starpu-max/mpi/src/starpu_mpi_insert_task.c

/* StarPU --- Runtime system for heterogeneous multicore architectures.
 *
 * Copyright (C) 2011, 2012  Centre National de la Recherche Scientifique
 * Copyright (C) 2011-2012  Université de Bordeaux 1
 *
 * StarPU 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 2.1 of the License, or (at
 * your option) any later version.
 *
 * StarPU 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 Lesser General Public License in COPYING.LGPL for more details.
 */

#include <stdarg.h>
#include <mpi.h>

#include <starpu.h>
#include <starpu_data.h>
#include <common/utils.h>
#include <common/uthash.h>
#include <util/starpu_insert_task_utils.h>
#include <datawizard/coherency.h>

//#define STARPU_MPI_VERBOSE 1
#include <starpu_mpi_private.h>

/* Whether we are allowed to keep copies of remote data. */
struct _starpu_data_entry
{
	UT_hash_handle hh;
	void *data;
};

static struct _starpu_data_entry **_cache_sent_data = NULL;
static struct _starpu_data_entry **_cache_received_data = NULL;
static int _cache_enabled=1;

void _starpu_mpi_cache_init(MPI_Comm comm)
{
	int nb_nodes;
	int i;

	_cache_enabled = starpu_get_env_number("STARPU_MPI_CACHE");
	if (_cache_enabled == -1)
	{
		_cache_enabled = 1;
	}

	if (_cache_enabled == 0)
	{
		if (!getenv("STARPU_SILENT")) fprintf(stderr,"Warning: StarPU MPI Communication cache is disabled\n");
		return;
	}

	MPI_Comm_size(comm, &nb_nodes);
	_STARPU_MPI_DEBUG("Initialising htable for cache\n");
	_cache_sent_data = malloc(nb_nodes * sizeof(struct _starpu_data_entry *));
	for(i=0 ; i<nb_nodes ; i++) _cache_sent_data[i] = NULL;
	_cache_received_data = malloc(nb_nodes * sizeof(struct _starpu_data_entry *));
	for(i=0 ; i<nb_nodes ; i++) _cache_received_data[i] = NULL;
}

void _starpu_mpi_cache_empty_tables(int world_size)
{
	int i;

	if (_cache_enabled == 0) return;

	_STARPU_MPI_DEBUG("Clearing htable for cache\n");

	for(i=0 ; i<world_size ; i++)
	{
		struct _starpu_data_entry *entry, *tmp;
		HASH_ITER(hh, _cache_sent_data[i], entry, tmp)
		{
			HASH_DEL(_cache_sent_data[i], entry);
			free(entry);
		}
		HASH_ITER(hh, _cache_received_data[i], entry, tmp)
		{
			HASH_DEL(_cache_received_data[i], entry);
			free(entry);
		}
	}
}

void _starpu_mpi_cache_free(int world_size)
{
	if (_cache_enabled == 0) return;

	_starpu_mpi_cache_empty_tables(world_size);
	free(_cache_sent_data);
	free(_cache_received_data);
}

void starpu_mpi_cache_flush_all_data(MPI_Comm comm)
{
	int nb_nodes;

	if (_cache_enabled == 0) return;

	MPI_Comm_size(comm, &nb_nodes);
	_starpu_mpi_cache_empty_tables(nb_nodes);
}

void starpu_mpi_cache_flush(MPI_Comm comm, starpu_data_handle_t data_handle)
{
	struct _starpu_data_entry *avail;
	int i, nb_nodes;

	if (_cache_enabled == 0) return;

	MPI_Comm_size(comm, &nb_nodes);
	for(i=0 ; i<nb_nodes ; i++)
	{
		HASH_FIND_PTR(_cache_sent_data[i], &data_handle, avail);
		if (avail)
		{
			_STARPU_MPI_DEBUG("Clearing send cache for data %p\n", data_handle);
			HASH_DEL(_cache_sent_data[i], avail);
		}
		HASH_FIND_PTR(_cache_received_data[i], &data_handle, avail);
		if (avail)
		{
			_STARPU_MPI_DEBUG("Clearing send cache for data %p\n", data_handle);
			HASH_DEL(_cache_received_data[i], avail);
		}
	}
}

static
void *_starpu_mpi_already_received(starpu_data_handle_t data, int mpi_rank)
{
	if (_cache_enabled == 0) return NULL;

