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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 19:51:24


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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>>>>> Link to this post:
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>>>>>
>>>>
>>>>
>>
>>
>> --
>> С Уважением, Поляков Артем Юрьевич
>> Best regards, Artem Y. Polyakov
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>>
>>
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>
>
>
> --
> С Уважением, Поляков Артем Юрьевич
> Best regards, Artem Y. Polyakov
>

-- 
С Уважением, Поляков Артем Юрьевич
Best regards, Artem Y. Polyakov