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

[Sasha Khapyorsky]

On 06:03 Thu 12 Jun     , Hal Rosenstock wrote:
> 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 ?

We can set with LMC=2, decrease it to 0, increase it to 1, etc..
Normally LIDs will not be reassigned in this case.

> > > > 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 ?

No, badness is bad balancing (see "port offseting" patchset thread).

> > 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 ?

Doesn't matter. LIDs still be "equial".

Sasha