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Tag matching logic
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==================
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The MPI standard defines a set of rules, known as tag-matching, for matching
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source send operations to destination receives. The following parameters must
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match the following source and destination parameters:
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* Communicator
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* User tag - wild card may be specified by the receiver
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* Source rank – wild car may be specified by the receiver
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* Destination rank – wild
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The ordering rules require that when more than one pair of send and receive
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message envelopes may match, the pair that includes the earliest posted-send
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and the earliest posted-receive is the pair that must be used to satisfy the
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matching operation. However, this doesn’t imply that tags are consumed in
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the order they are created, e.g., a later generated tag may be consumed, if
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earlier tags can’t be used to satisfy the matching rules.
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When a message is sent from the sender to the receiver, the communication
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library may attempt to process the operation either after or before the
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corresponding matching receive is posted. If a matching receive is posted,
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this is an expected message, otherwise it is called an unexpected message.
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Implementations frequently use different matching schemes for these two
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different matching instances.
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To keep MPI library memory footprint down, MPI implementations typically use
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two different protocols for this purpose:
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1. The Eager protocol- the complete message is sent when the send is
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processed by the sender. A completion send is received in the send_cq
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notifying that the buffer can be reused.
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2. The Rendezvous Protocol - the sender sends the tag-matching header,
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and perhaps a portion of data when first notifying the receiver. When the
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corresponding buffer is posted, the responder will use the information from
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the header to initiate an RDMA READ operation directly to the matching buffer.
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A fin message needs to be received in order for the buffer to be reused.
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Tag matching implementation
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===========================
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There are two types of matching objects used, the posted receive list and the
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unexpected message list. The application posts receive buffers through calls
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to the MPI receive routines in the posted receive list and posts send messages
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using the MPI send routines. The head of the posted receive list may be
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maintained by the hardware, with the software expected to shadow this list.
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When send is initiated and arrives at the receive side, if there is no
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pre-posted receive for this arriving message, it is passed to the software and
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placed in the unexpected message list. Otherwise the match is processed,
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including rendezvous processing, if appropriate, delivering the data to the
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specified receive buffer. This allows overlapping receive-side MPI tag
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matching with computation.
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When a receive-message is posted, the communication library will first check
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the software unexpected message list for a matching receive. If a match is
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found, data is delivered to the user buffer, using a software controlled
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protocol. The UCX implementation uses either an eager or rendezvous protocol,
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depending on data size. If no match is found, the entire pre-posted receive
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list is maintained by the hardware, and there is space to add one more
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pre-posted receive to this list, this receive is passed to the hardware.
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Software is expected to shadow this list, to help with processing MPI cancel
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operations. In addition, because hardware and software are not expected to be
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tightly synchronized with respect to the tag-matching operation, this shadow
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list is used to detect the case that a pre-posted receive is passed to the
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hardware, as the matching unexpected message is being passed from the hardware
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to the software.
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