<HTML>
<HEAD>
<TITLE>Some "stupid user" questions, comments.</TITLE>
</HEAD>
<BODY>
<FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>Hi folks,<BR>
<BR>
I'm still sorting things out in my mind, so perhaps this note is just me talking to myself. But should you feel so compelled to sort through it, I would appreciate any feedback you might offer; and it will make me a more informed participant. <BR>
<BR>
I see two main perspectives: the user and the implementer. I come from the user side, so I feel comfortable in positing that user confusion over the size of the standard is really a function of presentation. That is, most of us get our information regarding using MPI directly from the standard. For me, this is the _only_ standard I've ever actually read! Perhaps I am missing out on thousands of C and Fortran capabilities, but sometimes ignorance is bliss. That speaks highly to the MPI specification presentation; however it need not be the case. An easy solution to the "too many routines" complaint is a tutorial/book/chapter on the basics, with pointers to further information. And in fact these books exist. That said, I hope that MPI-3 deprecates a meaningful volume of functionality.<BR>
<BR>
>From the implementer perspective, there appear to be two goals. First is to ease the burden with regard to the amount of functionality that must be supported. (And we users don't want to hear of your whining, esp. from a company the size of Intel :) Second, which overlaps with user concerns, is performance. That is, by defining a small subset of functionality, strong performance (in some sense, e.g. speed or memory requirements) can be realized.<BR>
<BR>
At the risk of starting too detailed a discussion at this early point (as well as exposing my ignorance:), I will throw out a few situations for discussion.<BR>
<BR>
</SPAN></FONT><OL><LI><FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>What would such a subset would imply with regard to what I view as support functionality, such as user-defined datatypes, topologies, etc? Ie could this support be easily provided, say by cutting-and-pasting from the full implementation you will still provide? (I now see </SPAN></FONT><SPAN STYLE='font-size:12.0px'><FONT FACE="Monaco, Courier New">Torsten recommends excluding datatypes, but what of other stuff?)
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">Even more broadly (and perhaps very ignorantly), can I simply link in both libraries, like -lmpi_subset -lmpi, getting the good stuff from the former and the excluded functionality from the latter? In addition to the application developers use of MPI, all large application programs I’ve dealt with make some use of externally produced libraries (a “very good thing” imo), which probably exceed the functionality in a “subset” implementation.
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">I (basically) understand the adverse performance effects of allowing promiscuous receives (MPI_ANY_SOURCE). However, this is a powerful capability for many codes, and used only in moderation, eg for setting up communication requirements (such as communication partners in unstructured, semi-structured, and dynamic mesh computations). In this case the sender knows its partner, but the receiver does not. A reduction(sum) is used to let each process know the number of communication partners from which it will receive data, the process posts that many promiscuous receives, which when satisfied lets it from then on specify the sender. So would it be possible to include this capability in a separate function, say the blocking send/recv, but not allow it in the non-blocking version?
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">Collectives: I can't name a code I've ever worked with that doesn't require MPI_Allreduce (though I wouldn’t be surprised to hear of many), and this in a broad set of science areas. MPI_Bcast is also often used (but quite often only in the setup phase). I see MPI_Reduce used most often to collect timing information, so MPI_Allreduce would probably be fine as well. MPI_Gather is often quite useful, as is MPI_Scatter, but again often in setup. (Though often “setup” occurs once per time step.) Non-constant size versions are often used. And others can also no doubt offer strong opinions regarding inclusion of exclusion. But from an implementation perspective, what are the issues? In particular, is the basic infrastructure for these (and other collective operations) the same? A driving premise for supporting collectives is that the sort of performance driven capability under discussion is most needed by applications running at very large scale, which is where even very good collect implementations run into problems.
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">Language bindings and perhaps other things: With the expectation/hope that full implementations continue to be available, I could use them for code development, thus making use of things like type checking, etc. And does this latter use then imply the need for "stubs" for things like the (vaporous) Fortran bindings module, communicators (if only MPI_COMM_WORLD is supported), etc.? And presuming the answer to #2 is “no”, could/should the full implementation “warn” me (preferably at compile time) when I’m using functionality that rules out use of the subset?