	struct _starpu_data_entry *already_received;
	HASH_FIND_PTR(_cache_received_data[mpi_rank], &data, already_received);
	if (already_received == NULL)
	{
		struct _starpu_data_entry *entry = (struct _starpu_data_entry *)malloc(sizeof(*entry));
		entry->data = data;
		HASH_ADD_PTR(_cache_received_data[mpi_rank], data, entry);
	}
	else
	{
		_STARPU_MPI_DEBUG("Do not receive data %p from node %d as it is already available\n", data, mpi_rank);
	}
	return already_received;
}

static
void *_starpu_mpi_already_sent(starpu_data_handle_t data, int dest)
{
	if (_cache_enabled == 0) return NULL;

	struct _starpu_data_entry *already_sent;
	HASH_FIND_PTR(_cache_sent_data[dest], &data, already_sent);
	if (already_sent == NULL)
	{
		struct _starpu_data_entry *entry = (struct _starpu_data_entry *)malloc(sizeof(*entry));
		entry->data = data;
		HASH_ADD_PTR(_cache_sent_data[dest], data, entry);
		_STARPU_MPI_DEBUG("Noting that data %p has already been sent to %d\n", data, dest);
	}
	else
	{
		_STARPU_MPI_DEBUG("Do not send data %p to node %d as it has already been sent\n", data, dest);
	}
	return already_sent;
}

static
int _starpu_mpi_find_executee_node(starpu_data_handle_t data, enum starpu_access_mode mode, int me, int *do_execute, int *inconsistent_execute, int *dest, size_t *size_on_nodes)
{
	if (data && mode & STARPU_R)
	{
		struct starpu_data_interface_ops *ops;
		int rank = starpu_data_get_rank(data);

		ops = data->ops;
		size_on_nodes[rank] += ops->get_size(data);
	}

	if (mode & STARPU_W)
	{
		if (!data)
		{
			/* We don't have anything allocated for this.
			 * The application knows we won't do anything
			 * about this task */
			/* Yes, the app could actually not call
			 * insert_task at all itself, this is just a
			 * safeguard. */
			_STARPU_MPI_DEBUG("oh oh\n");
			_STARPU_MPI_LOG_OUT();
			return -EINVAL;
		}
		int mpi_rank = starpu_data_get_rank(data);
		if (mpi_rank == me)
		{
			if (*do_execute == 0)
			{
				*inconsistent_execute = 1;
			}
			else
			{
				*do_execute = 1;
			}
		}
		else if (mpi_rank != -1)
		{
			if (*do_execute == 1)
			{
				*inconsistent_execute = 1;
			}
			else
			{
				*do_execute = 0;
				*dest = mpi_rank;
				/* That's the rank which needs the data to be sent to */
			}
		}
		else
		{
			_STARPU_ERROR("rank invalid\n");
		}
	}
	return 0;
}

static
void _starpu_mpi_exchange_data_before_execution(starpu_data_handle_t data, enum starpu_access_mode mode, int me, int dest, int do_execute, MPI_Comm comm)
{
	if (data && mode & STARPU_R)
	{
		int mpi_rank = starpu_data_get_rank(data);
		int mpi_tag = starpu_data_get_tag(data);
		if(mpi_rank == -1)
		{
			fprintf(stderr,"StarPU needs to be told the MPI rank of this data, using starpu_data_set_rank\n");
			STARPU_ABORT();
		}
		if(mpi_tag == -1)
		{
			fprintf(stderr,"StarPU needs to be told the MPI tag of this data, using starpu_data_set_tag\n");
			STARPU_ABORT();
		}
		/* The task needs to read this data */
		if (do_execute && mpi_rank != me && mpi_rank != -1)
		{
			/* I will have to execute but I don't have the data, receive */
			void *already_received = _starpu_mpi_already_received(data, mpi_rank);
			if (already_received == NULL)
			{
				_STARPU_MPI_DEBUG("Receive data %p from %d\n", data, mpi_rank);
				starpu_mpi_irecv_detached(data, mpi_rank, mpi_tag, comm, NULL, NULL);
			}
		}
		if (!do_execute && mpi_rank == me)
		{
			/* Somebody else will execute it, and I have the data, send it. */
			void *already_sent = _starpu_mpi_already_sent(data, dest);
			if (already_sent == NULL)
			{
				_STARPU_MPI_DEBUG("Send data %p to %d\n", data, dest);
				starpu_mpi_isend_detached(data, dest, mpi_tag, comm, NULL, NULL);
			}
		}
	}
}

