engine.c

/*******************************************************************************
 * This file is part of SWIFT.
 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
 *                    Matthieu Schaller (matthieu.schaller@durham.ac.uk)
 *               2015 Peter W. Draper (p.w.draper@durham.ac.uk)
 *                    Angus Lepper (angus.lepper@ed.ac.uk)
 *               2016 John A. Regan (john.a.regan@durham.ac.uk)
 *                    Tom Theuns (tom.theuns@durham.ac.uk)
 *
 * 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.
 *
 * 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.
 *
 * 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/>.
 *
 ******************************************************************************/

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

/* Some standard headers. */
#include <float.h>
#include <limits.h>
#include <sched.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/* MPI headers. */
#ifdef WITH_MPI
#include <mpi.h>
#endif

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

/* This object's header. */
#include "engine.h"

/* Local headers. */
#include "atomic.h"
#include "cell.h"
#include "clocks.h"
#include "cycle.h"
#include "debug.h"
#include "error.h"
#include "hydro.h"
#include "minmax.h"
#include "parallel_io.h"
#include "part.h"
#include "partition.h"
#include "proxy.h"
#include "runner.h"
#include "serial_io.h"
#include "single_io.h"
#include "timers.h"
#include "units.h"

const char *engine_policy_names[13] = {"none",
                                       "rand",
                                       "steal",
                                       "keep",
                                       "block",
                                       "fix_dt",
                                       "cpu_tight",
                                       "mpi",
                                       "numa_affinity",
                                       "hydro",
                                       "self_gravity",
                                       "external_gravity",
                                       "cosmology_integration"};

/** The rank of the engine as a global variable (for messages). */
int engine_rank;

#ifdef HAVE_SETAFFINITY
/** The initial affinity of the main thread (set by engin_pin()) */
static cpu_set_t entry_affinity;
#endif

/**
 * @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.
 */
struct link *engine_addlink(struct engine *e, struct link *l, struct task *t) {

  const int ind = atomic_inc(&e->nr_links);
  if (ind >= e->size_links) {
    error("Link table overflow.");
  }
  struct link *res = &e->links[ind];
  res->next = l;
  res->t = t;
  return res;
}

/**
 * @brief Generate the gravity hierarchical tasks for a hierarchy of cells -
 * i.e. all the O(Npart) tasks.
 *
 * Tasks are only created here. The dependencies will be added later on.
 *
 * @param e The #engine.
 * @param c The #cell.
 * @param super The super #cell.
 */
void engine_make_gravity_hierarchical_tasks(struct engine *e, struct cell *c,
                                            struct cell *super) {

  struct scheduler *s = &e->sched;
  const int is_with_external_gravity =
      (e->policy & engine_policy_external_gravity) ==
      engine_policy_external_gravity;
  const int is_fixdt = (e->policy & engine_policy_fixdt) == engine_policy_fixdt;

  /* Am I the super-cell? */
  /* TODO(pedro): Add a condition for gravity tasks as well. */
  if (super == NULL &&
      (c->density != NULL || (!c->split && (c->count > 0 || c->gcount > 0)))) {

    /* This is the super cell, i.e. the first with density tasks attached. */
    super = c;

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

      /* Add the init task. */
      if (c->init == NULL)
        c->init = scheduler_addtask(s, task_type_init, task_subtype_none, 0, 0,
                                    c, NULL, 0);

      /* Add the drift task. */
      if (c->drift == NULL)
        c->drift = scheduler_addtask(s, task_type_drift, task_subtype_none, 0,
                                     0, c, NULL, 0);

      /* Add the kick task that matches the policy. */
      if (is_fixdt) {
        if (c->kick == NULL)
          c->kick = scheduler_addtask(s, task_type_kick_fixdt,
                                      task_subtype_none, 0, 0, c, NULL, 0);
      } else {
        if (c->kick == NULL)
          c->kick = scheduler_addtask(s, task_type_kick, task_subtype_none, 0,
                                      0, c, NULL, 0);
      }

      if (is_with_external_gravity)
        c->grav_external = scheduler_addtask(
            s, task_type_grav_external, task_subtype_none, 0, 0, c, NULL, 0);
    }
  }

