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 "active.h"
#include "atomic.h"
#include "cell.h"
#include "clocks.h"
#include "cycle.h"
#include "debug.h"
#include "error.h"
#include "gravity.h"
#include "hydro.h"
#include "minmax.h"
#include "parallel_io.h"
#include "part.h"
#include "partition.h"
#include "profiler.h"
#include "proxy.h"
#include "runner.h"
#include "serial_io.h"
#include "single_io.h"
#include "sort_part.h"
#include "statistics.h"
#include "timers.h"
#include "tools.h"
#include "units.h"
#include "version.h"

/* Particle cache size. */
#define CACHE_SIZE 512

const char *engine_policy_names[16] = {"none",
                                       "rand",
                                       "steal",
                                       "keep",
                                       "block",
                                       "cpu_tight",
                                       "mpi",
                                       "numa_affinity",
                                       "hydro",
                                       "self_gravity",
                                       "external_gravity",
                                       "cosmology_integration",
                                       "drift_all",
                                       "cooling",
                                       "sourceterms",
                                       "stars"};

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

/**
 * @brief Link a density/force task to a cell.
 *
 * @param e The #engine.
 * @param l A pointer to the #link, will be modified atomically.
 * @param t The #task.
 *
 * @return The new #link pointer.
 */
void engine_addlink(struct engine *e, struct link **l, struct task *t) {

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

  /* Set it atomically. */
  res->t = t;
  res->next = atomic_swap(l, res);
}

/**
 * @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.
 *
 * Note that there is no need to recurse below the super-cell.
 *
 * @param e The #engine.
 * @param c The #cell.
 */
void engine_make_hierarchical_tasks(struct engine *e, struct cell *c) {

  struct scheduler *s = &e->sched;
  const int is_hydro = (e->policy & engine_policy_hydro);
  const int is_self_gravity = (e->policy & engine_policy_self_gravity);
  const int is_with_cooling = (e->policy & engine_policy_cooling);
  const int is_with_sourceterms = (e->policy & engine_policy_sourceterms);

  /* Are we in a super-cell ? */
  if (c->super == c) {

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

      /* Add the two half kicks */
      c->kick1 = scheduler_addtask(s, task_type_kick1, task_subtype_none, 0, 0,
                                   c, NULL);

      c->kick2 = scheduler_addtask(s, task_type_kick2, task_subtype_none, 0, 0,
                                   c, NULL);

      /* Add the time-step calculation task and its dependency */
      c->timestep = scheduler_addtask(s, task_type_timestep, task_subtype_none,
                                      0, 0, c, NULL);

      scheduler_addunlock(s, c->kick2, c->timestep);
      scheduler_addunlock(s, c->timestep, c->kick1);

      if (is_self_gravity) {

        /* Initialisation of the multipoles */
        c->init_grav = scheduler_addtask(s, task_type_init_grav,
                                         task_subtype_none, 0, 0, c, NULL);

        /* Gravity non-neighbouring pm calculations */
        c->grav_long_range = scheduler_addtask(
            s, task_type_grav_long_range, task_subtype_none, 0, 0, c, NULL);

        /* Gravity top-level periodic calculation */
        c->grav_top_level = scheduler_addtask(s, task_type_grav_top_level,
                                              task_subtype_none, 0, 0, c, NULL);

        /* Gravity recursive down-pass */
        c->grav_down = scheduler_addtask(s, task_type_grav_down,
                                         task_subtype_none, 0, 0, c, NULL);

        scheduler_addunlock(s, c->init_grav, c->grav_long_range);
        scheduler_addunlock(s, c->init_grav, c->grav_top_level);
        scheduler_addunlock(s, c->grav_long_range, c->grav_down);
        scheduler_addunlock(s, c->grav_top_level, c->grav_down);
        scheduler_addunlock(s, c->grav_down, c->kick2);
      }

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

#ifdef EXTRA_HYDRO_LOOP
      /* Generate the extra ghost task. */
      if (is_hydro)
        c->extra_ghost = scheduler_addtask(s, task_type_extra_ghost,
                                           task_subtype_none, 0, 0, c, NULL);
#endif

      /* Cooling task */
      if (is_with_cooling) {
        c->cooling = scheduler_addtask(s, task_type_cooling, task_subtype_none,
                                       0, 0, c, NULL);

        scheduler_addunlock(s, c->cooling, c->kick2);
      }

      /* add source terms */
      if (is_with_sourceterms) {
        c->sourceterms = scheduler_addtask(s, task_type_sourceterms,
                                           task_subtype_none, 0, 0, c, NULL);
      }
    }

