csci5451/assignments/02/qs_mpi.c

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

// https://stackoverflow.com/a/75458495
#define check_mpi_error(n) __check_mpi_error(__FILE__, __LINE__, n)

void __check_mpi_error(const char *file, const int line, const int n) {
  char errbuffer[MPI_MAX_ERROR_STRING];
  int errlen;

  if (n != MPI_SUCCESS) {
    MPI_Error_string(n, errbuffer, &errlen);
    printf("MPI-error: %s\n", errbuffer);
    printf("Location: %s:%i\n", file, line);
    MPI_Abort(MPI_COMM_WORLD, n);
  }
}

#define ORDER_FORWARDS 1
#define ORDER_BACKWARDS 2
#define CTL_SIZE 4
#define ROOT_RANK 0

#define GENERIC_MAX(x, y) ((x) > (y) ? (x) : (y))
#define GENERIC_MIN(x, y) ((x) < (y) ? (x) : (y))

#define ENSURE_int(i) _Generic((i), int : (i))
#define ENSURE_float(f) _Generic((f), float : (f))

#define MAX(type, x, y) (type) GENERIC_MAX(ENSURE_##type(x), ENSURE_##type(y))
#define MIN(type, x, y) (type) GENERIC_MIN(ENSURE_##type(x), ENSURE_##type(y))

void init_ctl(int *ctl, int len);
void local_quicksort(int *arr, int lo, int hi);
char *string_of_list(int *arr, int len);
void recursive_quicksort(int *integers, int n, int segment_capac,
                         int segment_len, int *integers_out, MPI_Comm comm);

int main(int argc, char **argv) {
  int rank, p;
  MPI_Init(&argc, &argv);

  int n = atoi(argv[1]);

  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &p);

  // Generate integers
  int n_over_p = n / p;
  int integers[n_over_p];

  // Minor implementation detail: srand(0) is specially handled by glibc to
  // behave as if it was called with srand(1). To get around this, I'm seeding
  // with rank + 1
  //
  // See more: https://stackoverflow.com/a/27386563
  srand(rank + 1);

  for (int i = 0; i < n_over_p; ++i) {
    integers[i] = rand() % 101;
  }

  int new_integers[n_over_p];
  recursive_quicksort(integers, n, n_over_p, n_over_p, new_integers,
                      MPI_COMM_WORLD);

  // The first node is responsible for collecting all the data and then
  // printing it out to the file

  FILE *fp;
  if (rank == ROOT_RANK)
    fp = fopen(argv[2], "w");

  for (int i = 0; i < p; i += 1) {
    if (rank == ROOT_RANK) {
      if (i != ROOT_RANK) {
        MPI_Recv(new_integers, n_over_p, MPI_INT, i, 129, MPI_COMM_WORLD,
                 MPI_STATUS_IGNORE);
      }

      for (int j = 0; j < n_over_p; ++j) {
        fprintf(fp, "%d\n", new_integers[j]);
      }
    } else if (rank == i) {
      MPI_Send(new_integers, n_over_p, MPI_INT, ROOT_RANK, 129, MPI_COMM_WORLD);
    }
  }

  if (rank == ROOT_RANK)
    fclose(fp);

  MPI_Finalize();
  return 0;
}

// hi not inclusive
void local_quicksort(int *arr, int lo, int hi) {
  int temp;

  if (lo >= hi || lo < 0)
    return;

  int pivot = arr[hi - 1];
  int pivot_idx = lo - 1;
  for (int j = lo; j < hi; ++j) {
    if (arr[j] < pivot) {
      pivot_idx += 1;

      temp = arr[j];
      arr[j] = arr[pivot_idx];
      arr[pivot_idx] = temp;
    }
  }

  pivot_idx += 1;
  temp = arr[hi - 1];
  arr[hi - 1] = arr[pivot_idx];
  arr[pivot_idx] = temp;

  // Recursive call
  local_quicksort(arr, lo, pivot_idx);
  local_quicksort(arr, pivot_idx + 1, hi);
}

// char *string_of_list(int *arr, int len) {
//   char *buffer = calloc(sizeof(char), 1000);
//   int offset = 0; // Keep track of the current position in the buffer
//   for (int i = 0; i < len; i++) {
//     offset += sprintf(buffer + offset, "%d", arr[i]);
//     if (i < len - 1) {
//       // Add a separator (e.g., comma or space) if it's not the last
//       element offset += sprintf(buffer + offset, " ");
//     }
//   }

//   return buffer;
// }

void recursive_quicksort(int *integers, int total_elems, int segment_capac,
                         int segment_len, int *integers_out, MPI_Comm comm) {
  int err, rank, p;
  MPI_Comm_size(comm, &p);
  MPI_Comm_rank(comm, &rank);

  if (p <= 1) {
    // Recursion base case: just sort it serially
    local_quicksort(integers, 0, total_elems);
    for (int i = 0; i < total_elems; ++i) {
      integers_out[i] = integers[i];
    }
    return;
  }