  /* Set the super-cell. */
  c->super = super;

  /* Recurse. */
  if (c->split)
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL)
        engine_make_gravity_hierarchical_tasks(e, c->progeny[k], super);
}

/**
 * @brief Generate the hydro hierarchical tasks for a hierarchy of cells -
 * i.e. all the O(Npart) tasks.
 *
 * Tasks are only created here. The dependencies will be added later on.
 *
 * @param e The #engine.
 * @param c The #cell.
 * @param super The super #cell.
 */
void engine_make_hydro_hierarchical_tasks(struct engine *e, struct cell *c,
                                          struct cell *super) {

  struct scheduler *s = &e->sched;
  const int is_fixdt = (e->policy & engine_policy_fixdt) == engine_policy_fixdt;

  /* Is this the super-cell? */
  if (super == NULL && (c->density != NULL || (c->count > 0 && !c->split))) {

    /* This is the super cell, i.e. the first with density tasks attached. */
    super = c;

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

      /* Add the init task. */
      if (c->init == NULL)
        c->init = scheduler_addtask(s, task_type_init, task_subtype_none, 0, 0,
                                    c, NULL, 0);

      /* Add the drift task. */
      if (c->drift == NULL)
        c->drift = scheduler_addtask(s, task_type_drift, task_subtype_none, 0,
                                     0, c, NULL, 0);

      /* Add the kick task that matches the policy. */
      if (is_fixdt) {
        if (c->kick == NULL)
          c->kick = scheduler_addtask(s, task_type_kick_fixdt,
                                      task_subtype_none, 0, 0, c, NULL, 0);
      } else {
        if (c->kick == NULL)
          c->kick = scheduler_addtask(s, task_type_kick, task_subtype_none, 0,
                                      0, c, NULL, 0);
      }

      /* Generate the ghost task. */
      c->ghost = scheduler_addtask(s, task_type_ghost, task_subtype_none, 0, 0,
                                   c, NULL, 0);
    }
  }

  /* Set the super-cell. */
  c->super = super;

  /* Recurse. */
  if (c->split)
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL)
        engine_make_hydro_hierarchical_tasks(e, c->progeny[k], super);
}

/**
 * @brief Redistribute the particles amongst the nodes according
 *      to their cell's node IDs.
 *
 * The strategy here is as follows:
 * 1) Each node counts the number of particles it has to send to each other
 * node.
 * 2) The number of particles of each type is then exchanged.
 * 3) The particles to send are placed in a temporary buffer in which the
 * part-gpart links are preserved.
 * 4) Each node allocates enough space for the new particles.
 * 5) (Asynchronous) communications are issued to transfer the data.
 *
 *
 * @param e The #engine.
 */
void engine_redistribute(struct engine *e) {

#ifdef WITH_MPI

  const int nr_nodes = e->nr_nodes;
  const int nodeID = e->nodeID;
  struct space *s = e->s;
  struct cell *cells = s->cells;
  const int nr_cells = s->nr_cells;
  const int *cdim = s->cdim;
  const double ih[3] = {s->ih[0], s->ih[1], s->ih[2]};
  const double dim[3] = {s->dim[0], s->dim[1], s->dim[2]};
  struct part *parts = s->parts;
  struct xpart *xparts = s->xparts;
  struct gpart *gparts = s->gparts;
  ticks tic = getticks();

  /* Allocate temporary arrays to store the counts of particles to be sent
     and the destination of each particle */
  int *counts, *g_counts;
  if ((counts = (int *)malloc(sizeof(int) * nr_nodes * nr_nodes)) == NULL)
    error("Failed to allocate count temporary buffer.");
  if ((g_counts = (int *)malloc(sizeof(int) * nr_nodes * nr_nodes)) == NULL)
    error("Failed to allocate gcount temporary buffer.");
  bzero(counts, sizeof(int) * nr_nodes * nr_nodes);
  bzero(g_counts, sizeof(int) * nr_nodes * nr_nodes);