  } else { /* We are above the super-cell so need to go deeper */

#ifdef SWIFT_DEBUG_CHECKS
    if (c->super != NULL) error("Incorrectly set super pointer");
#endif

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

/**
 * @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_top;
  const int nr_cells = s->nr_cells;
  const int *cdim = s->cdim;
  const double iwidth[3] = {s->iwidth[0], s->iwidth[1], s->iwidth[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;
  struct spart *sparts = s->sparts;
  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, *s_counts;
  if ((counts = (int *)malloc(sizeof(int) * nr_nodes * nr_nodes)) == NULL)
    error("Failed to allocate counts temporary buffer.");
  if ((g_counts = (int *)malloc(sizeof(int) * nr_nodes * nr_nodes)) == NULL)
    error("Failed to allocate g_gcount temporary buffer.");
  if ((s_counts = (int *)malloc(sizeof(int) * nr_nodes * nr_nodes)) == NULL)
    error("Failed to allocate s_counts temporary buffer.");
  bzero(counts, sizeof(int) * nr_nodes * nr_nodes);
  bzero(g_counts, sizeof(int) * nr_nodes * nr_nodes);
  bzero(s_counts, sizeof(int) * nr_nodes * nr_nodes);

  /* Allocate the destination index arrays. */
  int *dest, *g_dest, *s_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.");
  if ((s_dest = (int *)malloc(sizeof(int) * s->nr_sparts)) == NULL)
    error("Failed to allocate s_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] * iwidth[0], parts[k].x[1] * iwidth[1],
                   parts[k].x[2] * iwidth[2]);
#ifdef SWIFT_DEBUG_CHECKS
    if (cid < 0 || cid >= s->nr_cells)
      error("Bad cell id %i for part %zu 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. */
  if (s->nr_parts > 0)
    space_parts_sort(s, dest, s->nr_parts, 0, nr_nodes - 1, e->verbose);

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify that the part have been sorted correctly. */
  for (size_t k = 0; k < s->nr_parts; k++) {
    const struct part *p = &s->parts[k];

    /* New cell index */
    const int new_cid =
        cell_getid(s->cdim, p->x[0] * s->iwidth[0], p->x[1] * s->iwidth[1],
                   p->x[2] * s->iwidth[2]);

    /* New cell of this part */
    const struct cell *c = &s->cells_top[new_cid];
    const int new_node = c->nodeID;

    if (dest[k] != new_node)
      error("part's new node index not matching sorted index.");

    if (p->x[0] < c->loc[0] || p->x[0] > c->loc[0] + c->width[0] ||
        p->x[1] < c->loc[1] || p->x[1] > c->loc[1] + c->width[1] ||
        p->x[2] < c->loc[2] || p->x[2] > c->loc[2] + c->width[2])
      error("part not sorted into the right top-level cell!");
  }
#endif

  /* 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 positive 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 s-particle */
  for (size_t k = 0; k < s->nr_sparts; k++) {

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

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

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

  /* Sort the particles according to their cell index. */
  if (s->nr_sparts > 0)
    space_sparts_sort(s, s_dest, s->nr_sparts, 0, nr_nodes - 1, e->verbose);

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify that the spart have been sorted correctly. */
  for (size_t k = 0; k < s->nr_sparts; k++) {
    const struct spart *sp = &s->sparts[k];

    /* New cell index */
    const int new_cid =
        cell_getid(s->cdim, sp->x[0] * s->iwidth[0], sp->x[1] * s->iwidth[1],
                   sp->x[2] * s->iwidth[2]);

    /* New cell of this spart */
    const struct cell *c = &s->cells_top[new_cid];
    const int new_node = c->nodeID;

    if (s_dest[k] != new_node)
      error("spart's new node index not matching sorted index.");

    if (sp->x[0] < c->loc[0] || sp->x[0] > c->loc[0] + c->width[0] ||
        sp->x[1] < c->loc[1] || sp->x[1] > c->loc[1] + c->width[1] ||
        sp->x[2] < c->loc[2] || sp->x[2] > c->loc[2] + c->width[2])
      error("spart not sorted into the right top-level cell!");
  }
#endif

  /* We need to re-link the gpart partners of sparts. */
  if (s->nr_sparts > 0) {
    int current_dest = s_dest[0];
    size_t count_this_dest = 0;
    for (size_t k = 0; k < s->nr_sparts; ++k) {
      if (s->sparts[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 positive id.
         * We will restore the pointers on the receiving node later on. */
        if (s_dest[k] != current_dest) {
          current_dest = s_dest[k];
          count_this_dest = 0;
        }