static
void _starpu_mpi_exchange_data_after_execution(starpu_data_handle_t data, enum starpu_access_mode mode, int me, int xrank, int dest, int do_execute, MPI_Comm comm)
{
	if (mode & STARPU_W)
	{
		int mpi_rank = starpu_data_get_rank(data);
		int mpi_tag = starpu_data_get_tag(data);
		if(mpi_rank == -1)
		{
			fprintf(stderr,"StarPU needs to be told the MPI rank of this data, using starpu_data_set_rank\n");
			STARPU_ABORT();
		}
		if(mpi_tag == -1)
		{
			fprintf(stderr,"StarPU needs to be told the MPI tag of this data, using starpu_data_set_tag\n");
			STARPU_ABORT();
		}
		if (mpi_rank == me)
		{
			if (xrank != -1 && me != xrank)
			{
				_STARPU_MPI_DEBUG("Receive data %p back from the task %d which executed the codelet ...\n", data, dest);
				starpu_mpi_irecv_detached(data, dest, mpi_tag, comm, NULL, NULL);
			}
		}
		else if (do_execute)
		{
			_STARPU_MPI_DEBUG("Send data %p back to its owner %d...\n", data, mpi_rank);
			starpu_mpi_isend_detached(data, mpi_rank, mpi_tag, comm, NULL, NULL);
		}
	}
}

void _starpu_mpi_clear_data_after_execution(starpu_data_handle_t data, enum starpu_access_mode mode, int me, int do_execute, MPI_Comm comm)
{
	if (_cache_enabled)
	{
		if (mode & STARPU_W || mode & STARPU_REDUX)
		{
			if (do_execute)
			{
				/* Note that all copies I've sent to neighbours are now invalid */
				int n, size;
				MPI_Comm_size(comm, &size);
				for(n=0 ; n<size ; n++)
				{
					struct _starpu_data_entry *already_sent;
					HASH_FIND_PTR(_cache_sent_data[n], &data, already_sent);
					if (already_sent)
					{
						_STARPU_MPI_DEBUG("Clearing send cache for data %p\n", data);
						HASH_DEL(_cache_sent_data[n], already_sent);
					}
				}
			}
			else
			{
				int mpi_rank = starpu_data_get_rank(data);
				struct _starpu_data_entry *already_received;
				HASH_FIND_PTR(_cache_received_data[mpi_rank], &data, already_received);
				if (already_received)
				{
#ifdef STARPU_DEVEL
#  warning TODO: Somebody else will write to the data, so discard our cached copy if any. starpu_mpi could just remember itself.
#endif
					_STARPU_MPI_DEBUG("Clearing receive cache for data %p\n", data);
					HASH_DEL(_cache_received_data[mpi_rank], already_received);
					starpu_data_invalidate_submit(data);
				}
			}
		}
	}
	else
	{
		/* We allocated a temporary buffer for the received data, now drop it */
		if ((mode & STARPU_R) && do_execute)
		{
			int mpi_rank = starpu_data_get_rank(data);
			if (mpi_rank != me && mpi_rank != -1)
			{
				starpu_data_invalidate_submit(data);
			}
		}
	}
}

int starpu_mpi_insert_task(MPI_Comm comm, struct starpu_codelet *codelet, ...)
{
	int arg_type;
	va_list varg_list;
	int me, do_execute, xrank, nb_nodes;
	size_t *size_on_nodes;
	size_t arg_buffer_size = 0;
	char *arg_buffer;
	int dest=0, inconsistent_execute;
	int current_data = 0;

	_STARPU_MPI_LOG_IN();

	MPI_Comm_rank(comm, &me);
	MPI_Comm_size(comm, &nb_nodes);

	size_on_nodes = (size_t *)calloc(1, nb_nodes * sizeof(size_t));

	/* Get the number of buffers and the size of the arguments */
	va_start(varg_list, codelet);
	arg_buffer_size = _starpu_insert_task_get_arg_size(varg_list);

	va_start(varg_list, codelet);
	_starpu_codelet_pack_args(arg_buffer_size, &arg_buffer, varg_list);

