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Subject: Re: [OMPI devel] PML selection logic
From: Brian W. Barrett (brbarret_at_[hidden])
Date: 2008-06-23 15:31:43


The problem is that we default to OB1, but that's not the right choice for
some platforms (like Pathscale / PSM), where there's a huge performance
hit for using OB1. So we run into a situation where user installs Open
MPI, starts running, gets horrible performance, bad mouths Open MPI, and
now we're in that game again. Yeah, the sys admin should know what to do,
but it doesn't always work that way.

Brian

On Mon, 23 Jun 2008, Ralph H Castain wrote:

> My fault - I should be more precise in my language. ;-/
>
> #1 is not adequate, IMHO, as it forces us to -always- do a modex. It seems
> to me that a simpler solution to what you describe is for the user to
> specify -mca pml ob1, or -mca pml cm. If the latter, then you could deal
> with the failed-to-initialize problem cleanly by having the proc directly
> abort.
>
> Again, sometimes I think we attempt to automate too many things. This seems
> like a pretty clear case where you know what you want - the sys admin, if
> nobody else, can certainly set that mca param in the default param file!
>
> Otherwise, it seems to me that you are relying on the modex to detect that
> your proc failed to init the correct subsystem. I hate to force a modex just
> for that - if so, then perhaps this could again be a settable option to
> avoid requiring non-scalable behavior for those of us who want scalability?
>
>
> On 6/23/08 1:21 PM, "Brian W. Barrett" <brbarret_at_[hidden]> wrote:
>
>> The selection code was added because frequently high speed interconnects
>> fail to initialize properly due to random stuff happening (yes, that's a
>> horrible statement, but true). We ran into a situation with some really
>> flaky machines where most of the processes would chose CM, but a couple
>> would fail to initialize the MTL and therefore chose OB1. This lead to a
>> hang situation, which is the worst of the worst.
>>
>> I think #1 is adequate, although it doesn't handle spawn particularly
>> well. And spawn is generally used in environments where such network
>> mismatches are most likely to occur.
>>
>> Brian
>>
>>
>> On Mon, 23 Jun 2008, Ralph H Castain wrote:
>>
>>> Since my goal is to eliminate the modex completely for managed
>>> installations, could you give me a brief understanding of this eventual PML
>>> selection logic? It would help to hear an example of how and why different
>>> procs could get different answers - and why we would want to allow them to
>>> do so.
>>>
>>> Thanks
>>> Ralph
>>>
>>>
>>>
>>> On 6/23/08 11:59 AM, "Aurélien Bouteiller" <bouteill_at_[hidden]> wrote:
>>>
>>>> The first approach sounds fair enough to me. We should avoid 2 and 3
>>>> as the pml selection mechanism used to be
>>>> more complex before we reduced it to accommodate a major design bug in
>>>> the BTL selection process. When using the complete PML selection, BTL
>>>> would be initialized several times, leading to a variety of bugs.
>>>> Eventually the PML selection should return to its old self, when the
>>>> BTL bug gets fixed.
>>>>
>>>> Aurelien
>>>>
>>>> Le 23 juin 08 à 12:36, Ralph H Castain a écrit :
>>>>
>>>>> Yo all
>>>>>
>>>>> I've been doing further research into the modex and came across
>>>>> something I
>>>>> don't fully understand. It seems we have each process insert into
>>>>> the modex
>>>>> the name of the PML module that it selected. Once the modex has
>>>>> exchanged
>>>>> that info, it then loops across all procs in the job to check their
>>>>> selection, and aborts if any proc picked a different PML module.
>>>>>
>>>>> All well and good...assuming that procs actually -can- choose
>>>>> different PML
>>>>> modules and hence create an "abort" scenario. However, if I look
>>>>> inside the
>>>>> PML's at their selection logic, I find that a proc can ONLY pick a
>>>>> module
>>>>> other than ob1 if:
>>>>>
>>>>> 1. the user specifies the module to use via -mca pml xyz or by using a
>>>>> module specific mca param to adjust its priority. In this case,
>>>>> since the
>>>>> mca param is propagated, ALL procs have no choice but to pick that
>>>>> same
>>>>> module, so that can't cause us to abort (we will have already
>>>>> returned an
>>>>> error and aborted if the specified module can't run).
>>>>>
>>>>> 2. the pml/cm module detects that an MTL module was selected, and
>>>>> that it is
>>>>> other than "psm". In this case, the CM module will be selected
>>>>> because its
>>>>> default priority is higher than that of OB1.
>>>>>
>>>>> In looking deeper into the MTL selection logic, it appears to me
>>>>> that you
>>>>> either have the required capability or you don't. I can see that in
>>>>> some
>>>>> environments (e.g., rsh across unmanaged collections of machines),
>>>>> it might
>>>>> be possible for someone to launch across a set of machines where
>>>>> some do and
>>>>> some don't have the required support. However, in all other cases,
>>>>> this will
>>>>> be homogeneous across the system.
>>>>>
>>>>> Given this analysis (and someone more familiar with the PML should
>>>>> feel free
>>>>> to confirm or correct it), it seems to me that this could be
>>>>> streamlined via
>>>>> one or more means:
>>>>>
>>>>> 1. at the most, we could have rank=0 add the PML module name to the
>>>>> modex,
>>>>> and other procs simply check it against their own and return an
>>>>> error if
>>>>> they differ. This accomplishes the identical functionality to what
>>>>> we have
>>>>> today, but with much less info in the modex.
>>>>>
>>>>> 2. we could eliminate this info from the modex altogether by
>>>>> requiring the
>>>>> user to specify the PML module if they want something other than the
>>>>> default
>>>>> OB1. In this case, there can be no confusion over what each proc is
>>>>> to use.
>>>>> The CM module will attempt to init the MTL - if it cannot do so,
>>>>> then the
>>>>> job will return the correct error and tell the user that CM/MTL
>>>>> support is
>>>>> unavailable.
>>>>>
>>>>> 3. we could again eliminate the info by not inserting it into the
>>>>> modex if
>>>>> (a) the default PML module is selected, or (b) the user specified
>>>>> the PML
>>>>> module to be used. In the first case, each proc can simply check to
>>>>> see if
>>>>> they picked the default - if not, then we can insert the info to
>>>>> indicate
>>>>> the difference. Thus, in the "standard" case, no info will be
>>>>> inserted.
>>>>>
>>>>> In the second case, we will already get an error if the specified
>>>>> PML module
>>>>> could not be used. Hence, the modex check provides no additional
>>>>> info or
>>>>> value.
>>>>>
>>>>> I understand the motivation to support automation. However, in this
>>>>> case,
>>>>> the automation actually doesn't seem to buy us very much, and it isn't
>>>>> coming "free". So perhaps some change in how this is done would be
>>>>> in order?
>>>>>
>>>>> Ralph
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> devel mailing list
>>>>> devel_at_[hidden]
>>>>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>>>
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