scheduler.c

/*******************************************************************************
 * This file is part of SWIFT.
 * Copyright (c) 2012 Pedro Gonnet (pedro.gonnet@durham.ac.uk)
 *                    Matthieu Schaller (matthieu.schaller@durham.ac.uk)
 *               2016 Peter W. Draper (p.w.draper@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 <limits.h>
#include <math.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

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

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

/* Local headers. */
#include "atomic.h"
#include "cycle.h"
#include "engine.h"
#include "error.h"
#include "intrinsics.h"
#include "kernel_hydro.h"
#include "queue.h"
#include "sort_part.h"
#include "space.h"
#include "task.h"
#include "timers.h"

/**
 * @brief Re-set the list of active tasks.
 */
void scheduler_clear_active(struct scheduler *s) { s->active_count = 0; }

/**
 * @brief Add an unlock_task to the given task.
 *
 * @param s The #scheduler.
 * @param ta The unlocking #task.
 * @param tb The #task that will be unlocked.
 */
void scheduler_addunlock(struct scheduler *s, struct task *ta,
                         struct task *tb) {
#ifdef SWIFT_DEBUG_CHECKS
  if (ta == NULL) error("Unlocking task is NULL.");
  if (tb == NULL) error("Unlocked task is NULL.");
#endif

  /* Get an index at which to store this unlock. */
  const int ind = atomic_inc(&s->nr_unlocks);

  /* Does the buffer need to be grown? */
  if (ind == s->size_unlocks) {
    /* Allocate the new buffer. */
    struct task **unlocks_new;
    int *unlock_ind_new;
    const int size_unlocks_new = s->size_unlocks * 2;
    if ((unlocks_new = (struct task **)malloc(sizeof(struct task *) *
                                              size_unlocks_new)) == NULL ||
        (unlock_ind_new = (int *)malloc(sizeof(int) * size_unlocks_new)) ==
            NULL)
      error("Failed to re-allocate unlocks.");

    /* Wait for all writes to the old buffer to complete. */
    while (s->completed_unlock_writes < ind)
      ;

    /* Copy the buffers. */
    memcpy(unlocks_new, s->unlocks, sizeof(struct task *) * ind);
    memcpy(unlock_ind_new, s->unlock_ind, sizeof(int) * ind);
    free(s->unlocks);
    free(s->unlock_ind);
    s->unlocks = unlocks_new;
    s->unlock_ind = unlock_ind_new;

    /* Publish the new buffer size. */
    s->size_unlocks = size_unlocks_new;
  }

  /* Wait for there to actually be space at my index. */
  while (ind > s->size_unlocks)
    ;

  /* Write the unlock to the scheduler. */
  s->unlocks[ind] = tb;
  s->unlock_ind[ind] = ta - s->tasks;
  atomic_inc(&s->completed_unlock_writes);
}

/**
 * @brief Split a hydrodynamic task if too large.
 *
 * @param t The #task
 * @param s The #scheduler we are working in.
 */
static void scheduler_splittask_hydro(struct task *t, struct scheduler *s) {

  /* Iterate on this task until we're done with it. */
  int redo = 1;
  while (redo) {

    /* Reset the redo flag. */
    redo = 0;

    /* Non-splittable task? */
    if ((t->ci == NULL) || (t->type == task_type_pair && t->cj == NULL)) {
      t->type = task_type_none;
      t->subtype = task_subtype_none;
      t->cj = NULL;
      t->skip = 1;
      break;
    }

    /* Self-interaction? */
    if (t->type == task_type_self) {

      /* Get a handle on the cell involved. */
      struct cell *ci = t->ci;

      /* Foreign task? */
      if (ci->nodeID != s->nodeID) {
        t->skip = 1;
        break;
      }

      /* Is this cell even split and the task does not violate h ? */
      if (cell_can_split_self_task(ci)) {

        /* Make a sub? */
        if (scheduler_dosub && ci->count < space_subsize_self) {

          /* convert to a self-subtask. */
          t->type = task_type_sub_self;

          /* Otherwise, make tasks explicitly. */
        } else {

          /* Take a step back (we're going to recycle the current task)... */
          redo = 1;

