Re: Rgerading RDMA/SDP and traditional socket apps

Sukanta ganguly said:
> Hi,
>   I do not think the issue that Mohan is raising is
> about more traffic across the wire. In essence the
> traffic on the wire will most probably be less that
> than the none SDP protocol is concerned.
>    The question of whether the application with
> synchronous socket approch will have to wait longer
> for the RNIC to do the DMA onto its local buffer
> before the app on the other side get controll of its
> buffer is concerned is a valid one.
>    So the zero copy effect is one aspect that will
> reduce the amount of real time the synchronous socket
> I/O app will face latency issues. As far as the reuse
> of the buffer is concerned, it will depend on a lot of
> elements, the LAN/WAN link speed will be one, the
> performance of the remote node will be another one.
>    So o clearly answer this the question, the
> application should be designed with more memory to be
> used as during such communication process it may not
> have the reusebility of its buffers at a very high
> rate. I think higher memroy consumption will be an
> element to wonder rather than latency, though latency
> is also an after effect.
>
>
> Thanks
> SG
>


Good point.

But it is important to remember that this can be more of an
accounting issue than an actual savings of memory.

Use of SDP, or for that matter an RDMA-aware interface,
will have the effect of replacing usage of system buffers
with usage of application buffers.

The benefit of zero-copying may result in resulting in
more total buffering, or it might merely be transferring
the same bytes from column A (system buffering) to
column B (application buffering).

The applicability statement covers these issues, though
mostly concentrating on receive buffering.  There are
applications that truly need the benefits of shared buffering
across multiple applications -- and they might be poor
candidates for RDMA.  For many other applications, and
I personally believe most, the system buffer is a bottleneck
rather than an enabler.  It is much easier to double the
amount of RAM in your system than to double its bandwidth.