[ofa-general] Re: [PATCH] opensm: preserve base lid routes

[Hal Rosenstock]

On Thu, 2008-06-12 at 15:11 +0300, Sasha Khapyorsky wrote:
> On 04:49 Thu 12 Jun     , Hal Rosenstock wrote:
> > On Thu, 2008-06-12 at 14:33 +0300, Sasha Khapyorsky wrote:
> > > On 03:59 Thu 12 Jun     , Hal Rosenstock wrote:
> > > > 
> > > > Would you elaborate on the motivation behind the requirement to
> > > > maintain/preserve the base LID routing ?
> > > 
> > > I see couple advantages:
> > > 
> > > 1. Application which works on base LIDs only will not be affected by LMC
> > 
> > Yes; that's the obvious one.
> > 
> > > 2. Changing LMC on a fabric will not change routing paths (when LIDs are
> > > not reassigned)
> > 
> > That's when LMC is reduced rather than increased.

> Also when increased and LIDs are not reassigned.

Is that possible for anything other than the last base LID assigned ?

> > > 3. Finally it does better balancing for secondary LIDs ("port
> > > offsetting")
> > 
> > Isn't that accomodated in the patch but separate from the base LID
> > preservation ?
> 
> It is integrated in the patch - balancing for each LID starts from
> its lower LID's port + 1.

Understood (mostly) with comment below.

> Not doing this would be really bad.

Is the badness disrupting the base LID traffic or something else ?

> > Preserving/maintaining base LIDs is a policy decision and perhaps this
> > should be an option with this as the default. For some balancing all
> > paths might be more important that not disrupting base LID traffic.
> 
> This is the trick - by preserving base LID traffic and offsetting over
> other LID ports we get better than before balancing. So right now it is
> hard to me to see when proposed option would be useful. 

When all ULPs use all LIDs "equally" and it's not just MPI ?

> But probably it would, I think we can add it then.

Sure; this can be viewed a future thing.

-- Hal

> Sasha