          /* Add the self tasks. */
          int first_child = 0;
          while (ci->progeny[first_child] == NULL) first_child++;
          t->ci = ci->progeny[first_child];
          for (int k = first_child + 1; k < 8; k++)
            if (ci->progeny[k] != NULL)
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_self, t->subtype, 0, 0,
                                    ci->progeny[k], NULL),
                  s);

          /* Make a task for each pair of progeny */
          for (int j = 0; j < 8; j++)
            if (ci->progeny[j] != NULL)
              for (int k = j + 1; k < 8; k++)
                if (ci->progeny[k] != NULL)
                  scheduler_splittask_hydro(
                      scheduler_addtask(s, task_type_pair, t->subtype,
                                        sub_sid_flag[j][k], 0, ci->progeny[j],
                                        ci->progeny[k]),
                      s);
        }
      } /* Cell is split */

    } /* Self interaction */

    /* Pair interaction? */
    else if (t->type == task_type_pair) {

      /* Get a handle on the cells involved. */
      struct cell *ci = t->ci;
      struct cell *cj = t->cj;

      /* Foreign task? */
      if (ci->nodeID != s->nodeID && cj->nodeID != s->nodeID) {
        t->skip = 1;
        break;
      }

      /* Get the sort ID, use space_getsid and not t->flags
         to make sure we get ci and cj swapped if needed. */
      double shift[3];
      const int sid = space_getsid(s->space, &ci, &cj, shift);

      /* Should this task be split-up? */
      if (cell_can_split_pair_task(ci) && cell_can_split_pair_task(cj)) {

        /* Replace by a single sub-task? */
        if (scheduler_dosub && ci->count &&
            cj->count && /* Use division to avoid integer overflow. */
            ci->count * sid_scale[sid] < space_subsize_pair / cj->count &&
            !sort_is_corner(sid)) {

          /* Make this task a sub task. */
          t->type = task_type_sub_pair;

          /* Otherwise, split it. */
        } else {

          /* Take a step back (we're going to recycle the current task)... */
          redo = 1;

          /* For each different sorting type... */
          switch (sid) {

            case 0: /* (  1 ,  1 ,  1 ) */
              t->ci = ci->progeny[7];
              t->cj = cj->progeny[0];
              t->flags = 0;
              break;

            case 1: /* (  1 ,  1 ,  0 ) */
              t->ci = ci->progeny[6];
              t->cj = cj->progeny[0];
              t->flags = 1;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 1, 0,
                                    ci->progeny[7], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[6], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              break;

            case 2: /* (  1 ,  1 , -1 ) */
              t->ci = ci->progeny[6];
              t->cj = cj->progeny[1];
              t->flags = 2;
              break;

            case 3: /* (  1 ,  0 ,  1 ) */
              t->ci = ci->progeny[5];
              t->cj = cj->progeny[0];
              t->flags = 3;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 3, 0,
                                    ci->progeny[7], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[5], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 6, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              break;

            case 4: /* (  1 ,  0 ,  0 ) */
              t->ci = ci->progeny[4];
              t->cj = cj->progeny[0];
              t->flags = 4;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 5, 0,
                                    ci->progeny[5], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[6], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 3, 0,
                                    ci->progeny[4], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 4, 0,
                                    ci->progeny[5], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 6, 0,
                                    ci->progeny[6], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[7], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 1, 0,
                                    ci->progeny[4], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[5], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 4, 0,
                                    ci->progeny[6], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 5, 0,
                                    ci->progeny[7], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[4], cj->progeny[3]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 1, 0,
                                    ci->progeny[5], cj->progeny[3]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 3, 0,
                                    ci->progeny[6], cj->progeny[3]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 4, 0,
                                    ci->progeny[7], cj->progeny[3]),
                  s);
              break;

            case 5: /* (  1 ,  0 , -1 ) */
              t->ci = ci->progeny[4];
              t->cj = cj->progeny[1];
              t->flags = 5;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 5, 0,
                                    ci->progeny[6], cj->progeny[3]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[4], cj->progeny[3]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[6], cj->progeny[1]),
                  s);
              break;

            case 6: /* (  1 , -1 ,  1 ) */
              t->ci = ci->progeny[5];
              t->cj = cj->progeny[2];
              t->flags = 6;
              break;

            case 7: /* (  1 , -1 ,  0 ) */
              t->ci = ci->progeny[4];
              t->cj = cj->progeny[3];
              t->flags = 6;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[5], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[4], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[5], cj->progeny[3]),
                  s);
              break;