  /* Allocate the destination index arrays. */
  int *dest, *g_dest;
  if ((dest = (int *)malloc(sizeof(int) * s->nr_parts)) == NULL)
    error("Failed to allocate dest temporary buffer.");
  if ((g_dest = (int *)malloc(sizeof(int) * s->nr_gparts)) == NULL)
    error("Failed to allocate g_dest temporary buffer.");

  /* Get destination of each particle */
  for (size_t k = 0; k < s->nr_parts; k++) {

    /* Periodic boundary conditions */
    for (int j = 0; j < 3; j++) {
      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];
    }
    const int cid = cell_getid(cdim, parts[k].x[0] * ih[0],
                               parts[k].x[1] * ih[1], parts[k].x[2] * ih[2]);
#ifdef SWIFT_DEBUG_CHECKS
    if (cid < 0 || cid >= s->nr_cells)
      error("Bad cell id %i for part %zi at [%.3e,%.3e,%.3e].", cid, k,
            parts[k].x[0], parts[k].x[1], parts[k].x[2]);
#endif

    dest[k] = cells[cid].nodeID;

    /* The counts array is indexed as count[from * nr_nodes + to]. */
    counts[nodeID * nr_nodes + dest[k]] += 1;
  }

  /* Sort the particles according to their cell index. */
  space_parts_sort(s, dest, s->nr_parts, 0, nr_nodes - 1, e->verbose);

  /* We need to re-link the gpart partners of parts. */
  if (s->nr_parts > 0) {
    int current_dest = dest[0];
    size_t count_this_dest = 0;
    for (size_t k = 0; k < s->nr_parts; ++k) {
      if (s->parts[k].gpart != NULL) {

        /* As the addresses will be invalidated by the communications, we will
         * instead store the absolute index from the start of the sub-array of
         * particles to be sent to a given node.
         * Recall that gparts without partners have a negative id.
         * We will restore the pointers on the receiving node later on. */
        if (dest[k] != current_dest) {
          current_dest = dest[k];
          count_this_dest = 0;
        }

#ifdef SWIFT_DEBUG_CHECKS
        if (s->parts[k].gpart->id_or_neg_offset >= 0)
          error("Trying to link a partnerless gpart !");
#endif

        s->parts[k].gpart->id_or_neg_offset = -count_this_dest;
        count_this_dest++;
      }
    }
  }

  /* Get destination of each g-particle */
  for (size_t k = 0; k < s->nr_gparts; k++) {

    /* Periodic boundary conditions */
    for (int j = 0; j < 3; j++) {
      if (gparts[k].x[j] < 0.0)
        gparts[k].x[j] += dim[j];
      else if (gparts[k].x[j] >= dim[j])
        gparts[k].x[j] -= dim[j];
    }
    const int cid = cell_getid(cdim, gparts[k].x[0] * ih[0],
                               gparts[k].x[1] * ih[1], gparts[k].x[2] * ih[2]);
#ifdef SWIFT_DEBUG_CHECKS
    if (cid < 0 || cid >= s->nr_cells)
      error("Bad cell id %i for part %zi at [%.3e,%.3e,%.3e].", cid, k,
            gparts[k].x[0], gparts[k].x[1], gparts[k].x[2]);
#endif

    g_dest[k] = cells[cid].nodeID;

    /* The counts array is indexed as count[from * nr_nodes + to]. */
    g_counts[nodeID * nr_nodes + g_dest[k]] += 1;
  }

  /* Sort the gparticles according to their cell index. */
  space_gparts_sort(s, g_dest, s->nr_gparts, 0, nr_nodes - 1, e->verbose);

  /* 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 all the g_counts from all the nodes. */
  if (MPI_Allreduce(MPI_IN_PLACE, g_counts, nr_nodes * nr_nodes, MPI_INT,
                    MPI_SUM, MPI_COMM_WORLD) != MPI_SUCCESS)
    error("Failed to allreduce gparticle transfer counts.");

  /* Each node knows how many parts and gparts will be transferred to every
     other node. We can start preparing to receive data */

  /* Get the new number of parts and gparts for this node */
  size_t nr_parts = 0, nr_gparts = 0;
  for (int k = 0; k < nr_nodes; k++) nr_parts += counts[k * nr_nodes + nodeID];
  for (int k = 0; k < nr_nodes; k++)
    nr_gparts += g_counts[k * nr_nodes + nodeID];

