fi_collective - Man Page

This call attaches an endpoint to a collective membership group. Libfabric treats collective members as a multicast group, and the fi_join_collective call attaches the endpoint to that multicast group. By default, the endpoint will join the group based on the data transfer capabilities of the endpoint. For example, if the endpoint has been configured to both send and receive data, then the endpoint will be able to initiate and receive transfers to and from the collective. The input flags may be used to restrict access to the collective group, subject to endpoint capability limitations.

Join collective operations complete asynchronously, and may involve fabric transfers, dependent on the provider implementation. An endpoint must be bound to an event queue prior to calling fi_join_collective. The result of the join operation will be reported to the EQ as an FI_JOIN_COMPLETE event. Applications cannot issue collective transfers until receiving notification that the join operation has completed. Note that an endpoint may begin receiving messages from the collective group as soon as the join completes, which can occur prior to the FI_JOIN_COMPLETE event being generated.

The join collective operation is itself a collective operation. All participating peers must call fi_join_collective before any individual peer will report that the join has completed. Application managed collective memberships are an exception. With application managed memberships, the fi_join_collective call may be completed locally without fabric communication. For provider managed memberships, the join collective call requires as input a coll_addr that refers to either an address associated with an AV set (see fi_av_set_addr) or an existing collective group (obtained through a previous call to fi_join_collective). The fi_join_collective call will create a new collective subgroup. If application managed memberships are used, coll_addr should be set to FI_ADDR_UNAVAIL.

Applications must call fi_close on the collective group to disconnect the endpoint from the group. After a join operation has completed, the fi_mc_addr call may be used to retrieve the address associated with the multicast group. See fi_cm(3) for additional details on fi_mc_addr().

The fi_barrier operation provides a mechanism to synchronize peers. Barrier does not result in any data being transferred at the application level. A barrier does not complete locally until all peers have invoked the barrier call. This signifies to the local application that work by peers that completed prior to them calling barrier has finished.

The fi_barrier2 operations is the same as fi_barrier, but with an extra parameter to pass in operation flags.

fi_broadcast transfers an array of data from a single sender to all other members of the collective group. The input buf parameter is treated as the transmit buffer if the local rank is the root, otherwise it is the receive buffer. The broadcast operation acts as an atomic write or read to a data array. As a result, the format of the data in buf is specified through the datatype parameter. Any non-void datatype may be broadcast.

The following diagram shows an example of broadcast being used to transfer an array of integers to a group of peers.

[1]  [1]  [1]
[5]  [5]  [5]
[9]  [9]  [9]
 |____^    ^
 |_________|
 broadcast

The fi_alltoall collective involves distributing (or scattering) different portions of an array of data to peers. It is best explained using an example. Here three peers perform an all to all collective to exchange different entries in an integer array.

[1]   [2]   [3]
[5]   [6]   [7]
[9]  [10]  [11]
   \   |   /
   All to all
   /   |   \
[1]   [5]   [9]
[2]   [6]  [10]
[3]   [7]  [11]

Each peer sends a piece of its data to the other peers.

All to all operations may be performed on any non-void datatype. However, all to all does not perform an operation on the data itself, so no operation is specified.

fi_allreduce can be described as all peers providing input into an atomic operation, with the result copied back to each peer. Conceptually, this can be viewed as each peer issuing a multicast atomic operation to all other peers, fetching the results, and combining them. The combining of the results is referred to as the reduction. The fi_allreduce() operation takes as input an array of data and the specified atomic operation to perform. The results of the reduction are written into the result buffer.

Any non-void datatype may be specified. Valid atomic operations are listed below in the fi_query_collective call. The following diagram shows an example of an all reduce operation involving summing an array of integers between three peers.

 [1]  [1]  [1]
 [5]  [5]  [5]
 [9]  [9]  [9]
   \   |   /
      sum
   /   |   \
 [3]  [3]  [3]
[15] [15] [15]
[27] [27] [27]
  All Reduce

Conceptually, all gather can be viewed as the opposite of the scatter component from reduce-scatter. All gather collects data from all peers into a single array, then copies that array back to each peer.

[1]  [5]  [9]
  \   |   /
 All gather
  /   |   \
[1]  [1]  [1]
[5]  [5]  [5]
[9]  [9]  [9]

All gather may be performed on any non-void datatype. However, all gather does not perform an operation on the data itself, so no operation is specified.