            case 8: /* (  1 , -1 , -1 ) */
              t->ci = ci->progeny[4];
              t->cj = cj->progeny[3];
              t->flags = 8;
              break;

            case 9: /* (  0 ,  1 ,  1 ) */
              t->ci = ci->progeny[3];
              t->cj = cj->progeny[0];
              t->flags = 9;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 9, 0,
                                    ci->progeny[7], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[3], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              break;

            case 10: /* (  0 ,  1 ,  0 ) */
              t->ci = ci->progeny[2];
              t->cj = cj->progeny[0];
              t->flags = 10;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 11, 0,
                                    ci->progeny[3], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[6], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 6, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 9, 0,
                                    ci->progeny[2], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 10, 0,
                                    ci->progeny[3], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[6], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 7, 0,
                                    ci->progeny[7], cj->progeny[1]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 1, 0,
                                    ci->progeny[2], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[3], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 10, 0,
                                    ci->progeny[6], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 11, 0,
                                    ci->progeny[7], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[2], cj->progeny[5]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 1, 0,
                                    ci->progeny[3], cj->progeny[5]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 9, 0,
                                    ci->progeny[6], cj->progeny[5]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 10, 0,
                                    ci->progeny[7], cj->progeny[5]),
                  s);
              break;

            case 11: /* (  0 ,  1 , -1 ) */
              t->ci = ci->progeny[2];
              t->cj = cj->progeny[1];
              t->flags = 11;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 11, 0,
                                    ci->progeny[6], cj->progeny[5]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[2], cj->progeny[5]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 6, 0,
                                    ci->progeny[6], cj->progeny[1]),
                  s);
              break;

            case 12: /* (  0 ,  0 ,  1 ) */
              t->ci = ci->progeny[1];
              t->cj = cj->progeny[0];
              t->flags = 12;
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 11, 0,
                                    ci->progeny[3], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 5, 0,
                                    ci->progeny[5], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 2, 0,
                                    ci->progeny[7], cj->progeny[0]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 9, 0,
                                    ci->progeny[1], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 12, 0,
                                    ci->progeny[3], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 8, 0,
                                    ci->progeny[5], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 5, 0,
                                    ci->progeny[7], cj->progeny[2]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 3, 0,
                                    ci->progeny[1], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 6, 0,
                                    ci->progeny[3], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 12, 0,
                                    ci->progeny[5], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 11, 0,
                                    ci->progeny[7], cj->progeny[4]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                    ci->progeny[1], cj->progeny[6]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 3, 0,
                                    ci->progeny[3], cj->progeny[6]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 9, 0,
                                    ci->progeny[5], cj->progeny[6]),
                  s);
              scheduler_splittask_hydro(
                  scheduler_addtask(s, task_type_pair, t->subtype, 12, 0,
                                    ci->progeny[7], cj->progeny[6]),
                  s);
              break;
          } /* switch(sid) */
        }

        /* Otherwise, break it up if it is too large? */
      } else if (scheduler_doforcesplit && ci->split && cj->split &&
                 ci->count && cj->count &&
                 (ci->count > space_maxsize / cj->count)) {

        // message( "force splitting pair with %i and %i parts." , ci->count ,
        // cj->count );

        /* Replace the current task. */
        t->type = task_type_none;

        for (int j = 0; j < 8; j++)
          if (ci->progeny[j] != NULL)
            for (int k = 0; k < 8; k++)
              if (cj->progeny[k] != NULL) {
                struct task *tl =
                    scheduler_addtask(s, task_type_pair, t->subtype, 0, 0,
                                      ci->progeny[j], cj->progeny[k]);
                scheduler_splittask_hydro(tl, s);
                tl->flags = space_getsid(s->space, &t->ci, &t->cj, shift);
              }
      }
    } /* pair interaction? */
  }   /* iterate over the current task. */
}

/**
 * @brief Split a gravity task if too large.
 *
 * @param t The #task
 * @param s The #scheduler we are working in.
 */
static void scheduler_splittask_gravity(struct task *t, struct scheduler *s) {

  /* Iterate on this task until we're done with it. */
  int redo = 1;
  while (redo) {

    /* Reset the redo flag. */
    redo = 0;

