[openib-general] [PATCH] osm: PathRecord prefer 1K MTU for MT23108 devices

[Michael S. Tsirkin]

Quoting r. Rimmer, Todd <trimmer at silverstorm.com>:
> Subject: RE: [openib-general] [PATCH] osm: PathRecord prefer 1K MTU for MT23108 devices
> 
> > From: Eitan Zahavi [mailto:eitan at mellanox.co.il]
> > Sent: Monday, September 18, 2006 11:20 AM
> > To: Rimmer, Todd
> > Cc: Or Gerlitz; Michael S. Tsirkin; OPENIB
> > Subject: Re: [openib-general] [PATCH] osm: PathRecord prefer 1K MTU
> for
> > MT23108 devices
> > 
> > Hi Todd,
> > 
> > Seems like your knowledge about the specific MTU best for the
> > application (MPI) you are running is good
> > enough such that you will be able to include the MTU in the PathRecord
> > request and thus the patch describe in here will not affect your MPI
> at
> > all.
> > The patch only applies if your request does not  provide any MTU & MTU
> > SEL comp_mask
> 
> Eitan,
> 
> The question is not about "our MPI", rather its to ensure the Open
> Fabrics and OFED included MPIs and ULPs are capable of being tuned for
> optimal performance.  When a fabric runs more than 1 application, its
> necessary to be able to tune this at the MPI, SDP, etc level, not at the
> SM level.

We did not remove this ability at all. So it's there.

> This patch turns on a non-standard behaviour in the SM for the entire
> fabric such that some applications will have better performance while
> others will suffer.

I disagree. The behaviour is perfectly standards compliant.

> In order to be complete, this patch would need to
> include ULP level tunability in all the relevant ULPs (MPI, SDP, uDAPL,
> etc) to select the "MAX MTU" to use or to request.

This tunability is already there - that's what MTU selector in path queries
does.

> This then begs the question, if proper tuning requires all the ULPs to
> have a configurable MAX MTU, why should the SA need to implement the
> quirk at all?
> 
> Todd Rimmer

If ULP wants MAX MTU, it must set MTU selector to 3 in path query.

If MTU selector is disabled in the query, SM will guess which MTU is best to
select. SM used a specific heuristic to perform that guess.  All we did is,
provide an option to use a different heuristic.

This is useful because, SM has data on the whole fabric as opposed to ULPs
which often only have data on the endnode.

-- 
MST