	/* Find out whether we are to execute the data because we own the data to be written to. */
	inconsistent_execute = 0;
	do_execute = -1;
	xrank = -1;
	va_start(varg_list, codelet);
	while ((arg_type = va_arg(varg_list, int)) != 0)
	{
		if (arg_type==STARPU_EXECUTE_ON_NODE)
		{
			xrank = va_arg(varg_list, int);
			_STARPU_MPI_DEBUG("Executing on node %d\n", xrank);
			do_execute = 1;
		}
		else if (arg_type==STARPU_EXECUTE_ON_DATA)
		{
			starpu_data_handle_t data = va_arg(varg_list, starpu_data_handle_t);
			xrank = starpu_data_get_rank(data);
			_STARPU_MPI_DEBUG("Executing on data node %d\n", xrank);
			STARPU_ASSERT(xrank <= nb_nodes);
			do_execute = 1;
		}
		else if (arg_type==STARPU_R || arg_type==STARPU_W || arg_type==STARPU_RW || arg_type==STARPU_SCRATCH || arg_type==STARPU_REDUX)
		{
			starpu_data_handle_t data = va_arg(varg_list, starpu_data_handle_t);
			enum starpu_access_mode mode = (enum starpu_access_mode) arg_type;
			int ret = _starpu_mpi_find_executee_node(data, mode, me, &do_execute, &inconsistent_execute, &dest, size_on_nodes);
			if (ret == -EINVAL)
			{
				free(size_on_nodes);
				return ret;
			}
			current_data ++;
		}
		else if (arg_type == STARPU_DATA_ARRAY)
		{
			starpu_data_handle_t *datas = va_arg(varg_list, starpu_data_handle_t *);
			int nb_handles = va_arg(varg_list, int);
			int i;
			for(i=0 ; i<nb_handles ; i++)
			{
				enum starpu_access_mode mode = codelet->modes[current_data];
				int ret = _starpu_mpi_find_executee_node(datas[i], mode, me, &do_execute, &inconsistent_execute, &dest, size_on_nodes);
				if (ret == -EINVAL)
				{
					free(size_on_nodes);
					return ret;
				}
				current_data ++;
			}
		}
		else if (arg_type==STARPU_VALUE)
		{
			va_arg(varg_list, void *);
			va_arg(varg_list, size_t);
		}
		else if (arg_type==STARPU_CALLBACK)
		{
			va_arg(varg_list, void (*)(void *));
		}
		else if (arg_type==STARPU_CALLBACK_WITH_ARG)
		{
			va_arg(varg_list, void (*)(void *));
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_CALLBACK_ARG)
		{
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_PRIORITY)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_NODE)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_DATA)
		{
			va_arg(varg_list, int);
		}
	}
	va_end(varg_list);

	if (do_execute == -1)
	{
		int i;
		size_t max_size = 0;
		for(i=0 ; i<nb_nodes ; i++)
		{
			if (size_on_nodes[i] > max_size)
			{
				max_size = size_on_nodes[i];
				xrank = i;
			}
		}
		free(size_on_nodes);
		if (xrank != -1)
		{
			_STARPU_MPI_DEBUG("Node %d is having the most R data\n", xrank);
			do_execute = 1;
		}
	}

	STARPU_ASSERT_MSG(do_execute != -1, "StarPU needs to see a W or a REDUX data which will tell it where to execute the task");

	if (inconsistent_execute == 1)
	{
		if (xrank == -1)
		{
			_STARPU_MPI_DEBUG("Different tasks are owning W data. Needs to specify which one is to execute the codelet, using STARPU_EXECUTE_ON_NODE or STARPU_EXECUTE_ON_DATA\n");
			free(size_on_nodes);
			return -EINVAL;
		}
		else
		{
			do_execute = (me == xrank);
			dest = xrank;
		}
	}
	else if (xrank != -1)
	{
		do_execute = (me == xrank);
		dest = xrank;
	}

	/* Send and receive data as requested */
	va_start(varg_list, codelet);
	current_data = 0;
	while ((arg_type = va_arg(varg_list, int)) != 0)
	{
		if (arg_type==STARPU_R || arg_type==STARPU_W || arg_type==STARPU_RW || arg_type==STARPU_SCRATCH || arg_type==STARPU_REDUX)
		{
			starpu_data_handle_t data = va_arg(varg_list, starpu_data_handle_t);
			enum starpu_access_mode mode = (enum starpu_access_mode) arg_type;

			_starpu_mpi_exchange_data_before_execution(data, mode, me, dest, do_execute, comm);
			current_data ++;

