[openib-general] basic IB doubt

[Greg Lindahl]

On Thu, Aug 24, 2006 at 03:43:38PM -0700, Sean Hefty wrote:
> >Actually, if a hardware implementation provided the same performance
> >(in this case latency) by polling on a CQ as one where polling on
> >memory was guaranteed to work, the customer may actually prefer the
> >"standard" implementation.
> 
> Polling on a CQ involves a function call, synchronization to the CQ, and
> formatting a structure to return to the user.  I don't see this ever being
> faster than polling memory.

Why don't you measure it, then? For example, an iWarp implementation
is going to be slowed down if it has to reorder segments to deliver
the last byte last. This expense might be more than the function call.
You guess not, but...

You're also assuming that programs are only checking the last byte of
the buffer. For all you know, Mellanox is delivering the whole buffer
in ascending order, and the user is checking bytes in the middle, too.
Which is a hazard of not-yet-specified standards extensions.

-- greg