[openib-general] [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K

[Fabian Tillier]

On 9/13/06, Michael S. Tsirkin <mst at mellanox.co.il> wrote:
> Quoting r. Hal Rosenstock <halr at voltaire.com>:
> > Subject: Re: [openib-general] [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K
> >
> > On Wed, 2006-09-13 at 11:57, Michael S. Tsirkin wrote:
> > > Tavor systems get better performance with 1K MTU. Since there does
> > > not seem to be any way to find out whether the remote system uses Tavor,
> > > add an option to limit the MTU globally.
> >
> > Can't Tavor be determined locally ?
>
> It can, but we need this for remote tavor as well, anyway.
>
> > And couldn't the remote end negotiate the MTU down (if Tavor) as well ?
>
> The way to do this is would be for SA to select 1K MTU if it detects Tavor on one side
> and if this does not conflict with MTU selector.

You can't do this because the SA doesn't have a way to tell if a path
query is going to be used for RC or UD, and IPoIB needs paths with 2K
MTU.

Would be nice if the CM REP would allow the MTU to be negotiated down.
 There is plenty of space in the REP if we were to use up some of the
reserved fields.

- Fab