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Subject: Re: [OMPI devel] PML selection logic
From: Brian W. Barrett (brbarret_at_[hidden])
Date: 2008-06-24 10:22:50


yeah, that could be a problem, but it's such a minority case and we've got
to draw the line somewhere.

Of course, it seems like this is a never ending battle between two
opposing forces... The desire to do the "right thing" all the time at
small and medium scale and the desire to scale out to the "big thing".
It seems like in the quest to kill off the modex, we've run into these
pretty often.

The modex doesn't hurt us at small scale (indeed, we're probably ok with
the routed communication pattern up to 512 nodes or so if we don't do
anything stupid, maybe further). Is it time to admit defeat in this
argument and have a configure option that turns off the modex (at the cost
of some of these correctness checks) for the large machines, but keeps
things simple for the common case? I'm sure there are other things where
this will come up, so perhaps a --enable-large-scale? Maybe it's a dumb
idea, but it seems like we've made a lot of compromises lately around
this, where no one ends up really happy with the solution :/.

Brian

On Tue, 24 Jun 2008, George Bosilca wrote:

> Brian hinted a possible bug in one of his replies. How does this work in the
> case of dynamic processes? We can envision several scenarios, but lets take a
> simple: 2 jobs that get connected with connect/accept. One might publish the
> PML name (simply because the -mca argument was on) and one might not?
>
> george.
>
> On Jun 24, 2008, at 8:28 AM, Jeff Squyres wrote:
>
>> Also sounds good to me.
>>
>> Note that the most difficult part of the forward-looking plan is that we
>> usually can't tell the difference between "something failed to initialize"
>> and "you don't have support for feature X".
>>
>> I like the general philosophy of: running out of the box always works just
>> fine, but if you/the sysadmin is smart, you can get performance
>> improvements.
>>
>>
>> On Jun 23, 2008, at 4:18 PM, Shipman, Galen M. wrote:
>>
>>> I concur
>>> - galen
>>>
>>> On Jun 23, 2008, at 3:44 PM, Brian W. Barrett wrote:
>>>
>>>> That sounds like a reasonable plan to me.
>>>>
>>>> Brian
>>>>
>>>> On Mon, 23 Jun 2008, Ralph H Castain wrote:
>>>>
>>>>> Okay, so let's explore an alternative that preserves the support you are
>>>>> seeking for the "ignorant user", but doesn't penalize everyone else.
>>>>> What we
>>>>> could do is simply set things up so that:
>>>>>
>>>>> 1. if -mca plm xyz is provided, then no modex data is added
>>>>>
>>>>> 2. if it is not provided, then only rank=0 inserts the data. All other
>>>>> procs
>>>>> simply check their own selection against the one given by rank=0
>>>>>
>>>>> Now, if a knowledgeable user or sys admin specifies what to use for
>>>>> their
>>>>> system, we won't penalize their startup time. A user who doesn't know
>>>>> what
>>>>> to do gets to run, albeit less scalably on startup.
>>>>>
>>>>> Looking forward from there, we can look to a day where failing to
>>>>> initialize
>>>>> something that exists on the system could be detected in some other
>>>>> fashion,
>>>>> letting the local proc abort since it would know that other procs that
>>>>> detected similar capabilities may well have selected that PML. For now,
>>>>> though, this would solve the problem.
>>>>>
>>>>> Make sense?
>>>>> Ralph
>>>>>
>>>>>
>>>>>
>>>>> On 6/23/08 1:31 PM, "Brian W. Barrett" <brbarret_at_[hidden]> wrote:
>>>>>
>>>>>> 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
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> _______________________________________________
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>>>>>>>>>>
>>>>>>>>>>
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>>
>> --
>> Jeff Squyres
>> Cisco Systems
>>
>>
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