  // Select a pivot.
  // This pivot is broadcasted to all nodes
  int pivot;
  {
    // First, select a random element
    int rand_el = integers[rand() % segment_len];

    // Gather it
    int rand_els[p];
    MPI_Gather(&rand_el, 1, MPI_INT, rand_els, 1, MPI_INT, ROOT_RANK, comm);

    // Get the median
    if (rank == ROOT_RANK) {
      // Get the middle element after sorting
      local_quicksort(rand_els, 0, p);
      pivot = rand_els[p / 2];
    }

    MPI_Bcast(&pivot, 1, MPI_INT, ROOT_RANK, comm);
  }

  // Determine where the boundary between S (lower) and L (higher) lies
  int boundary = 0;
  for (int i = 0; i < segment_len; ++i) {
    if (integers[i] >= pivot) {
      boundary = i;
      break;
    }
  }

  int S_lo = 0, S_hi = boundary;
  int L_lo = boundary, L_hi = segment_len;
  int S_size = S_hi - S_lo, L_size = L_hi - L_lo;

  // Perform global arrangement
  int S_global_end = -1, L_reverse_end = -1, S_global_max_end = -1;
  MPI_Scan(&S_size, &S_global_end, 1, MPI_INT, MPI_SUM, comm);
  MPI_Scan(&L_size, &L_reverse_end, 1, MPI_INT, MPI_SUM, comm);

  int index;
  MPI_Scan(&segment_len, &index, 1, MPI_INT, MPI_SUM, comm);
  MPI_Allreduce(&S_global_end, &S_global_max_end, 1, MPI_INT, MPI_MAX, comm);

  int S_global_start = S_global_end - S_size,
      L_reverse_start = L_reverse_end - L_size,
      L_global_start = total_elems - L_reverse_end,
      L_global_end = total_elems - L_reverse_start;

  // Determine which process S's and L's destination will start in,
  // respectively
  int S_starting_process, L_starting_process;
  int p_of_split, split_point;
  int indexes[p];
  {
    MPI_Allgather(&index, 1, MPI_INT, indexes, 1, MPI_INT, comm);

    for (int i = 0; i < p; ++i) {
      int lo = i == 0 ? 0 : indexes[i - 1];
      int hi = indexes[i];
      if (S_global_start >= lo && S_global_start < hi)
        S_starting_process = i;
      if (L_global_start >= lo && L_global_start < hi)
        L_starting_process = i;
      if (S_global_max_end >= lo && S_global_max_end < hi) {
        p_of_split = i;
        split_point = S_global_max_end - lo;
      }
    }
  }

  int S_offset = S_global_start % segment_len,
      L_offset = L_global_start % segment_len;

  int S_ctl[p * CTL_SIZE];
  int L_ctl[p * CTL_SIZE];
  int S_send_ctl[p * CTL_SIZE];
  int L_send_ctl[p * CTL_SIZE];
  int ctl_send_counts[p];
  int ctl_send_displs[p];

  int send_counts[p];
  int send_displs[p];
  int recv_counts[p];
  int recv_displs[p];

  init_ctl(S_ctl, p);
  init_ctl(L_ctl, p);
  init_ctl(S_send_ctl, p);
  init_ctl(L_send_ctl, p);

  int SPACE = segment_capac;

  for (int i = 0; i < p; ++i) {
    send_counts[i] = SPACE;
    send_displs[i] = i * SPACE;

    ctl_send_counts[i] = CTL_SIZE;
    ctl_send_displs[i] = i * CTL_SIZE;
    recv_counts[i] = CTL_SIZE;
    recv_displs[i] = i * CTL_SIZE;
  }

  // Send S to the correct target
  if (S_size) {
    for (int i = S_lo, dest_pos = S_global_start,
             processor = S_starting_process;
         i < S_hi;) {
      int next_break =
          MIN(int, S_global_end,
              MIN(int, dest_pos + (S_hi - S_lo),
                  (dest_pos / segment_len) * segment_len + segment_len));
      int count = next_break - dest_pos;

      int from_local_start = i, from_local_end = i + count;
      int from_global_start = rank * segment_len + from_local_start,
          from_global_end = from_global_start + count;

      int to_global_start = dest_pos, to_global_end = dest_pos + count;
      int to_local_start = to_global_start - processor * segment_len,
          to_local_end = to_global_end - processor * segment_len;