The fi_reduce_scatter collective is similar to an fi_allreduce operation, followed by all to all. With reduce scatter, all peers provide input into an atomic operation, similar to all reduce. However, rather than the full result being copied to each peer, each participant receives only a slice of the result.

This is shown by the following example:

[1]  [1]  [1]
[5]  [5]  [5]
[9]  [9]  [9]
  \   |   /
     sum (reduce)
      |
     [3]
    [15]
    [27]
      |
   scatter
  /   |   \
[3] [15] [27]

The reduce scatter call supports the same datatype and atomic operation as fi_allreduce.

The fi_reduce collective is the first half of an fi_allreduce operation. With reduce, all peers provide input into an atomic operation, with the the results collected by a single `root' endpoint.

This is shown by the following example, with the leftmost peer identified as the root:

[1]  [1]  [1]
[5]  [5]  [5]
[9]  [9]  [9]
  \   |   /
     sum (reduce)
    /
 [3]
[15]
[27]

The reduce call supports the same datatype and atomic operation as fi_allreduce.

The fi_scatter collective is the second half of an fi_reduce_scatter operation. The data from a single `root' endpoint is split and distributed to all peers.

This is shown by the following example:

 [3]
[15]
[27]
    \
   scatter
  /   |   \
[3] [15] [27]

The scatter operation is used to distribute results to the peers. No atomic operation is performed on the data.

The fi_gather operation is used to collect (gather) the results from all peers and store them at a `root' peer.

This is shown by the following example, with the leftmost peer identified as the root.

[1]  [5]  [9]
  \   |   /
    gather
   /
[1]
[5]
[9]

The gather operation does not perform any operation on the data itself.

The fi_query_collective call reports which collective operations are supported by the underlying provider, for suitably configured endpoints. Collective operations needed by an application that are not supported by the provider must be implemented by the application. The query call checks whether a provider supports a specific collective operation for a given datatype and operation, if applicable.

The name of the collective, as well as the datatype and associated operation, if applicable, and are provided as input into fi_query_collective.

The coll parameter may reference one of these collectives: FI_BARRIER, FI_BROADCAST, FI_ALLTOALL, FI_ALLREDUCE, FI_ALLGATHER, FI_REDUCE_SCATTER, FI_REDUCE, FI_SCATTER, or FI_GATHER. Additional details on the collective operation is specified through the struct fi_collective_attr parameter. For collectives that act on data, the operation and related data type must be specified through the given attributes.

struct fi_collective_attr {
    enum fi_op op;
    enum fi_datatype datatype;
    struct fi_atomic_attr datatype_attr;
    size_t max_members;
      uint64_t mode;
};

For a description of struct fi_atomic_attr, see fi_atomic(3).

op

On input, this specifies the atomic operation involved with the collective call. This should be set to one of the following values: FI_MIN, FI_MAX, FI_SUM, FI_PROD, FI_LOR, FI_LAND, FI_BOR, FI_BAND, FI_LXOR, FI_BXOR, FI_ATOMIC_READ, FI_ATOMIC_WRITE, of FI_NOOP. For collectives that do not exchange application data (fi_barrier), this should be set to FI_NOOP.

datatype

On onput, specifies the datatype of the data being modified by the collective. This should be set to one of the following values: FI_INT8, FI_UINT8, FI_INT16, FI_UINT16, FI_INT32, FI_UINT32, FI_INT64, FI_UINT64, FI_FLOAT, FI_DOUBLE, FI_FLOAT_COMPLEX, FI_DOUBLE_COMPLEX, FI_LONG_DOUBLE, FI_LONG_DOUBLE_COMPLEX, or FI_VOID. For collectives that do not exchange application data (fi_barrier), this should be set to FI_VOID.

datatype_attr.count

The maximum number of elements that may be used with the collective.

datatype_attr.size

The size of the datatype as supported by the provider. Applications should validate the size of datatypes that differ based on the platform, such as FI_LONG_DOUBLE.

max_members

The maximum number of peers that may participate in a collective operation.

mode

This field is reserved and should be 0.

If a collective operation is supported, the query call will return FI_SUCCESS, along with attributes on the limits for using that collective operation through the provider.

Collective operations map to underlying fi_atomic operations. For a discussion of atomic completion semantics, see fi_atomic(3). The completion, ordering, and atomicity of collective operations match those defined for point to point atomic operations.