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

[Michael S. Tsirkin]

Quoting r. Hal Rosenstock <halr at voltaire.com>:
> Subject: Re: [PATCH for-2.6.18] IB/cma: option to limitMTU to 1K
> 
> On Wed, 2006-09-13 at 12:22, Michael S. Tsirkin 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.
> 
> But it only needs the MTU on each local side (once for the REQ and on
> the remote side for the REP). It would mean that if the local side were
> capable of larger MTU and the remote side were Tavor, that the REQ would
> be REJ with MTU too large and need to be retried at a smaller MTU.

This has 3 implications that make it impractical:
. connection rate will suffer greatly
. this will need ot be done in each ulp, and it's a lot of code
. protocols such as sdp explicitly say what to do on rej
  and do not seem to speak about retries

> > However
> > 1. Even opensm does not implement this optimization yet
> 
> What optimization ? I don't understand what you are saying OpenSM
> doesn't support.
> 
> > 2. We need to work with existing SMs too
> 
> Not sure what the SA issue is here.

If path MTU selector in path query allows MTU 1K (e.g. "best MTU")
and one of the sides is Tavor, select the best MTU that is 1K
and not the largest possible.

If path MTU selector requires 2K MTU, return path with 2K MTU.

-- 
MST