[ofa-general] Re: [PATCH draft, untested] ehca srq emulation (for IPoIB CM)

[Bernard King-Smith]

Roland Dreier <rdreier at cisco.com> wrote on 06/20/2007 04:40:03 PM:

>  > We are already running with the non-SRQ patch here and the results 
are 
>  > very good. Changing to a different approach is not the right thing to 
do 
>  > at this time.
> 
> Why not, if a different approach is better?
> 
>  - R.

It is not clear if anything is better yet, but instead you have to go back 
to the IPoIB-CM  RFC 4755 that we wrote. In the spec you will see that the 
approach for this driver is to have the IPoIB driver select the most 
appropriate method of connecting. If RC was not available then UD was 
used. You can extend that to UC mode as Michael proposed, as long as you 
allow selecting the most appropriate method of connection. By pushing the 
issue of SRQ or not SRQ to the driver you have broken the IPoIB-CM 
original design. Since SRQ was not a required function in the IB spec we 
never addressed that issue in the RFC along with UC. I think we can agree 
that adding UC is a good thing and follows the approach in the original 
spec. Including SRQ as one of the tests for the best possible connection 
method follows this same approach.

If you really want to start splitting up which layer has part of the 
decision on how to connect, then you need to propose a totally different 
RFC. I prefer the approach where as few as possible places are required to 
make a connection type decision. When you change the options supported, 
then you potentially have several places that you have to address the 
changes, opening up a possible maintenance headache that Pradeep 
mentioned.

I would be interested in hearing a better approach, as long as we start 
with the approach in RFC 4755. However, for now I have not seen anything 
that says supporting both SRQ and non-SRQ in the same IPoIB-CM driver has 
disastrous impact.

Regards.


Bernie King-Smith 
IBM Corporation
Server Group
Cluster System Performance 
wombat2 at us.ibm.com    (845)433-8483
Tie. 293-8483 or wombat2 on NOTES 

"We are not responsible for the world we are born into, only for the world 
we leave when we die.
So we have to accept what has gone before us and work to change the only 
thing we can,
-- The Future." William Shatner
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