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Mercury Bulk Layer

In addition to the previous layer, some RPCs may require the transfer of larger amounts of data. For these RPCs, the bulk layer can be used. It is built on top of the RMA protocol defined in the network abstraction layer and prevents intermediate memory copies.

HG Bulk Interface

The interface allows origin processes to expose a memory region to the target by creating a bulk descriptor (which contains virtual memory address information, size of the memory region that is being exposed, and other parameters that depend on the underlying network implementation). The bulk descriptor can be serialized and sent to the target along with the RPC request arguments (using the RPC layer). When the target gets the input parameters, it can deserialize the bulk descriptor, get the size of the memory buffer that has to be transferred, and initiate the transfer. Only targets should initiate one-sided transfers so that they can, as well as controlling the data flow, protect their memory from concurrent accesses.

As no explicit ack message is sent on transfer completion, the origin process can only assume that accesses to its local memory are completed once it receives the RPC response from the target. Therefore, in the case of an RPC with no response, great care should be taken when initiating a bulk transfer to ensure that the origin gets notified when its exposed memory can be safely released and accessed.


The interface uses the class and execution context that are defined by the HG RPC layer. To initiate a bulk transfer, one needs to create a bulk descriptor on both the origin and the target, which will later be passed to the HG_Bulk_transfer() call.

HG_Bulk_create(hg_class_t *hg_class, hg_uint32_t count,
               void **buf_ptrs, const hg_size_t *buf_sizes,
               hg_uint8_t flags, hg_bulk_t *handle);

The bulk descriptor can be released by using:

hg_return_t HG_Bulk_free(hg_bulk_t handle);

For convenience, memory pointers from an existing bulk descriptor can be accessed with:

HG_Bulk_access(hg_bulk_t handle, hg_size_t offset, hg_size_t size,
               hg_uint8_t flags, hg_uint32_t max_count, void **buf_ptrs,
               hg_size_t *buf_sizes, hg_uint32_t *actual_count);

Additionally, it is also possible to bind the origin's address to the bulk handle through the HG_Bulk_bind() function at the cost of an additional overhead for serializing and deserializing addressing information. This should only be necessary when the source address retrieved from the HG_Get_info() call is different from the one that must be used for the transfer (e.g., multiple sources).

HG_Bulk_bind(hg_bulk_t handle, hg_context_t *context);

In that particular case, the address information can be directly retrieved using:

HG_Bulk_get_addr(hg_bulk_t handle);


Serialization and deserialization of bulk handles should never be explicitly done by users and we instead encourage the use of the mercury proc routines that provide an hg_proc_hg_bulk_t routine:

hg_proc_hg_bulk_t(hg_proc_t proc, void *data);

For more details on RPC argument serialization, please refer to this section.

Bulk Transfer

When the bulk descriptor from the origin has been received, the target can initiate the bulk transfer to/from its own bulk descriptor. Virtual offsets can be used to transfer data pieces from a non-contiguous block transparently. The call is non-blocking. When the operation completes, the user callback is placed onto the context's completion queue.

HG_Bulk_transfer(hg_context_t *context, hg_bulk_cb_t callback, void *arg,
                 hg_bulk_op_t op, hg_addr_t origin_addr,
                 hg_bulk_t origin_handle, hg_size_t origin_offset,
                 hg_bulk_t local_handle, hg_size_t local_offset,
                 hg_size_t size, hg_op_id_t *op_id);

Note that for convenience, as the transfer needs to be realized within the RPC callback on the RPC target, the routine HG_Get_info() enables easy retrieval of classes, contexts and source address:

struct hg_info {
    hg_class_t *hg_class;               /* HG class */
    hg_context_t *context;              /* HG context */
    hg_addr_t addr;                     /* HG address */
    hg_id_t id;                         /* RPC ID */

struct hg_info *
HG_Get_info(hg_handle_t handle);

Last update: December 4, 2021