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

[Hal Rosenstock]

Hi Fab,

On Wed, 2006-09-13 at 12:39, Fabian Tillier wrote:
> 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.

Are you referring to IPoIB-CM ?

The patch appears to be for the SA PR request prior to the CM REQ. I
don't think it affects IPoIB SA PR requests.

-- Hal


> 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