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

        s->sparts[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] * iwidth[0], gparts[k].x[1] * iwidth[1],
                   gparts[k].x[2] * iwidth[2]);
#ifdef SWIFT_DEBUG_CHECKS
    if (cid < 0 || cid >= s->nr_cells)
      error("Bad cell id %i for part %zu 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. */
  if (s->nr_gparts > 0)
    space_gparts_sort(s, g_dest, s->nr_gparts, 0, nr_nodes - 1, e->verbose);

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify that the gpart have been sorted correctly. */
  for (size_t k = 0; k < s->nr_gparts; k++) {
    const struct gpart *gp = &s->gparts[k];

    /* New cell index */
    const int new_cid =
        cell_getid(s->cdim, gp->x[0] * s->iwidth[0], gp->x[1] * s->iwidth[1],
                   gp->x[2] * s->iwidth[2]);

    /* New cell of this gpart */
    const struct cell *c = &s->cells_top[new_cid];
    const int new_node = c->nodeID;

    if (g_dest[k] != new_node)
      error("gpart's new node index not matching sorted index.");

    if (gp->x[0] < c->loc[0] || gp->x[0] > c->loc[0] + c->width[0] ||
        gp->x[1] < c->loc[1] || gp->x[1] > c->loc[1] + c->width[1] ||
        gp->x[2] < c->loc[2] || gp->x[2] > c->loc[2] + c->width[2])
      error("gpart not sorted into the right top-level cell!");
  }
#endif

  /* 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 s_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.");

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

  /* Report how many particles will be moved. */
  if (e->verbose) {
    if (e->nodeID == 0) {
      size_t total = 0, g_total = 0, s_total = 0;
      size_t unmoved = 0, g_unmoved = 0, s_unmoved = 0;
      for (int p = 0, r = 0; p < nr_nodes; p++) {
        for (int s = 0; s < nr_nodes; s++) {
          total += counts[r];
          g_total += g_counts[r];
          s_total += s_counts[r];
          if (p == s) {
            unmoved += counts[r];
            g_unmoved += g_counts[r];
            s_unmoved += s_counts[r];
          }
          r++;
        }
      }
      if (total > 0)
        message("%ld of %ld (%.2f%%) of particles moved", total - unmoved,
                total, 100.0 * (double)(total - unmoved) / (double)total);
      if (g_total > 0)
        message("%ld of %ld (%.2f%%) of g-particles moved", g_total - g_unmoved,
                g_total,
                100.0 * (double)(g_total - g_unmoved) / (double)g_total);
      if (s_total > 0)
        message("%ld of %ld (%.2f%%) of s-particles moved", s_total - s_unmoved,
                s_total,
                100.0 * (double)(s_total - s_unmoved) / (double)s_total);
    }
  }

  /* Each node knows how many parts, sparts 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, nr_sparts = 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];
  for (int k = 0; k < nr_nodes; k++)
    nr_sparts += s_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;
  struct spart *sparts_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.");
  if (posix_memalign((void **)&sparts_new, spart_align,
                     sizeof(struct spart) * nr_sparts *
                         engine_redistribute_alloc_margin) != 0)
    error("Failed to allocate new spart data.");

  /* Prepare MPI requests for the asynchronous communications */
  MPI_Request *reqs;
  if ((reqs = (MPI_Request *)malloc(sizeof(MPI_Request) * 8 * nr_nodes)) ==
      NULL)
    error("Failed to allocate MPI request list.");
  for (int k = 0; k < 8 * 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;
  size_t s_offset_send = 0, s_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, 4 * ind_send + 0, MPI_COMM_WORLD,
                      &reqs[8 * k + 0]) != 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, 4 * ind_send + 1, MPI_COMM_WORLD,
                      &reqs[8 * 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, 4 * ind_send + 2, MPI_COMM_WORLD,
                      &reqs[8 * k + 2]) != MPI_SUCCESS)
          error("Failed to isend gparts to node %i.", k);
        g_offset_send += g_counts[ind_send];
      }
    }

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

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

      /* If the send is to the same node, just copy */
      if (k == nodeID) {
        memcpy(&sparts_new[s_offset_recv], &s->sparts[s_offset_send],
               sizeof(struct spart) * s_counts[ind_recv]);
        s_offset_send += s_counts[ind_send];
        s_offset_recv += s_counts[ind_recv];

