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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 11:48:51


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
>
>

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