[openib-general] Re: calculate inline data size

[Fab Tillier]

> From: Sean Hefty [mailto:sean.hefty at intel.com]
> Sent: Friday, July 08, 2005 11:43 AM
> 
> >>     Gleb> But how should I know what maximum size that I can use? By
> >>     Gleb> trial and error?
> >>
> >> This is a good question that I don't have an answer to right now.
> >
> >Why not add the max inline size to the CA attributes?  HCAs that don't
> >support inline data would set it to zero.
> >
> >It does require the user to query the CA attributes to get the limit, but
> >that's probably OK.
> 
> In general I agree that the ib_device_attr is a good place to store this.  The
> problem is that the maximum changes depending on the work request type (send
> or RDMA write), if immediate data is used, and number of sge's.

As far as I know, the maximum size supported by the Mellanox HCAs is larger than
optimum - that is, the performance goes down due to the data copy into the WQE
as compared to formatting the data segment and letting the HCA DMA the data.  

If we expect all implementation that support inline data to do so beyond the
optimal point, I don't see an issue with the maximum being the smallest maximum
of all the different types of work requests.

So my vote is for adding the max inline to the device attributes, and making it
the maximum for the most constrained work request (probably RDMA Write with
immediate?).

- Fab