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Subject: Re: [OMPI devel] [RFC] Non-blocking collectives (LibNBC) merge to trunk
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-02-08 12:16:42


Terry -- I reluctantly agree. :-) What I envision is not difficult
(a first cut/feature-lean version is probably only several hundred
lines of perl?), but I don't have the cycles (at present) to implement
it -- my priorities are elsewhere at the moment.

If anyone is interested in this, I would gladly talk them through what
[I think] needs to be done.

That being said, for NBC, per Terry's points:

- if it's not compiled/installed by default
- if we can make a big enough red flag for users that it's an R&D
effort that is subject to change (perhaps 3'x5'?)

Then I think it would not be a bad thing to include NBC. But then I
think we need to disallow any other contrib/ projects until someone
can find the cycles to implement a better solution (such as an
ompi_contrib executable/system).

On Feb 7, 2008, at 1:18 PM, Terry Dontje wrote:

> Jeff, the below sounds good if we really believe there is going to
> be a
> whole bunch of addons. I am not sure NBC really constitute as an
> addon
> than more some research work that might become an official API. So I
> look at the NBC stuff more like a BTL or PM that is in progress of
> being
> developed/refined for prime time. So would a new PM or BTL be added
> via
> ompi_contrib? I wouldn't think they would.
>
> The ompi_contrib sounds like a nice utility but I have feeling there
> are
> bigger fish to fry unless we really believe there will be a lot of
> addons that we will need to support.
>
> --td
>
> Jeff Squyres wrote:
>> All these comments are good. I confess that although I should
>> have, I
>> really did not previously consider the complexity of adding in N
>> contrib packages to OMPI.
>>
>> The goal of the contrib packages is to easily allow additional
>> functionality that is nicely integrated with Open MPI. An obvious
>> way
>> to do this is to include the code in the Open MPI tarball, but that
>> leads to the logistics and other issues that have been identified.
>>
>> Ralph proposes a good way around this. But what about going farther
>> than that: what we if we offer a standardized set of hooks for
>> including contrib functionality *after* core OMPI has been installed?
>> Yes, it's one more step after OMPI has been installed -- but if we
>> can
>> keep it as *one* step, perhaps the user onus is not that bad. Let me
>> explain.
>>
>> Consider a new standalone executable: ompi_contrib. You would run
>> ompi_contrib to install and uninstall contrib functionality into your
>> existing OMPI:
>>
>> ompi_contrib --install http://www.example.com/nbc/nbc-ompi-contrib.tar.gz
>> or ompi_contrib --install file:///home/htor/nbc-ompi-contrib.tar.gz
>>
>> This will download NBC (if http), build it, and install it into the
>> current OMPI. It is likely that the nbc-ompi-contrib.tar.gz file
>> will
>> contain the real NBC tarball (or maybe just a reference to it?)
>> plus a
>> small number of hook/glue scripts for OMPI integration (perhaps quite
>> similar to what is in the contrib/ tree [on the branch] today for
>> NBC?). Likewise, after NBC is installed into the local OMPI
>> installation, ompi_info should be able to show "nbc" as installed
>> contrib functionality. It then follows that we might be able to do:
>>
>> ompi_contrib --uninstall nbc
>>
>> to uninstall contrib NBC from the local OMPI installation.
>>
>> This kind of approach would seem to have several benefits:
>>
>> - Keep a clear[er] distinction between core OMPI and contributed
>> packages.
>>
>> - Allow simple integration of MPI libraries, tools, and even
>> applications (!) (think: numerical libraries, boost C++ libraries,
>> etc. -- how many of your users install additional tools on top of MPI
>> incorrectly?). Anything
>>
>> - Allow 3rd parties to have "contrib" code to Open MPI without
>> needing
>> to get into our code tree (and sign the 3rd party agreements, etc.),
>> keeping our distribution size down, avoiding release schedule
>> logistical issues, keeping our "core" build time down, etc.
>>
>> - Allow integration of contrib functionality at both a per-user and
>> system-wide basis.
>>
>> What I'm really proposing here is that OMPI becomes a system that can
>> have additional functionality installed / uninstalled. Based on the
>> infrastructure that we already have, this is not as much of a stretch
>> as one would think.
>>
>> Comments?
>>
>> ("who's going to write this" is a question that will also have to be
>> answered, but perhaps we can discuss the code concept/idea first...)
>>
>>
>>
>> On Feb 7, 2008, at 10:11 AM, Ralph H Castain wrote:
>>
>>
>>> I believe Brian and Terry raise good points. May I offer a possible