  /* Allocate the new arrays with some extra margin */
  struct part *parts_new = NULL;
  struct xpart *xparts_new = NULL;
  struct gpart *gparts_new = NULL;
  if (posix_memalign((void **)&parts_new, part_align,
                     sizeof(struct part) * nr_parts *
                         engine_redistribute_alloc_margin) != 0)
    error("Failed to allocate new part data.");
  if (posix_memalign((void **)&xparts_new, xpart_align,
                     sizeof(struct xpart) * nr_parts *
                         engine_redistribute_alloc_margin) != 0)
    error("Failed to allocate new xpart data.");
  if (posix_memalign((void **)&gparts_new, gpart_align,
                     sizeof(struct gpart) * nr_gparts *
                         engine_redistribute_alloc_margin) != 0)
    error("Failed to allocate new gpart data.");

  /* Prepare MPI requests for the asynchronous communications */
  MPI_Request *reqs;
  if ((reqs = (MPI_Request *)malloc(sizeof(MPI_Request) * 6 * nr_nodes)) ==
      NULL)
    error("Failed to allocate MPI request list.");
  for (int k = 0; k < 6 * nr_nodes; k++) reqs[k] = MPI_REQUEST_NULL;

  /* Emit the sends and recvs for the particle and gparticle data. */
  size_t offset_send = 0, offset_recv = 0;
  size_t g_offset_send = 0, g_offset_recv = 0;
  for (int k = 0; k < nr_nodes; k++) {

    /* Indices in the count arrays of the node of interest */
    const int ind_send = nodeID * nr_nodes + k;
    const int ind_recv = k * nr_nodes + nodeID;

    /* Are we sending any part/xpart ? */
    if (counts[ind_send] > 0) {

      /* message("Sending %d part to node %d", counts[ind_send], k); */

      /* If the send is to the same node, just copy */
      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, emit some communications */
      } else {
        if (MPI_Isend(&s->parts[offset_send], counts[ind_send], part_mpi_type,
                      k, 3 * ind_send + 0, MPI_COMM_WORLD,
                      &reqs[6 * k]) != MPI_SUCCESS)
          error("Failed to isend parts to node %i.", k);
        if (MPI_Isend(&s->xparts[offset_send], counts[ind_send], xpart_mpi_type,
                      k, 3 * ind_send + 1, MPI_COMM_WORLD,
                      &reqs[6 * k + 1]) != MPI_SUCCESS)
          error("Failed to isend xparts to node %i.", k);
        offset_send += counts[ind_send];
      }
    }

    /* Are we sending any gpart ? */
    if (g_counts[ind_send] > 0) {

      /* message("Sending %d gpart to node %d", g_counts[ind_send], k); */

      /* If the send is to the same node, just copy */
      if (k == nodeID) {
        memcpy(&gparts_new[g_offset_recv], &s->gparts[g_offset_send],
               sizeof(struct gpart) * g_counts[ind_recv]);
        g_offset_send += g_counts[ind_send];
        g_offset_recv += g_counts[ind_recv];

        /* Else, emit some communications */
      } else {
        if (MPI_Isend(&s->gparts[g_offset_send], g_counts[ind_send],
                      gpart_mpi_type, k, 3 * ind_send + 2, MPI_COMM_WORLD,
                      &reqs[6 * k + 2]) != MPI_SUCCESS)
          error("Failed to isend gparts to node %i.", k);
        g_offset_send += g_counts[ind_send];
      }
    }

    /* Now emit the corresponding Irecv() */

    /* Are we receiving any part/xpart from this node ? */
    if (k != nodeID && counts[ind_recv] > 0) {
      if (MPI_Irecv(&parts_new[offset_recv], counts[ind_recv], part_mpi_type, k,
                    3 * ind_recv + 0, MPI_COMM_WORLD,
                    &reqs[6 * k + 3]) != MPI_SUCCESS)
        error("Failed to emit irecv of parts from node %i.", k);
      if (MPI_Irecv(&xparts_new[offset_recv], counts[ind_recv], xpart_mpi_type,
                    k, 3 * ind_recv + 1, MPI_COMM_WORLD,
                    &reqs[6 * k + 4]) != MPI_SUCCESS)
        error("Failed to emit irecv of xparts from node %i.", k);
      offset_recv += counts[ind_recv];
    }

