[ofa-general] IB credit-based flow control

[Roland Dreier]

 > How would I verify that? I'm using current HCAs (mlx4), so I'm
 > assuming if the spec says an HCA must support something, is is
 > supported?
 > 
 > We definitely still need ulp-level flow control for iwarp so it's not
 > wasted work. But if IB doesn't, then it would be great to not incur
 > the overhead.

mlx4 HCAs do support end-to-end credits.  However, as you've discovered,
that transport level flow control is not necessarily that useful: if a
sender overruns the receives that are posted, then it triggers an RNR
NAK which leads to a large delay in the connection, which can be very
bad for throughput.  So for best performance, application level flow
control is required, even with IB end-to-end credit flow control at the
transport level.

 - R.