[ofiwg] EP_RDM question

[Jeff Squyres (jsquyres)]

EP_RDM is described thusly in fi_endpoint(3):

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FI_EP_RDM
Reliable datagram message. Provides a reliable, unconnected data transfer service with flow control that maintains message boundaries.
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Consider this scenario:

1. sender sends message A on EP_RDM endpoint at time X
2. when message A arrives at the target, there are no receive buffers posted

What happens?

1. The lack of receive buffers at the target should trigger an error at the sender indicating that message A was not delivered.

2. Or:

2a. If receive buffers are eventually posted at the target, message A will be delivered successfully.
2b. If the target endpoint is closed before receive buffers are available at the target for message A, an error is triggered at the sender indicating that message A was not delivered
2c. If message A has not been delivered within a given timeout (for any reason -- to include lack of buffers at the target), an error is triggered at the sender indicating that message A was not delivered

In short: assuming a receiver a) continually posts receive buffers, and b) doesn't close its endpoint, do senders need to worry about credits with RDM endpoints?

This question obviously only makes sense when only a single sender is sending to a RDM target endpoint.

But I also ask because a popular technique for IB SRQ with multiple senders is to set the sender retry to an infinite value. I.e., since there are multiple senders, credit schemes don't help, and the most hardware-offload-ish scheme is to just have the hardware keep re-sending until a message gets through.  Does RDM offer that capability?

-- 
Jeff Squyres
jsquyres at cisco.com
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