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Subject: Re: [OMPI devel] if btl->add_procs() fails...?
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-08-04 09:40:56


On Aug 2, 2008, at 2:34 PM, Brian Barrett wrote:

>> I am curious how all of the above affects client/server or spawned
>> jobs. If you finalize a BTL then do a connect to a process that
>> would use that BTL would it reinitialize itself?
>
> To deal with all the dynamics issues, I wouldn't finalized the BTL.
> The BML should handle the progress stuff, just as if the add_procs
> succeeded but returned no active peers. But I'd guess that's part of
> the bit that doesn't work today. I would further suspect that a BTL
> will need to have a working progress function in the face of
> add_procs failures to cope with all the dynamics options. I'm
> travelling this weekend, so I can't verify any of this at the moment.

This seems a little different than the rest of the code base -- we're
talking about having the BTL return an error but have the upper level
not treat it as a fatal error.

I think we actually have a few different situations ("fail" means "not
returning OMPI_SUCCESS"):

1. btl component init fails (only during MPI_INIT): the API supports
no notion of failure -- it either returns modules or not (i.e., valid
pointers or NULL). If NULL is returned, the component is ignored and
unloaded.
2. btl add_procs during MPI_INIT fails: this is under debate
3. btl add_procs during MPI-2 dynamics fails: this is under debate

For #2 and #3, I suspect that only the BTL knows if it can continue or
not. For example, a failure in #3 may cause the entire BTL to be
hosed such that it can no longer communicate with procs that it
previously successfully added (e.g., in MPI_INIT). So we really need
add_procs to be able to return multiple things:

A. OMPI_SUCCESS / all was good
B. a non-fatal error occurred such that this BTL cannot communicate
with the desired peers, but the upper level PML can continue
C. a fatal error has occurred such that the upper level should abort
(or, more specifically, do whatever the error manager says)

I think that for B in both #2 and #3, we can just have the BTL set all
the reachability bits to 0 and return OMPI_SUCCESS. But for C, the
BTL should return != OMPI_SUCCESS. The PML should treat it as a fatal
error and therefore call the error manager.

I think that this is in-line with Brian's original comments, right?

-- 
Jeff Squyres
Cisco Systems