Subject: RE: [rddp] Alignment and TCP modification

From: "Williams, Jim" <[email-masked]>
Date: Fri, 10 Jan 2003 11:44:04 -0500

> So, I believe the primary issue before the WG is whether to accept:
> 	- The RDDP protocols must allow implementations over
> 		unmodified TCP/IP stacks.
>           as a requirement.  

I would say yes to this.  A few more thoughts follow.

What is the required performance of an RDMA receiver when
receiving data from a transmitter using an unmodified TCP/IP stack?

    1.  Only functional correctness matters, performance is not an issue.

    2.  Performance must be as good as conventional (non-RDMA) TCP.

    3.  Performance must be significantly better than conventional
        (non-RDMA) TCP otherwise RDMA should not be used in this case.

A second question is:

       - Is the case of a large number of slow clients using
         unmodified TCP/IP stacks talking to a fast server
         using an RDMA accelerated NIC an important design point?

If the answer to the second question is YES, then I believe
that the answer to the first question is #3.

If the answer to the second question is NO, then I believe
the answer to the first question is #1, since in practice
no one will use RDMA over an unmodified TCP/IP stack.
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---

From: Caitlin Bestler <[email-masked]>
Date: Fri, 10 Jan 2003 11:32:10 -0600

On 1/10/03, Williams, Jim wrote:

> 3.  Performance must be significantly better than
> conventional (non-RDMA) TCP otherwise RDMA should not be
> used in this case.
>
I could see performance falling to 99% of TCP, where the TCP
stack had pre-posted reads. In a worst case scenario the
lack of allignment eliminates all optimizations for that
one-directional stream.

Then, no matter how valiant the DDP stack tried, it would be
hard to overcome the inherent penalties of the MPA markers.
That would especially be true if the connection were very
reliable (in the statistical sense) and dropped packets were
very rare.

That's essentially agreeing with #2, with hints of #3 if
compared to a non-optimized TCP stack. #1 is clearly
unacceptable from any viewpoint other than it could be the
result of absolute minimalism in drafting the standard. I
would favor a SHOULD that unalligned performance be at least
functionally equivalent of TCP performance.

> unmodified TCP/IP stacks talking to a fast server using
> an RDMA accelerated NIC an important design point?
>

The fast server would be using an aligned stack. Maintaining
alignment on reception over an unmodified TCP stack is far
easier (there is less for the local stack to optimize). So
the full benefits of alignment would be achieved in at least
one direction.

The server also recieves the benefit of choosing what order
to deliver tagged buffers. As previously discussed, for a
storage oriented server this can be a considerable benefit.

BTW, am I correct that the only spec issue that depends upon
these questions is that of the header CRC? A header CRC is
indeed justified if we determine the need for higher
performance on an unalligned stream.

Caitlin Bestler
http://asomi.com/CaitlinBestler/
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