    /* Non-splittable task? */
    if ((t->ci == NULL) || (t->type == task_type_pair && t->cj == NULL)) {
      t->type = task_type_none;
      t->subtype = task_subtype_none;
      t->cj = NULL;
      t->skip = 1;
      break;
    }

    /* Self-interaction? */
    if (t->type == task_type_self) {

      /* Get a handle on the cell involved. */
      struct cell *ci = t->ci;

      /* Foreign task? */
      if (ci->nodeID != s->nodeID) {
        t->skip = 1;
        break;
      }

      /* Is this cell even split? */
      if (ci->split) {

        /* Make a sub? */
        if (scheduler_dosub && ci->gcount < space_subsize_self) {

          /* convert to a self-subtask. */
          t->type = task_type_sub_self;

          /* Make sure we have a drift task (MATTHIEU temp. fix) */
          lock_lock(&ci->lock);
          if (ci->drift_gpart == NULL)
            ci->drift_gpart = scheduler_addtask(
                s, task_type_drift_gpart, task_subtype_none, 0, 0, ci, NULL);
          lock_unlock_blind(&ci->lock);

          /* Otherwise, make tasks explicitly. */
        } else {

          /* Take a step back (we're going to recycle the current task)... */
          redo = 1;

          /* Add the self tasks. */
          int first_child = 0;
          while (ci->progeny[first_child] == NULL) first_child++;
          t->ci = ci->progeny[first_child];
          for (int k = first_child + 1; k < 8; k++)
            if (ci->progeny[k] != NULL)
              scheduler_splittask_gravity(
                  scheduler_addtask(s, task_type_self, t->subtype, 0, 0,
                                    ci->progeny[k], NULL),
                  s);

          /* Make a task for each pair of progeny */
          if (t->subtype != task_subtype_external_grav) {
            for (int j = 0; j < 8; j++)
              if (ci->progeny[j] != NULL)
                for (int k = j + 1; k < 8; k++)
                  if (ci->progeny[k] != NULL)
                    scheduler_splittask_gravity(
                        scheduler_addtask(s, task_type_pair, t->subtype,
                                          sub_sid_flag[j][k], 0, ci->progeny[j],
                                          ci->progeny[k]),
                        s);
          }
        }
      } /* Cell is split */

      /* Otherwise, make sure the self task has a drift task */
      else {

        lock_lock(&ci->lock);

        if (ci->drift_gpart == NULL)
          ci->drift_gpart = scheduler_addtask(
              s, task_type_drift_gpart, task_subtype_none, 0, 0, ci, NULL);
        lock_unlock_blind(&ci->lock);
      }
    } /* Self interaction */

    /* Pair interaction? */
    else if (t->type == task_type_pair) {

      /* Get a handle on the cells involved. */
      struct cell *ci = t->ci;
      struct cell *cj = t->cj;

      /* Foreign task? */
      if (ci->nodeID != s->nodeID && cj->nodeID != s->nodeID) {
        t->skip = 1;
        break;
      }

      /* Should this task be split-up? */
      if (ci->split && cj->split) {

        // MATTHIEU: nothing here for now

      } else {

        /* Create the drift for ci. */
        lock_lock(&ci->lock);
        if (ci->drift_gpart == NULL && ci->nodeID == engine_rank)
          ci->drift_gpart = scheduler_addtask(
              s, task_type_drift_gpart, task_subtype_none, 0, 0, ci, NULL);
        lock_unlock_blind(&ci->lock);

        /* Create the drift for cj. */
        lock_lock(&cj->lock);
        if (cj->drift_gpart == NULL && cj->nodeID == engine_rank)
          cj->drift_gpart = scheduler_addtask(
              s, task_type_drift_gpart, task_subtype_none, 0, 0, cj, NULL);
        lock_unlock_blind(&cj->lock);
      }
    } /* pair interaction? */
  }   /* iterate over the current task. */
}

/**
 * @brief Mapper function to split tasks that may be too large.
 *
 * @param map_data the tasks to process
 * @param num_elements the number of tasks.
 * @param extra_data The #scheduler we are working in.
 */
void scheduler_splittasks_mapper(void *map_data, int num_elements,
                                 void *extra_data) {

  /* Extract the parameters. */
  struct scheduler *s = (struct scheduler *)extra_data;
  struct task *tasks = (struct task *)map_data;

  for (int ind = 0; ind < num_elements; ind++) {
    struct task *t = &tasks[ind];