      S_send_ctl[processor * CTL_SIZE] = count;
      S_send_ctl[processor * CTL_SIZE + 1] = from_global_start;
      S_send_ctl[processor * CTL_SIZE + 2] = to_local_start;
      S_send_ctl[processor * CTL_SIZE + 3] = from_local_start;

      i += count;
      dest_pos += count;
      processor += 1;
    }
  }

  MPI_Alltoallv(S_send_ctl, ctl_send_counts, ctl_send_displs, MPI_INT, S_ctl,
                recv_counts, recv_displs, MPI_INT, comm);

  // Send L to the correct target
  if (L_size) {
    for (int i = L_lo, dest_pos = L_global_start,
             processor = L_starting_process;
         i < L_hi;) {
      int next_break =
          MIN(int, L_global_end,
              MIN(int, dest_pos + (L_hi - L_lo),
                  (dest_pos / segment_len) * segment_len + segment_len));
      int count = next_break - dest_pos;

      int from_local_start = i, from_local_end = i + count;
      int from_global_start = rank * segment_len + from_local_start,
          from_global_end = from_global_start + count;

      int to_global_start = dest_pos, to_global_end = dest_pos + count;
      int to_local_start = to_global_start - processor * segment_len,
          to_local_end = to_global_end - processor * segment_len;

      L_send_ctl[processor * CTL_SIZE] = count;
      L_send_ctl[processor * CTL_SIZE + 1] = from_global_start;
      L_send_ctl[processor * CTL_SIZE + 2] = to_local_start;
      L_send_ctl[processor * CTL_SIZE + 3] = from_local_start;

      i += count;
      dest_pos += count;
      processor += 1;
    }
  }

  MPI_Alltoallv(L_send_ctl, ctl_send_counts, ctl_send_displs, MPI_INT, L_ctl,
                recv_counts, recv_displs, MPI_INT, comm);

  // After sending S and L information

  for (int i = 0; i < p; ++i) {
    recv_counts[i] = segment_len;
    recv_displs[i] = i * segment_len;
  }

  // Algorithm for sending S and L between all processes without O(n)

  int integers_recv_2[segment_capac];
  int integers_recv_3[segment_capac];
  for (int i = 0; i < segment_len; ++i) {
    integers_recv_2[i] = -1;
    integers_recv_3[i] = integers[i];
  }

  for (int host_p = 0; host_p < p; ++host_p) {
    if (rank == host_p) {
      // Your {S,L}_ctl is a mapping from source_processor -> ctl
      // Everyone already knows who needs to send to who now
      for (int sender_p = 0; sender_p < p; ++sender_p) {
        int S_count = S_ctl[sender_p * CTL_SIZE];
        if (S_count > 0) {
          int to_local_start = S_ctl[sender_p * CTL_SIZE + 2];
          int from_local_start = S_ctl[sender_p * CTL_SIZE + 3];

          if (sender_p == host_p) {
            for (int k = 0; k < S_count; ++k) {
              integers_recv_3[to_local_start + k] =
                  integers[from_local_start + k];
            }
            continue;
          }

          err = MPI_Recv(&integers_recv_2[to_local_start], S_count, MPI_INT,
                         sender_p, 124, comm, MPI_STATUS_IGNORE);
          check_mpi_error(err);
          for (int k = 0; k < S_count; ++k) {
            integers_recv_3[to_local_start + k] =
                integers_recv_2[to_local_start + k];
          }
        }
      }
    } else {
      // Your {S,L}_send_ctl contains a mapping from dest_processor -> ctl
      for (int dest_p = 0; dest_p < p; ++dest_p) {
        int S_count = S_send_ctl[dest_p * CTL_SIZE];
        if (S_count > 0 && dest_p == host_p) {
          int from_local_start = S_send_ctl[dest_p * CTL_SIZE + 3];
          MPI_Send(&integers[from_local_start], S_count, MPI_INT, dest_p, 124,
                   comm);
        }
      }
    }
  }

  for (int host_p = 0; host_p < p; ++host_p) {
    if (rank == host_p) {
      // Your {S,L}_ctl is a mapping from source_processor -> ctl
      // Everyone already knows who needs to send to who now
      for (int sender_p = 0; sender_p < p; ++sender_p) {
        int L_count = L_ctl[sender_p * CTL_SIZE];
        if (L_count > 0) {
          int to_local_start = L_ctl[sender_p * CTL_SIZE + 2];
          int from_local_start = L_ctl[sender_p * CTL_SIZE + 3];

          if (sender_p == host_p) {
            for (int k = 0; k < L_count; ++k) {
              integers_recv_3[to_local_start + k] =
                  integers[from_local_start + k];
            }
            continue;
          }

          err = MPI_Recv(&integers_recv_2[to_local_start], L_count, MPI_INT,
                         sender_p, 125, comm, MPI_STATUS_IGNORE);
          check_mpi_error(err);
          for (int k = 0; k < L_count; ++k) {
            integers_recv_3[to_local_start + k] =
                integers_recv_2[to_local_start + k];
          }
        }
      }
    } else {
      // Your {S,L}_send_ctl contains a mapping from dest_processor -> ctl
      for (int dest_p = 0; dest_p < p; ++dest_p) {
        int L_count = L_send_ctl[dest_p * CTL_SIZE];
        if (L_count > 0 && dest_p == host_p) {
          int from_local_start = L_send_ctl[dest_p * CTL_SIZE + 3];
          MPI_Send(&integers[from_local_start], L_count, MPI_INT, dest_p, 125,
                   comm);
        }
      }
    }
  }

