[openib-general] [PATCH 0/6] Tranport Neutral Verbs Proposal.

[Greg Lindahl]

On Mon, Jul 31, 2006 at 01:25:39PM -0400, James Lentini wrote:

> I agree that the term RDMA SEND is confusing. However, the data in an 
> RDMA SEND is deposited directly (zero copy) into the users memory. 

There are many mechanisms other than DMA or RDMA which have this
property. You're confusing specification with implementation, too.
When I read from a disk on modern Unix, the data is deposited into
the user's memory, whether it's DMA or PIO.

The defining characteristic of RDMA is that it deposits or reads data
based on address provided by the other side, *and* that it has one-sided
semantics. In ordinary messaging, data is transferred from buffers
which are much less flexibly addressed, and semantics are two-sided.

> > Here's why it's a problem: I've repeatedly seen people try to use RDMA
> > (get and put) all the time because they think it must be faster than
> 
> I'm assuming RDMA get/put correspond to RDMA READ/WRITE.

Yes, "get" and "put" are what the general community have traditionally
called these operations. These names emphasize the one-sided nature of
the operation, unlike the new official(tm) names.

> > simple send and receive... that's what the slogans tell them. But then
> > they discover that they need to use ordinary SEND/RECV for shorter
> > messages and for conversations with a lot of participants. 
> 
> By ordinary SEND/RECV, do you mean IB/iWARP SEND/RECV or traditional 
> (sockets) networking send(2)/recv(2)?

I was actually thinking of OpenIB SEND/RECV.

> > That's a technical screwup caused by the marketing slogan.
> 
> The terms RDMA read and RDMA write are technically accurate.

It seems we have different defintions of "technical", then. Slogans
don't make good engineering.

> Perhaps someone can think of a better prefix. How about dav_ (direct 
> access verb)?

That's much better than rdma_, but do you really think the Linux folks
are going to be happy about OpenFabrics calls with a prefix that
doesn't look anything like "Open Fabrics"?

-- greg