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Subject: Re: [OMPI devel] RFC: Force Slurm to use PMI-1 unless PMI-2 is specifically requested
From: Artem Polyakov (artpol84_at_[hidden])
Date: 2014-05-07 20:08:25


I like #2 too.
But my question was slightly different. Can we incapsulate PMI logic that
OMPI use in common/pmi as #2 suggests but have 2 different implementations
of this component say common/pmi and common/pmi2? I am asking because I
have concerns that this kind of component is not supposed to be duplicated.
In this case we could have one common MCA parameter and 2 components as it
was suggested by Jeff.

2014-05-08 7:01 GMT+07:00 Ralph Castain <rhc_at_[hidden]>:

> The desired solution is to have the ability to select pmi-1 vs pmi-2 at
> runtime. This can be done in two ways:
>
> 1. you could have separate pmi1 and pmi2 components in each framework.
> You'd want to define only one common MCA param to direct the selection,
> however.
>
> 2. you could have a single pmi component in each framework, calling code
> in the appropriate common/pmi location. You would then need a runtime MCA
> param to select whether pmi-1 or pmi-2 was going to be used, and have the
> common code check before making the desired calls.
>
> The choice of method is left up to you. They each have their negatives. If
> it were me, I'd probably try #2 first, assuming the codes are mostly common
> in the individual frameworks.
>
>
> On May 7, 2014, at 4:51 PM, Artem Polyakov <artpol84_at_[hidden]> wrote:
>
> Just reread your suggestions in our out-of-list discussion and found that
> I misunderstand it. So no parallel PMI! Take all possible code into
> opal/mca/common/pmi.
> To additionally clarify what is the preferred way:
> 1. to create one joined PMI module having a switches to decide what
> functiononality to implement.
> 2. or to have 2 separate common modules for PMI1 and one for PMI2, and
> does this fit opal/mca/common/ ideology at all?
>
>
> 2014-05-08 6:44 GMT+07:00 Artem Polyakov <artpol84_at_[hidden]>:
>
>>
>> 2014-05-08 5:54 GMT+07:00 Ralph Castain <rhc_at_[hidden]>:
>>
>> Ummm....no, I don't think that's right. I believe we decided to instead
>>> create the separate components, default to PMI-2 if available, print nice
>>> error message if not, otherwise use PMI-1.
>>>
>>> I don't want to initialize both PMIs in parallel as most installations
>>> won't support it.
>>>
>>
>> Ok, I agree. Beside the lack of support there can be a performance hit
>> caused by PMI1 initialization at scale. This is not a case of SLURM PMI1
>> since it is quite simple and local. But I didn't consider other
>> implementations.
>>
>> On May 7, 2014, at 3:49 PM, Artem Polyakov <artpol84_at_[hidden]> wrote:
>>>
>>> We discussed with Ralph Joshuas concerns and decided to try automatic
>>> PMI2 correctness first as it was initially intended. Here is my idea. The
>>> universal way to decide if PMI2 is correct is to compare PMI_Init(..,
>>> &rank, &size, ...) and PMI2_Init(.., &rank, &size, ...). Size and rank
>>> should be equal. In this case we proceed with PMI2 finalizing PMI1.
>>> Otherwise we finalize PMI2 and proceed with PMI1.
>>> I need to clarify with SLURM guys if parallel initialization of both
>>> PMIs are legal. If not - we'll do that sequentially.
>>> In other places we'll just use the flag saying what PMI version to use.
>>> Does that sounds reasonable?
>>>
>>> 2014-05-07 23:10 GMT+07:00 Artem Polyakov <artpol84_at_[hidden]>:
>>>
>>>> That's a good point. There is actually a bunch of modules in ompi, opal
>>>> and orte that has to be duplicated.
>>>>
>>>> среда, 7 мая 2014 г. пользователь Joshua Ladd написал:
>>>>
>>>>> +1 Sounds like a good idea - but decoupling the two and adding all the
>>>>> right selection mojo might be a bit of a pain. There are several places in
>>>>> OMPI where the distinction between PMI1and PMI2 is made, not only in
>>>>> grpcomm. DB and ESS frameworks off the top of my head.
>>>>>
>>>>> Josh
>>>>>
>>>>>
>>>>> On Wed, May 7, 2014 at 11:48 AM, Artem Polyakov <artpol84_at_[hidden]>
>>>>> wrote:
>>>>>
>>>>>> Good idea :)!
>>>>>>
>>>>>> среда, 7 мая 2014 г. пользователь Ralph Castain написал:
>>>>>>
>>>>>> Jeff actually had a useful suggestion (gasp!).He proposed that we
>>>>>> separate the PMI-1 and PMI-2 codes into separate components so you could
>>>>>> select them at runtime. Thus, we would build both (assuming both PMI-1 and
>>>>>> 2 libs are found), default to PMI-1, but users could select to try PMI-2.
