not have a proxy for the requested nodeID %i for part with "
          "id=%llu, x=[%e,%e,%e].",
          node_id, s->parts[offset + k].id, s->parts[offset + k].x[0],
          s->parts[offset + k].x[1], s->parts[offset + k].x[2]);
    proxy_parts_load(&e->proxies[pid], &s->parts[offset + k],
                     &s->xparts[offset + k], 1);
  }

  /* Launch the proxies. */
  for (k = 0; k < e->nr_proxies; k++) {
    proxy_parts_exch1(&e->proxies[k]);
    reqs_in[k] = e->proxies[k].req_parts_count_in;
    reqs_out[k] = e->proxies[k].req_parts_count_out;
  }

  /* Wait for each count to come in and start the recv. */
  for (k = 0; k < e->nr_proxies; k++) {
    if (MPI_Waitany(e->nr_proxies, reqs_in, &pid, &status) != MPI_SUCCESS ||
        pid == MPI_UNDEFINED)
      error("MPI_Waitany failed.");
    // message( "request from proxy %i has arrived." , pid );
    proxy_parts_exch2(&e->proxies[pid]);
  }
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  /* Wait for all the sends to have finished too. */
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  if (MPI_Waitall(e->nr_proxies, reqs_out, MPI_STATUSES_IGNORE) != MPI_SUCCESS)
    error("MPI_Waitall on sends failed.");

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  /* Count the total number of incoming particles and make sure we have
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     enough space to accommodate them. */
  int count_in = 0;
  for (k = 0; k < e->nr_proxies; k++) count_in += e->proxies[k].nr_parts_in;
  message("sent out %i particles, got %i back.", N, count_in);
  if (offset + count_in > s->size_parts) {
    s->size_parts = (offset + count_in) * 1.05;
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    struct part *parts_new = NULL;
    struct xpart *xparts_new = NULL;
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    if (posix_memalign((void **)&parts_new, part_align,
                       sizeof(struct part) * s->size_parts) != 0 ||
        posix_memalign((void **)&xparts_new, part_align,
                       sizeof(struct xpart) * s->size_parts) != 0)
      error("Failed to allocate new part data.");
    memcpy(parts_new, s->parts, sizeof(struct part) * offset);
    memcpy(xparts_new, s->xparts, sizeof(struct xpart) * offset);
    free(s->parts);
    free(s->xparts);
    s->parts = parts_new;
    s->xparts = xparts_new;
  }

  /* Collect the requests for the particle data from the proxies. */
  for (k = 0; k < e->nr_proxies; k++) {
    if (e->proxies[k].nr_parts_in > 0) {
      reqs_in[2 * k] = e->proxies[k].req_parts_in;
      reqs_in[2 * k + 1] = e->proxies[k].req_xparts_in;
      nr_in += 1;
    } else
      reqs_in[2 * k] = reqs_in[2 * k + 1] = MPI_REQUEST_NULL;
    if (e->proxies[k].nr_parts_out > 0) {
      reqs_out[2 * k] = e->proxies[k].req_parts_out;
      reqs_out[2 * k + 1] = e->proxies[k].req_xparts_out;
      nr_out += 1;
    } else
      reqs_out[2 * k] = reqs_out[2 * k + 1] = MPI_REQUEST_NULL;
  }

  /* Wait for each part array to come in and collect the new
     parts from the proxies. */
  for (k = 0; k < 2 * (nr_in + nr_out); k++) {
    int err;
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