        /* Else, emit some communications */
      } else {
        if (MPI_Isend(&s->sparts[s_offset_send], s_counts[ind_send],
                      spart_mpi_type, k, 4 * ind_send + 3, MPI_COMM_WORLD,
                      &reqs[8 * k + 3]) != MPI_SUCCESS)
          error("Failed to isend gparts to node %i.", k);
        s_offset_send += s_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,
                    4 * ind_recv + 0, MPI_COMM_WORLD,
                    &reqs[8 * k + 4]) != 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, 4 * ind_recv + 1, MPI_COMM_WORLD,
                    &reqs[8 * k + 5]) != 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, 4 * ind_recv + 2, MPI_COMM_WORLD,
                    &reqs[8 * k + 6]) != MPI_SUCCESS)
        error("Failed to emit irecv of gparts from node %i.", k);
      g_offset_recv += g_counts[ind_recv];
    }

    /* Are we receiving any spart from this node ? */
    if (k != nodeID && s_counts[ind_recv] > 0) {
      if (MPI_Irecv(&sparts_new[s_offset_recv], s_counts[ind_recv],
                    spart_mpi_type, k, 4 * ind_recv + 3, MPI_COMM_WORLD,
                    &reqs[8 * k + 7]) != MPI_SUCCESS)
        error("Failed to emit irecv of sparts from node %i.", k);
      s_offset_recv += s_counts[ind_recv];
    }
  }

  /* Wait for all the sends and recvs to tumble in. */
  MPI_Status stats[8 * nr_nodes];
  int res;
  if ((res = MPI_Waitall(8 * nr_nodes, reqs, stats)) != MPI_SUCCESS) {
    for (int k = 0; k < 8 * 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.");
  }

  /* All particles have now arrived. Time for some final operations on the
     stuff we just received */

  /* Restore the part<->gpart and spart<->gpart links */
  size_t offset_parts = 0, offset_sparts = 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];
    const size_t count_sparts = s_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 gas partner ? */
      if (gparts_new[k].type == swift_type_gas) {

        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];
      }

      /* Does this gpart have a star partner ? */
      if (gparts_new[k].type == swift_type_star) {

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

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

    offset_parts += count_parts;
    offset_gparts += count_gparts;
    offset_sparts += count_sparts;
  }

#ifdef SWIFT_DEBUG_CHECKS
  /* Verify that all parts are in the right place. */
  for (size_t k = 0; k < nr_parts; k++) {
    const int cid = cell_getid(cdim, parts_new[k].x[0] * iwidth[0],
                               parts_new[k].x[1] * iwidth[1],
                               parts_new[k].x[2] * iwidth[2]);
    if (cells[cid].nodeID != nodeID)
      error("Received particle (%zu) that does not belong here (nodeID=%i).", k,
            cells[cid].nodeID);
  }
  for (size_t k = 0; k < nr_gparts; k++) {
    const int cid = cell_getid(cdim, gparts_new[k].x[0] * iwidth[0],
                               gparts_new[k].x[1] * iwidth[1],
                               gparts_new[k].x[2] * iwidth[2]);
    if (cells[cid].nodeID != nodeID)
      error("Received g-particle (%zu) that does not belong here (nodeID=%i).",
            k, cells[cid].nodeID);
  }
  for (size_t k = 0; k < nr_sparts; k++) {
    const int cid = cell_getid(cdim, sparts_new[k].x[0] * iwidth[0],
                               sparts_new[k].x[1] * iwidth[1],
                               sparts_new[k].x[2] * iwidth[2]);
    if (cells[cid].nodeID != nodeID)
      error("Received s-particle (%zu) that does not belong here (nodeID=%i).",
            k, cells[cid].nodeID);
  }

  /* Verify that the links are correct */
  part_verify_links(parts_new, gparts_new, sparts_new, nr_parts, nr_gparts,
                    nr_sparts, e->verbose);
#endif

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

  /* 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 %zu parts, %zu sparts and %zu gparts in %i cells.",
            nodeID, nr_parts, nr_sparts, 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();

#ifdef SWIFT_DEBUG_CHECKS
  /* Be verbose about this. */
  if (e->nodeID == 0 || e->verbose) message("repartitioning space");
  fflush(stdout);

  /* Check that all cells have been drifted to the current time */
  space_check_drift_point(e->s, e->ti_old,
                          e->policy & engine_policy_self_gravity);
#endif

  /* Clear the repartition flag. */
  e->forcerepart = 0;

  /* 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(e->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