    /* Are we receiving any gpart from this node ? */
    if (k != nodeID && g_counts[ind_recv] > 0) {
      if (MPI_Irecv(&gparts_new[g_offset_recv], g_counts[ind_recv],
                    gpart_mpi_type, k, 3 * ind_recv + 2, MPI_COMM_WORLD,
                    &reqs[6 * k + 5]) != MPI_SUCCESS)
        error("Failed to emit irecv of gparts from node %i.", k);
      g_offset_recv += g_counts[ind_recv];
    }
  }

  /* Wait for all the sends and recvs to tumble in. */
  MPI_Status stats[6 * nr_nodes];
  int res;
  if ((res = MPI_Waitall(6 * nr_nodes, reqs, stats)) != MPI_SUCCESS) {
    for (int k = 0; k < 6 * nr_nodes; k++) {
      char buff[MPI_MAX_ERROR_STRING];
      MPI_Error_string(stats[k].MPI_ERROR, buff, &res);
      message("request %i has error '%s'.", k, buff);
    }
    error("Failed during waitall for part data.");
  }

  /* We now need to restore the part<->gpart links */
  size_t offset_parts = 0, offset_gparts = 0;
  for (int node = 0; node < nr_nodes; ++node) {

    const int ind_recv = node * nr_nodes + nodeID;
    const size_t count_parts = counts[ind_recv];
    const size_t count_gparts = g_counts[ind_recv];

    /* Loop over the gparts received from that node */
    for (size_t k = offset_gparts; k < offset_gparts + count_gparts; ++k) {

      /* Does this gpart have a partner ? */
      if (gparts_new[k].id_or_neg_offset <= 0) {

        const ptrdiff_t partner_index =
            offset_parts - gparts_new[k].id_or_neg_offset;

        /* Re-link */
        gparts_new[k].id_or_neg_offset = -partner_index;
        parts_new[partner_index].gpart = &gparts_new[k];
      }
    }

    offset_parts += count_parts;
    offset_gparts += count_gparts;
  }

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify that all parts are in the right place. */
  for (int k = 0; k < nr_parts; k++) {
    int 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);
  }

  /* Verify that the links are correct */
  for (size_t k = 0; k < nr_gparts; ++k) {

    if (gparts_new[k].id_or_neg_offset <= 0) {

      struct part *part = &parts_new[-gparts_new[k].id_or_neg_offset];

      if (part->gpart != &gparts_new[k]) error("Linking problem !");

      if (gparts_new[k].x[0] != part->x[0] ||
          gparts_new[k].x[1] != part->x[1] || gparts_new[k].x[2] != part->x[2])
        error("Linked particles are not at the same position !");
    }
  }
  for (size_t k = 0; k < nr_parts; ++k) {

    if (parts_new[k].gpart != NULL &&
        parts_new[k].gpart->id_or_neg_offset != -k) {
      error("Linking problem !");
    }
  }
#endif

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

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

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

  if (e->verbose)
    message("took %.3f %s.", clocks_from_ticks(getticks() - tic),
            clocks_getunit());
#else
  error("SWIFT was not compiled with MPI support.");
#endif
}

/**
 * @brief Repartition the cells amongst the nodes.
 *
 * @param e The #engine.
 */
void engine_repartition(struct engine *e) {

#if defined(WITH_MPI) && defined(HAVE_METIS)

  ticks tic = getticks();

  /* Clear the repartition flag. */
  enum repartition_type reparttype = e->forcerepart;
  e->forcerepart = REPART_NONE;

  /* Nothing to do if only using a single node. Also avoids METIS
   * bug that doesn't handle this case well. */
  if (e->nr_nodes == 1) return;

  /* Do the repartitioning. */
  partition_repartition(reparttype, e->nodeID, e->nr_nodes, e->s,
                        e->sched.tasks, e->sched.nr_tasks);

