Re: Rgerading RDMA/SDP and traditional socket apps

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


--- Caitlin Bestler <[email protected]> wrote:

> 
> On Oct 4, 2004, at 5:04 AM, Mohan Parthasarathy
> wrote:
> 
> >  Hi Mike/Caitlin,
> >
> >    Thanks for the answers. So, what I understand
> is that my assumption
> >    that existing socket apps might not have
> ownership of the buffer 
> > when
> >    the send completes, was wrong.
> >    But, doesn't this mean that send calls will
> take more time to 
> > complete,
> >    if the RNIC has to DMA the buffers before
> control to the buffer is
> >    returned to the caller. Are there any potential
> performance issues 
> > here
> >    for existing synchronous sockets ?
> >
> >  Regards,
> >  Mohan
> 
> 
> SDP defines multiple transfer procedures precisely
> to avoid
> this problem.  SDP should be configured so that
> zero-copy
> transfers are only done when doing so will improve
> performance.
> 
> For example,  when the receive  is posted long
> enough before
> the send for the data sink buffer to have been
> advertised the
> data source can RDMA Write to the final target
> buffer without
> any delay.
> 
> SDP inherently involves more wire traffic, but
> depending on
> the timing of sends and receives this extra traffic
> may have no
> impact on latency.  The benefits of direct placement
> should
> overcome any penalty for the wire traffic.  But that
> penalty is
> there, which is why local-only optimizations that
> achieve direct
> placement without wire traffic might be more
> efficient, and why
> full adaptation to RDMA-aware application logic will
> almost
> always be more efficient.
> 
> 
> 
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> 


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