>>>>>> If the PMI-2 component failed, we would emit a show_help indicating that
>>>>>> they probably have a broken PMI-2 version and should try PMI-1.
>>>>>>
>>>>>> Make sense?
>>>>>> Ralph
>>>>>>
>>>>>> On May 7, 2014, at 8:00 AM, Ralph Castain <rhc_at_[hidden]> wrote:
>>>>>>
>>>>>>
>>>>>> On May 7, 2014, at 7:56 AM, Joshua Ladd <jladd.mlnx_at_[hidden]> wrote:
>>>>>>
>>>>>> Ah, I see. Sorry for the reactionary comment - but this feature falls
>>>>>> squarely within my "jurisdiction", and we've invested a lot in improving
>>>>>> OMPI jobstart under srun.
>>>>>>
>>>>>> That being said (now that I've taken some deep breaths and carefully
>>>>>> read your original email :)), what you're proposing isn't a bad idea. I
>>>>>> think it would be good to maybe add a "--with-pmi2" flag to configure since
>>>>>> "--with-pmi" automagically uses PMI2 if it finds the header and lib. This
>>>>>> way, we could experiment with PMI1/PMI2 without having to rebuild SLURM or
>>>>>> hack the installation.
>>>>>>
>>>>>>
>>>>>> That would be a much simpler solution than what Artem proposed
>>>>>> (off-list) where we would try PMI2 and then if it didn't work try to figure
>>>>>> out how to fall back to PMI1. I'll add this for now, and if Artem wants to
>>>>>> try his more automagic solution and can make it work, then we can
>>>>>> reconsider that option.
>>>>>>
>>>>>> Thanks
>>>>>> Ralph
>>>>>>
>>>>>>
>>>>>> Josh
>>>>>>
>>>>>>
>>>>>> On Wed, May 7, 2014 at 10:45 AM, Ralph Castain <rhc_at_[hidden]>
>>>>>> wrote:
>>>>>>
>>>>>> Okay, then we'll just have to develop a workaround for all those
>>>>>> Slurm releases where PMI-2 is borked :-(
>>>>>>
>>>>>> FWIW: I think people misunderstood my statement. I specifically did
>>>>>> *not* propose to *lose* PMI-2 support. I suggested that we change it to
>>>>>> "on-by-request" instead of the current "on-by-default" so we wouldn't keep
>>>>>> getting asked about PMI-2 bugs in Slurm. Once the Slurm implementation
>>>>>> stabilized, then we could reverse that policy.
>>>>>>
>>>>>> However, given that both you and Chris appear to prefer to keep it
>>>>>> "on-by-default", we'll see if we can find a way to detect that PMI-2 is
>>>>>> broken and then fall back to PMI-1.
>>>>>>
>>>>>>
>>>>>> On May 7, 2014, at 7:39 AM, Joshua Ladd <jladd.mlnx_at_[hidden]> wrote:
>>>>>>
>>>>>> Just saw this thread, and I second Chris' observations: at scale we
>>>>>> are seeing huge gains in jobstart performance with PMI2 over PMI1. We
>>>>>> *CANNOT* loose this functionality. For competitive reasons, I
>>>>>> cannot provide exact numbers, but let's say the difference is in the
>>>>>> ballpark of a full order-of-magnitude on 20K ranks versus PMI1. PMI1 is
>>>>>> completely unacceptable/unusable at scale. Certainly PMI2 still has scaling
>>>>>> issues, but there is no contest between PMI1 and PMI2. We (MLNX) are
>>>>>> actively working to resolve some of the scalability issues in PMI2.
>>>>>>
>>>>>> Josh
>>>>>>
>>>>>> Joshua S. Ladd
>>>>>> Staff Engineer, HPC Software
>>>>>> Mellanox Technologies
>>>>>>
>>>>>> Email: joshual_at_[hidden]
>>>>>>
>>>>>>
>>>>>> On Wed, May 7, 2014 at 4:00 AM, Ralph Castain <rhc_at_[hidden]>
>>>>>> wrote:
>>>>>>
>>>>>> Interesting - how many nodes were involved? As I said, the bad
>>>>>> scaling becomes more evident at a fairly high node count.
>>>>>>
>>>>>> On May 7, 2014, at 12:07 AM, Christopher Samuel <
>>>>>> samuel_at_[hidden]> wrote:
>>>>>>
>>>>>> > -----BEGIN PGP SIGNED MESSAGE-----
>>>>>> > Hash: SHA1
>>>>>> >
>>>>>> > Hiya Ralph,
>>>>>> >
>>>>>> > On 07/05/14 14:49, Ralph Castain wrote:
>>>>>> >
>>>>>> >> I should have looked closer to see the numbers you posted, Chris -
>>>>>> >> those include time for MPI wireup. So what you are seeing is that
>>>>>> >> mpirun is much more efficient at exchanging the MPI endpoint info
>>>>>> >> than PMI. I suspect that PMI2 is not much better as the primary
>>>>>> >> reason for the difference is that mpriun sends blobs, while PMI
>>>>>> >> requires that everything b
>>>>>>
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>>>>>>
>>>>>
>>>>>
>>>
>>>
>>> --
>>> С Уважением, Поляков Артем Юрьевич
>>> Best regards, Artem Y. Polyakov
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>>
>>
>>
>> --
>> С Уважением, Поляков Артем Юрьевич
>> Best regards, Artem Y. Polyakov
>>
>
>
>
> --
> С Уважением, Поляков Артем Юрьевич
> Best regards, Artem Y. Polyakov
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-- 
С Уважением, Поляков Артем Юрьевич
Best regards, Artem Y. Polyakov