  /* 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
     the parts array, and emitting the sends and receives.
     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;

  if (e->verbose)
    message("took %.3f %s.", clocks_from_ticks(getticks() - tic),
            clocks_getunit());
#else
  error("SWIFT was not compiled with MPI and METIS support.");
#endif
}

/**
 * @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.
 */
void engine_addtasks_grav(struct engine *e, struct cell *c, struct task *up,
                          struct task *down) {

  /* 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);
}

/**
 * @brief Add send tasks to a hierarchy of cells.
 *
 * @param e The #engine.
 * @param ci The sending #cell.
 * @param cj Dummy cell containing the nodeID of the receiving node.
 * @param t_xv The send_xv #task, if it has already been created.
 * @param t_rho The send_rho #task, if it has already been created.
 * @param t_ti The send_ti #task, if required and has already been created.
 */
void engine_addtasks_send(struct engine *e, struct cell *ci, struct cell *cj,
                          struct task *t_xv, struct task *t_rho,
                          struct task *t_ti) {

#ifdef WITH_MPI
  struct link *l = NULL;
  struct scheduler *s = &e->sched;
  const int nodeID = cj->nodeID;

  /* 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 == nodeID ||
        (l->t->cj != NULL && l->t->cj->nodeID == nodeID))
      break;

  /* If so, attach send tasks. */
  if (l != NULL) {

    /* Create the tasks and their dependencies? */
    if (t_xv == NULL) {
      t_xv = scheduler_addtask(s, task_type_send, task_subtype_none,
                               3 * ci->tag, 0, ci, cj, 0);
      t_rho = scheduler_addtask(s, task_type_send, task_subtype_none,
                                3 * ci->tag + 1, 0, ci, cj, 0);
      if (!(e->policy & engine_policy_fixdt))
        t_ti = scheduler_addtask(s, task_type_send, task_subtype_tend,
                                 3 * ci->tag + 2, 0, ci, cj, 0);

      /* The send_rho task depends on the cell's ghost task. */
      scheduler_addunlock(s, ci->super->ghost, t_rho);

      /* The send_rho task should unlock the super-cell's kick task. */
      scheduler_addunlock(s, t_rho, ci->super->kick);

      /* The send_xv task should unlock the super-cell's ghost task. */
      scheduler_addunlock(s, t_xv, ci->super->ghost);

      /* The super-cell's kick task should unlock the send_ti task. */
      if (t_ti != NULL) scheduler_addunlock(s, ci->super->kick, t_ti);
    }

    /* Add them to the local cell. */
    ci->send_xv = engine_addlink(e, ci->send_xv, t_xv);
    ci->send_rho = engine_addlink(e, ci->send_rho, t_rho);
    if (t_ti != NULL) ci->send_ti = engine_addlink(e, ci->send_ti, t_ti);
  }

  /* Recurse? */
  if (ci->split)
    for (int k = 0; k < 8; k++)
      if (ci->progeny[k] != NULL)
        engine_addtasks_send(e, ci->progeny[k], cj, t_xv, t_rho, t_ti);

#else
  error("SWIFT was not compiled with MPI support.");
#endif
}

/**
 * @brief Add recv tasks to a hierarchy of cells.
 *
 * @param e The #engine.
 * @param c The foreign #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.
 * @param t_ti The recv_ti #task, if required and has already been created.
 */
void engine_addtasks_recv(struct engine *e, struct cell *c, struct task *t_xv,
                          struct task *t_rho, struct task *t_ti) {

#ifdef WITH_MPI
  struct scheduler *s = &e->sched;

  /* Do we need to construct a recv task?
     Note that since c is a foreign cell, all its density tasks will involve
     only the current rank, and thus we don't have to check them.*/
  if (t_xv == NULL && c->density != NULL) {