  // ###################################################################################
  // SUBDIVIDING

  // Now, determine which processes should be responsible for taking the S and
  // L arrays. Specifically, the part where it's split, break the tie to see
  // if it goes down or up

  int child_len = segment_len;
  int difference = segment_len - split_point;
  int transfer[split_point];

  int has_split = 0;
  if (p_of_split == 0 || p_of_split == p - 1) {
    // Super unfortunate, bad pivot
  } else if (split_point == 0) {
    // Super lucky, it's split evenly!
  } else {
    has_split = 1;
    // Let's just say that if there's any split, the block itself counts as L
    // and then add the rest to the previous block
    if (rank == p_of_split - 1) {
      child_len += split_point;
      err = MPI_Recv(transfer, split_point, MPI_INT, p_of_split, 126, comm,
                     MPI_STATUS_IGNORE);
      check_mpi_error(err);
    } else if (rank == p_of_split) {
      child_len = difference;
      err = MPI_Send(integers, split_point, MPI_INT, p_of_split - 1, 126, comm);
      check_mpi_error(err);
    }
  }

  // Which group is this child going into?
  int color;
  if (rank < p_of_split)
    color = 100;
  else
    color = 200;

  MPI_Comm child_comm;
  MPI_Comm_split(comm, color, rank, &child_comm);

  // Figure out what the max is
  int max_child_buf_len, total_child_elems;
  err = MPI_Allreduce(&child_len, &max_child_buf_len, 1, MPI_INT, MPI_MAX,
                      child_comm);
  check_mpi_error(err);
  err = MPI_Allreduce(&child_len, &total_child_elems, 1, MPI_INT, MPI_SUM,
                      child_comm);
  check_mpi_error(err);

  // Copy into a new buf
  int new_buf[max_child_buf_len];
  int whichCase = 999;
  for (int i = 0; i < max_child_buf_len; ++i) {
    if (has_split && rank == p_of_split - 1) {
      whichCase = 1001;
      if (i < segment_len)
        new_buf[i] = integers_recv_3[i];
      else if (i < segment_len + split_point)
        new_buf[i] = transfer[i - segment_len];
      else
        new_buf[i] = -1;
    } else if (has_split && rank == p_of_split) {
      whichCase = 1002;
      if (i < difference)
        new_buf[i] = integers_recv_3[i + split_point];
      else
        new_buf[i] = -1;
    } else {
      whichCase = 1003;
      if (i < child_len)
        new_buf[i] = integers_recv_3[i];
      else
        new_buf[i] = -1;
    }
  }

  int integers_out_buf[total_child_elems];
  recursive_quicksort(new_buf, total_child_elems, max_child_buf_len, child_len,
                      integers_out_buf, child_comm);

  // Ok now copy the new items back
  switch (whichCase) {
  case 1001:
    // In this case, p is right before the split, so it got extra elements
    // To reverse this, we can send the elements back to the second
    for (int i = 0; i < total_child_elems; ++i) {
      if (i < segment_len)
        integers_out[i] = integers_out_buf[i];
      else
        transfer[i - segment_len] = integers_out_buf[i];
    }
    MPI_Send(transfer, split_point, MPI_INT, p_of_split, 127, comm);
    break;
  case 1002:
    // The original array got shortened, so copy the transferred ones back in
    // first, then copy the result from the child quicksorting after it
    MPI_Recv(transfer, split_point, MPI_INT, p_of_split - 1, 127, comm,
             MPI_STATUS_IGNORE);
    for (int i = 0; i < split_point; ++i) {
      integers_out[i] = transfer[i];
    }
    for (int i = 0; i < total_child_elems; ++i) {
      integers_out[i + split_point] = integers_out_buf[i];
    }
    break;
  case 1003:
    // This is just the regular case
    for (int i = 0; i < total_child_elems; ++i) {
      integers_out[i] = integers_out_buf[i];
    }
    break;
  }

  MPI_Comm_free(&child_comm);
}

void init_ctl(int *ctl, int len) {
  for (int i = 0; i < len; ++i) {
    ctl[i * CTL_SIZE] = 0;
    for (int j = 1; j < CTL_SIZE; ++j) {
      ctl[i * CTL_SIZE + j] = -1;
    }
  }
}