>>> alternative? What if we only include in Open MPI an include file
>>> that
>>> contains the "hooks" to libNBC, and have the build system only "see"
>>> those
>>> if someone specifies --with-NBC (or whatever option name you like).
>>> If you
>>> like, you can make the inclusion automatic if libNBC is detected on
>>> the
>>> system. It would make sense to also add -libNBC to the mpicc et al
>>> wrappers
>>> as well when the build system includes the function definitions.
>>>
>>> This would allow those users that want (or can) to use that library
>>> link
>>> against it, without adding a bunch of source code to our release. I
>>> suspect
>>> there are complications that will have to be dealt with, but offer
>>> it as
>>> something to consider.
>>>
>>>
>>> Also, remember that there is an added burden when we add source code
>>> to Open
>>> MPI that we haven't discussed - we are now adding coordination
>>> issues to our
>>> own release cycle. If libNBC changes, are we now going to be pressed
>>> to
>>> issue another OMPI release so that the new NBC version is included?
>>> Do we
>>> now need to coordinate our releases with theirs so that things
>>> align?
>>>
>>> And if we have an increasing number of such "included" packages, how
>>> complex
>>> is -that- release discussion going to get?!?
>>>
>>>
>>> On 2/7/08 4:48 AM, "Terry Dontje" <Terry.Dontje_at_[hidden]> wrote:
>>>
>>>
>>>> Torsten Hoefler wrote:
>>>>
>>>>> Hi Brian,
>>>>>
>>>>>
>>>>>> Let me start by reminding everyone that I have no vote, so this
>>>>>> should
>>>>>> probably be sent to /dev/null.
>>>>>>
>>>>>>
>>>>> thanks for your comment and this will not go to /dev/null!
>>>>>
>>>>>
>>>>>
>>>>>> I think Ralph raised some good points. I'd like to raise
>>>>>> another.
>>>>>>
>>>>>>
>>>>> yes [will reply to this in a separate thread]
>>>>>
>>>>>
>>>>>
>>>>>> Does it make sense to bring LibNBC into the release at this
>>>>>> point,
>>>>>> given the current standardization process of non-blocking
>>>>>> collectives?
>>>>>>
>>>>>> My feeling is no, based on the long term support costs. We had
>>>>>> this
>>>>>> problem with a function in LAM/MPI -- MPIL_SPAWN, I believe it
>>>>>> was --
>>>>>> that was almost but not quite MPI_COMM_SPAWN. It was added to
>>>>>> allow
>>>>>> spawn before the standard was finished for dynamics. The problem
>>>>>> is,
>>>>>> it wasn't quite MPI_COMM_SPAWN, so we were now stuck with yet
>>>>>> another
>>>>>> function to support (in a touchy piece of code) for infinity and
>>>>>> beyond.
>>>>>>
>>>>>> I worry that we'll have the same with LibNBC -- a piece of code
>>>>>> that
>>>>>> solves an immediate problem (no non-blocking collectives in MPI)
>>>>>> but
>>>>>> will become a long-term support anchor. Since this is something
>>>>>> we'll
>>>>>> be encouraging users to write code to, it's not like support for
>>>>>> mvapi, where we can just deprecate it and users won't really
>>>>>> notice.
>>>>>> It's one thing to tell them to update their cluster software
>>>>>> stack --
>>>>>> it's another to tell them to rewrite their applications.
>>>>>>
>>>>>>
>>>>> I think this is a very good and valid point. However, I would like
>>>>> to
>>>>> deprecate the NBC_* things as soon as non-blocking collectives
>>>>> are a
>>>>> part of the standard. Of course, this would probably need two
>>>>> minor
>>>>> versions to "clean" the code-base, but this is (will be) our
>>>>> normal
>>>>> procedure (just what happened to MVAPI).
>>>>>
>>>>>
>>>>>
>>>> Though it doesn't seem to me that NBC is a slam dunk to get into
>>>> the MPI
>>>> spec and I could
>>>> imagine it changing significantly due to someone elses opinion/
>>>> needs.
>>>>
>>>>> And rewriting the user's application will not be that hard, it'll
>>>>> mainly
>>>>> be vim:%s/NBC_/MPI_/g. Even if we change the interface (e.g. add
>>>>> tags or
>>>>> decide to use the more limited split collective approach), this
>>>>> task is
>>>>> rather easy and can be automated easily. It's not a functionality
>>>>> change, just an interface.
>>>>>
>>>>>
>>>>>
>>>> Though if NBC is built by default for release builds I think that
>>>> raises
>>>> the bar saying that we
>>>> OMPI believe this should be used by all of our users without any
>>>> concerns that the API may
>>>> change or it might have significant issues.
>>>>
>>>> On a similar track do you have any tests that validate the
>>>> functionality/correctness of NBC
>>>> that can be ran as a part of the MTT nightly tests?
>>>>
>>>> My opinion is I have no problem with NBC being merged in just
>>>> that I
>>>> don't think it should be
>>>> built by default.
>>>>
>>>> --td
>>>>
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>>
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-- 
Jeff Squyres
Cisco Systems