    /* Create the tasks. */
    t_xv = scheduler_addtask(s, task_type_recv, task_subtype_none, 3 * c->tag,
                             0, c, NULL, 0);
    t_rho = scheduler_addtask(s, task_type_recv, task_subtype_none,
                              3 * c->tag + 1, 0, c, NULL, 0);
    if (!(e->policy & engine_policy_fixdt))
      t_ti = scheduler_addtask(s, task_type_recv, task_subtype_tend,
                               3 * c->tag + 2, 0, c, NULL, 0);
  }
  c->recv_xv = t_xv;
  c->recv_rho = t_rho;
  c->recv_ti = t_ti;

  /* 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 (t_ti != NULL) scheduler_addunlock(s, l->t, t_ti);
  }
  if (c->sorts != NULL) scheduler_addunlock(s, t_xv, c->sorts);

  /* Recurse? */
  if (c->split)
    for (int k = 0; k < 8; k++)
      if (c->progeny[k] != NULL)
        engine_addtasks_recv(e, c->progeny[k], t_xv, t_rho, t_ti);

#else
  error("SWIFT was not compiled with MPI support.");
#endif
}

/**
 * @brief Exchange cell structures with other nodes.
 *
 * @param e The #engine.
 */
void engine_exchange_cells(struct engine *e) {

#ifdef WITH_MPI

  struct space *s = e->s;
  struct cell *cells = s->cells;
  const int nr_cells = s->nr_cells;
  const 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;
  ticks tic = getticks();

  /* Run through the cells and get the size of the ones that will be sent off.
   */
  int count_out = 0;
  for (int k = 0; k < nr_cells; k++) {
    offset[k] = count_out;
    if (cells[k].sendto)
      count_out += (cells[k].pcell_size = cell_getsize(&cells[k]));
  }

  /* Allocate the pcells. */
  struct pcell *pcells;
  if ((pcells = (struct pcell *)malloc(sizeof(struct pcell) * count_out)) ==
      NULL)
    error("Failed to allocate pcell buffer.");

  /* Pack the cells. */
  cell_next_tag = 0;
  for (int 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 (int 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 (int k = 0; k < nr_proxies; k++) {
    int pid = MPI_UNDEFINED;
    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]);
  }

  /* Wait for all the sends to have finished too. */
  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 (int 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 (int k = 0; k < nr_proxies; k++) {
    int pid = MPI_UNDEFINED;
    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 (int 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);
  }

  /* Wait for all the sends to have finished too. */
  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. */
  int count_parts_in = 0, count_gparts_in = 0;
  for (int k = 0; k < nr_proxies; k++)
    for (int j = 0; j < e->proxies[k].nr_cells_in; j++) {
      count_parts_in += e->proxies[k].cells_in[j]->count;
      count_gparts_in += e->proxies[k].cells_in[j]->gcount;
    }
  if (count_parts_in > s->size_parts_foreign) {
    if (s->parts_foreign != NULL) free(s->parts_foreign);
    s->size_parts_foreign = 1.1 * count_parts_in;
    if (posix_memalign((void **)&s->parts_foreign, part_align,
                       sizeof(struct part) * s->size_parts_foreign) != 0)
      error("Failed to allocate foreign part data.");
  }
  if (count_gparts_in > s->size_gparts_foreign) {
    if (s->gparts_foreign != NULL) free(s->gparts_foreign);
    s->size_gparts_foreign = 1.1 * count_gparts_in;
    if (posix_memalign((void **)&s->gparts_foreign, gpart_align,
                       sizeof(struct gpart) * s->size_gparts_foreign) != 0)
      error("Failed to allocate foreign gpart data.");
  }

  /* Unpack the cells and link to the particle data. */
  struct part *parts = s->parts_foreign;
  struct gpart *gparts = s->gparts_foreign;
  for (int k = 0; k < nr_proxies; k++) {
    for (int j = 0; j < e->proxies[k].nr_cells_in; j++) {
      cell_link_parts(e->proxies[k].cells_in[j], parts);
      cell_link_gparts(e->proxies[k].cells_in[j], gparts);
      parts = &parts[e->proxies[k].cells_in[j]->count];
      gparts = &gparts[e->proxies[k].cells_in[j]->gcount];
    }
  }
  s->nr_parts_foreign = parts - s->parts_foreign;
  s->nr_gparts_foreign = gparts - s->gparts_foreign;