    /* Invoke the correct splitting strategy */
    if (t->subtype == task_subtype_density) {
      scheduler_splittask_hydro(t, s);
    } else if (t->subtype == task_subtype_external_grav) {
      scheduler_splittask_gravity(t, s);
    } else if (t->subtype == task_subtype_grav) {
      scheduler_splittask_gravity(t, s);
    } else if (t->type == task_type_grav_top_level ||
               t->type == task_type_grav_ghost) {
      // MATTHIEU: for the future
    } else {
      error("Unexpected task sub-type");
    }
  }
}

/**
 * @brief Splits all the tasks in the scheduler that are too large.
 *
 * @param s The #scheduler.
 */
void scheduler_splittasks(struct scheduler *s) {

  /* Call the mapper on each current task. */
  threadpool_map(s->threadpool, scheduler_splittasks_mapper, s->tasks,
                 s->nr_tasks, sizeof(struct task), 1000, s);
}

/**
 * @brief Add a #task to the #scheduler.
 *
 * @param s The #scheduler we are working in.
 * @param type The type of the task.
 * @param subtype The sub-type of the task.
 * @param flags The flags of the task.
 * @param implicit If true, only use this task to unlock dependencies, i.e.
 *        this task is never enqueued.
 * @param ci The first cell to interact.
 * @param cj The second cell to interact.
 */
struct task *scheduler_addtask(struct scheduler *s, enum task_types type,
                               enum task_subtypes subtype, int flags,
                               int implicit, struct cell *ci, struct cell *cj) {

#ifdef SWIFT_DEBUG_CHECKS
  if (ci == NULL && cj != NULL)
    error("Added a task with ci==NULL and cj!=NULL type=%s/%s",
          taskID_names[type], subtaskID_names[subtype]);
#endif

  /* Get the next free task. */
  const int ind = atomic_inc(&s->tasks_next);

  /* Overflow? */
  if (ind >= s->size) error("Task list overflow.");

  /* Get a pointer to the new task. */
  struct task *t = &s->tasks[ind];

  /* Copy the data. */
  t->type = type;
  t->subtype = subtype;
  t->flags = flags;
  t->wait = 0;
  t->ci = ci;
  t->cj = cj;
  t->skip = 1; /* Mark tasks as skip by default. */
  t->implicit = implicit;
  t->weight = 0;
  t->rank = 0;
  t->nr_unlock_tasks = 0;
#ifdef SWIFT_DEBUG_TASKS
  t->rid = -1;
  t->tic = 0;
  t->toc = 0;
#endif

  /* Add an index for it. */
  // lock_lock( &s->lock );
  s->tasks_ind[atomic_inc(&s->nr_tasks)] = ind;
  // lock_unlock_blind( &s->lock );

  /* Return a pointer to the new task. */
  return t;
}

/**
 * @brief Set the unlock pointers in each task.
 *
 * @param s The #scheduler.
 */
void scheduler_set_unlocks(struct scheduler *s) {

  /* Store the counts for each task. */
  short int *counts;
  if ((counts = (short int *)malloc(sizeof(short int) * s->nr_tasks)) == NULL)
    error("Failed to allocate temporary counts array.");
  bzero(counts, sizeof(short int) * s->nr_tasks);
  for (int k = 0; k < s->nr_unlocks; k++) {
    counts[s->unlock_ind[k]] += 1;

#ifdef SWIFT_DEBUG_CHECKS
    /* Check that we are not overflowing */
    if (counts[s->unlock_ind[k]] < 0)
      error("Task unlocking more than %d other tasks!",
            (1 << (8 * sizeof(short int) - 1)) - 1);

#endif
  }

  /* Compute the offset for each unlock block. */
  int *offsets;
  if ((offsets = (int *)malloc(sizeof(int) * (s->nr_tasks + 1))) == NULL)
    error("Failed to allocate temporary offsets array.");
  offsets[0] = 0;
  for (int k = 0; k < s->nr_tasks; k++) {
    offsets[k + 1] = offsets[k] + counts[k];

#ifdef SWIFT_DEBUG_CHECKS
    /* Check that we are not overflowing */
    if (offsets[k + 1] < 0) error("Task unlock offset array overflowing");
#endif
  }