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Subject: Re: [OMPI devel] initial SCTP BTL commit comments?
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2007-12-01 07:10:54


On Nov 13, 2007, at 3:41 PM, Brad Penoff wrote:

> A question I had was with respect to what to set for the default value
> of btl_sctp_exclusivity... I had wanted the exclusivity to be
> "slightly less than TCP" so it was available but not the default. In
> the code I set btl_sctp_exclusivity to this:
> MCA_BTL_EXCLUSIVITY_LOW - 1
> ...however MCA_BTL_EXCLUSIVITY_LOW is defined as 0 and ompi_info says
> that exclusivity must be >= 0... a -1 exclusivity doesn't seem to
> break anything though... If two BTLs have the same exclusivity, what
> is the tie-break? Alphabetic order?

There is no tie break; they are both used. During BML selection, only
the BTLs that share the highest exclusivity settings are saved and
used later.

FWIW, here's what I see as some of the current values:

[3:54] svbu-mpi:~ % ompi_info --param btl all --parsable | grep
exclusivity | grep value
mca:btl:openib:param:btl_openib_exclusivity:value:1024
mca:btl:self:param:btl_self_exclusivity:value:65536
mca:btl:sm:param:btl_sm_exclusivity:value:65535
mca:btl:tcp:param:btl_tcp_exclusivity:value:0

Perhaps we should make TCP be the LOW value + 100 or somesuch. This
would give others the flexibility to do exactly what Brad proposes --
available, but not the default.

Alternatively, you could do what the ofud BTL does (a currently
experimental BTL): look for the string "ofud" in the "btl" MCA
parameter -- i.e., see if the user explicitly asked for the ofud BTL.
If not found (doing the Right Things with the "^" operator, of
course), then disable the ofud BTL by returning NULL from the
component_init() function.

Either seems fine to me; the ofud method seems a little less elegant
-- was there a reason not to use exclusivity here? Was it just the
fact that TCP's exclusivity is already the lowest possible value (0)?

-- 
Jeff Squyres
Cisco Systems