[ofiwg] mapping adapter memory

[Reese Faucette (rfaucett)]

Many adapters allow mapping memory regions on the adapter into user-space for efficient PIO-based sending and receiving.  This is often a big performance win and seems worthwhile finding an efficient abstract for.  Has this been discussed at all?

The approach of "let the provider find the best way to use the hardware resources and hide that from the user" will not always work in this case, as the constraints on adapter-mapped memory may prevent it from being a general solution.  For example, it may be a limited resource, and so not every EP will have access to it, and those that do may have restrictions that other EPs do not have (e.g. fewer send credits available, smaller MTU).

Any thoughts on whether this is worth trying to come up with some abstraction for?  From Cisco's point of view, it definitely is - it requires some level of EP differentiation such that it cannot be totally transparent to the app, but it can drop latency by 20%, a worthwhile goal for us.

As another example, the new Solarflare NICs are pushing this as a feature also:
https://support.solarflare.com/index.php?option=com_cognidox&file=SF-111142-TC-1_Introduction_to_SFC91xx_ASIC_Family.pdf&task=download&format=raw&Itemid=17

See section on "PIO and Templates" - it's not just us :)

-reese

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