[ofa-general] IB credit-based flow control

[Steve Wise]

Andy Grover wrote:
> Sean Hefty wrote:
>> My assumption is that if no credits are available when the SEND 
>> request arrives,
>> then the receiver generates a RNR message, but I didn't read through 
>> the entire
>> section to verify this.
>>
>> This is totally a guess, but there needs to be some sort of recovery 
>> mechanism
>> in place to handle a lost credit update message.  Allowing the 
>> requester to
>> issue a limited request in the absence of credits will force a credit 
>> update if
>> any are available.
>>
>> Did you verify that the HCAs you're using implement e2e flow control?
>
> 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.
>

 From what I've seen in the various IB ULPs, the only way to remove RNRs 
is to do correct ULP flow control. 

But I never know about this IB transport level credit stuff until you 
brought it up! :)

Steve