		}
		else if (arg_type == STARPU_DATA_ARRAY)
		{
			starpu_data_handle_t *datas = va_arg(varg_list, starpu_data_handle_t *);
			int nb_handles = va_arg(varg_list, int);
			int i;

			for(i=0 ; i<nb_handles ; i++)
			{
				_starpu_mpi_exchange_data_before_execution(datas[i], codelet->modes[current_data], me, dest, do_execute, comm);
				current_data++;
			}
		}
		else if (arg_type==STARPU_VALUE)
		{
			va_arg(varg_list, void *);
			va_arg(varg_list, size_t);
		}
		else if (arg_type==STARPU_CALLBACK)
		{
			va_arg(varg_list, void (*)(void *));
		}
		else if (arg_type==STARPU_CALLBACK_WITH_ARG)
		{
			va_arg(varg_list, void (*)(void *));
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_CALLBACK_ARG)
		{
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_PRIORITY)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_NODE)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_DATA)
		{
			va_arg(varg_list, starpu_data_handle_t);
		}
	}
	va_end(varg_list);

	if (do_execute)
	{
		_STARPU_MPI_DEBUG("Execution of the codelet %p (%s)\n", codelet, codelet->name);
		va_start(varg_list, codelet);
		struct starpu_task *task = starpu_task_create();
		int ret = _starpu_insert_task_create_and_submit(arg_buffer, arg_buffer_size, codelet, &task, varg_list);
		_STARPU_MPI_DEBUG("ret: %d\n", ret);
		STARPU_ASSERT(ret==0);
	}

	if (inconsistent_execute)
	{
		va_start(varg_list, codelet);
		current_data = 0;
		while ((arg_type = va_arg(varg_list, int)) != 0)
		{
			if (arg_type==STARPU_R || arg_type==STARPU_W || arg_type==STARPU_RW || arg_type==STARPU_SCRATCH || arg_type==STARPU_REDUX)
			{
				starpu_data_handle_t data = va_arg(varg_list, starpu_data_handle_t);
				enum starpu_access_mode mode = (enum starpu_access_mode) arg_type;

				_starpu_mpi_exchange_data_after_execution(data, mode, me, xrank, dest, do_execute, comm);
				current_data++;
			}
			else if (arg_type == STARPU_DATA_ARRAY)
			{
				starpu_data_handle_t *datas = va_arg(varg_list, starpu_data_handle_t *);
				int nb_handles = va_arg(varg_list, int);
				int i;

				for(i=0 ; i<nb_handles ; i++)
				{
					_starpu_mpi_exchange_data_after_execution(datas[i], codelet->modes[current_data], me, xrank, dest, do_execute, comm);
					current_data++;
				}
			}
			else if (arg_type==STARPU_VALUE)
			{
				va_arg(varg_list, void *);
				va_arg(varg_list, size_t);
			}
			else if (arg_type==STARPU_CALLBACK)
			{
				va_arg(varg_list, void (*)(void *));
			}
			else if (arg_type==STARPU_CALLBACK_WITH_ARG)
			{
				va_arg(varg_list, void (*)(void *));
				va_arg(varg_list, void *);
			}
			else if (arg_type==STARPU_CALLBACK_ARG)
			{
				va_arg(varg_list, void *);
			}
			else if (arg_type==STARPU_PRIORITY)
			{
				va_arg(varg_list, int);
			}
			else if (arg_type==STARPU_EXECUTE_ON_NODE)
			{
				va_arg(varg_list, int);
			}
			else if (arg_type==STARPU_EXECUTE_ON_DATA)
			{
				va_arg(varg_list, starpu_data_handle_t);
			}
		}
		va_end(varg_list);
	}

	va_start(varg_list, codelet);
	current_data = 0;
	while ((arg_type = va_arg(varg_list, int)) != 0)
	{
		if (arg_type==STARPU_R || arg_type==STARPU_W || arg_type==STARPU_RW || arg_type == STARPU_SCRATCH || arg_type == STARPU_REDUX)
		{
			starpu_data_handle_t data = va_arg(varg_list, starpu_data_handle_t);
			enum starpu_access_mode mode = (enum starpu_access_mode) arg_type;

			_starpu_mpi_clear_data_after_execution(data, mode, me, do_execute, comm);
			current_data++;
		}
		else if (arg_type == STARPU_DATA_ARRAY)
		{
			starpu_data_handle_t *datas = va_arg(varg_list, starpu_data_handle_t *);
			int nb_handles = va_arg(varg_list, int);
			int i;

			for(i=0 ; i<nb_handles ; i++)
			{
				_starpu_mpi_clear_data_after_execution(datas[i], codelet->modes[current_data], me, do_execute, comm);
				current_data++;
			}
		}
		else if (arg_type==STARPU_VALUE)
		{
			va_arg(varg_list, void *);
			va_arg(varg_list, size_t);
		}
		else if (arg_type==STARPU_CALLBACK)
		{
			va_arg(varg_list, void (*)(void *));
		}
		else if (arg_type==STARPU_CALLBACK_WITH_ARG)
		{
			va_arg(varg_list, void (*)(void *));
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_CALLBACK_ARG)
		{
			va_arg(varg_list, void *);
		}
		else if (arg_type==STARPU_PRIORITY)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_NODE)
		{
			va_arg(varg_list, int);
		}
		else if (arg_type==STARPU_EXECUTE_ON_DATA)
		{
			va_arg(varg_list, starpu_data_handle_t);
		}
	}

	va_end(varg_list);
	_STARPU_MPI_LOG_OUT();
	return 0;
}

void starpu_mpi_get_data_on_node_detached(MPI_Comm comm, starpu_data_handle_t data_handle, int node, void (*callback)(void*), void *arg)
{
	int me, rank, tag;

	rank = starpu_data_get_rank(data_handle);
	tag = starpu_data_get_tag(data_handle);
	if (rank == -1)
	{
		_STARPU_ERROR("StarPU needs to be told the MPI rank of this data, using starpu_data_set_rank\n");
	}
	if (tag == -1)
	{
		_STARPU_ERROR("StarPU needs to be told the MPI tag of this data, using starpu_data_set_tag\n");
	}
	MPI_Comm_rank(comm, &me);

	if (node == rank) return;

	if (me == node)
	{
		starpu_mpi_irecv_detached(data_handle, rank, tag, comm, callback, arg);
	}
	else if (me == rank)
	{
		starpu_mpi_isend_detached(data_handle, node, tag, comm, NULL, NULL);
	}
}

void starpu_mpi_get_data_on_node(MPI_Comm comm, starpu_data_handle_t data_handle, int node)
{
	int me, rank, tag;

	rank = starpu_data_get_rank(data_handle);
	tag = starpu_data_get_tag(data_handle);
	if (rank == -1)
	{
		fprintf(stderr,"StarPU needs to be told the MPI rank of this data, using starpu_data_set_rank\n");
		STARPU_ABORT();
	}
	if (tag == -1)
	{
		fprintf(stderr,"StarPU needs to be told the MPI tag of this data, using starpu_data_set_tag\n");
		STARPU_ABORT();
	}
	MPI_Comm_rank(comm, &me);

	if (node == rank) return;

	if (me == node)
	{
		MPI_Status status;
		starpu_mpi_recv(data_handle, rank, tag, comm, &status);
	}
	else if (me == rank)
	{
		starpu_mpi_send(data_handle, node, tag, comm);
	}
}

void starpu_mpi_redux_data(MPI_Comm comm, starpu_data_handle_t data_handle)
{
	int me, rank, tag, nb_nodes;

	rank = starpu_data_get_rank(data_handle);
	tag = starpu_data_get_tag(data_handle);
	if (rank == -1)
	{
		fprintf(stderr,"StarPU needs to be told the MPI rank of this data, using starpu_data_set_rank\n");
		STARPU_ABORT();
	}
	if (tag == -1)
	{
		fprintf(stderr,"StarPU needs to be told the MPI tag of this data, using starpu_data_set_tag\n");
		STARPU_ABORT();
	}

	MPI_Comm_rank(comm, &me);
	MPI_Comm_size(comm, &nb_nodes);

	_STARPU_MPI_DEBUG("Doing reduction for data %p on node %d with %d nodes ...\n", data_handle, rank, nb_nodes);

	// need to count how many nodes have the data in redux mode
	if (me == rank)
	{
		int i;

		for(i=0 ; i<nb_nodes ; i++)
		{
			if (i != rank)
			{
				starpu_data_handle_t new_handle;

				starpu_data_register_same(&new_handle, data_handle);

				_STARPU_MPI_DEBUG("Receiving redux handle from %d in %p ...\n", i, new_handle);

				starpu_mpi_irecv_detached(new_handle, i, tag, comm, NULL, NULL);
				starpu_insert_task(data_handle->redux_cl,
						   STARPU_RW, data_handle,
						   STARPU_R, new_handle,
						   0);
				starpu_data_unregister_submit(new_handle);
			}
		}
	}
	else
	{
		_STARPU_MPI_DEBUG("Sending redux handle to %d ...\n", rank);
		starpu_mpi_isend_detached(data_handle, rank, tag, comm, NULL, NULL);
		starpu_insert_task(data_handle->init_cl, STARPU_W, data_handle, 0);
	}
}