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">Will the profile layer still be supported? Generating usage can still be quantified using a full implementation, but performance would not be (at least in this manner), which would rule out an apples-to-apples comparison between a full implementation and the subset version with its advertised superior performance. (Of course an overall runtime could be compared, which is the final word, but a more detailed analysis is often preferred.)
</FONT></SPAN><LI><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial">If blocking and non-blocking are required of the subset, aren't these blocking semantics?<BR>
</FONT></SPAN></OL><SPAN STYLE='font-size:12.0px'><FONT FACE="Verdana, Helvetica, Arial"><BR>
MPI_Send: MPI_Isend ( ..., &req ); MPI_Wait ( ..., &req );<BR>
-----<BR>
MPI_Recv: MPI_Irecv ( ..., &req ); MPI_Wait ( &req );<BR>
<BR>
- And speaking of this, are there performance issues associated with variants of MPI_Wait, eg MPI_Waitany, MPI_Waitsome? <BR>
<BR>
Finally, I’ll officially register my concern with what I see as an increasing complexity in this effort, esp wrt “multiple subsets”. I don’t intend this comment to suppress ideas, but to help keep the beating the drum for simplicity, which I see as a key goal of this effort. <BR>
<BR>
If you read this far, thanks! My apologies if some of these issues have been previously covered. And if I've simply exposed myself as ignorant, I feel confident is stating that I am not alone - these questions will persist from others. :)<BR>
<BR>
Richard<BR>
-- <BR>
Richard Barrett<BR>
Future Technologies Group, Computer Science and Mathematics Division, and<BR>
Scientific Computing Group, National Center for Computational Science<BR>
Oak Ridge National Laboratory<BR>
<BR>
<a href="http://ft.ornl.gov/~rbarrett">http://ft.ornl.gov/~rbarrett</a><BR>
<BR>
<BR>
On 2/28/08 1:04 PM, "mpi3-subsetting-request@lists.mpi-forum.org" <mpi3-subsetting-request@lists.mpi-forum.org> wrote:<BR>
<BR>
<FONT COLOR="#0000FF"><BR>
> Thank you for your time today. It was a very good discussion. Here's<BR>
> what I captured (please add/modify what I may have missed):<BR>
> <BR>
> Present: Leonid Meyerguz (Microsoft), Rich Graham (ORNL), Richard<BR>
> Barrett (ORNL), Torsten Hoefler (ISU), Alexander Supalov (Intel)<BR>
> <BR>
> - Opens & introductions <BR>
> <BR>
> - Scope of the effort <BR>
> - Rich<BR>
> - Minimum subset consistent with the rest of MPI, for<BR>
> performance/memory footprint optimization<BR>
> - Danger of splitting MPI, hence against optional features in the<BR>
> standard<BR>
> - Both blocking & nonblocking belong to the core<BR>
> - Torsten<BR>
> - Some collectives may go into selectable subsets<BR>
> - MPI_ANY_SOURCE considered harmful<BR>
> - Leonid<BR>
> - Flexible support for optional features, means for choosing and<BR>
> advertising level of compliance/set of features<BR>
> - See enclosed email for Alexander's POV<BR>
> <BR>
> - General discussion snapshots<BR>
> - Support of subsets: some or all? If some, possible linkage problems<BR>
> in static apps (or dead calls). If all, where's the gain?<BR>
> - Optional: really optional (may be not present) or selectable (are<BR>
> present but may be unused)?<BR>
> - Performance penalty for unused subsets: implementation matter or<BR>
> standard choice?<BR>
> - Portability may be limited to certain class of applications (think<BR>
> FT, master-slave runs)<BR>
> - All we design needs to be implementable, complexity needs to be<BR>
> controlled<BR>
> - An ability to use certain set of subsets should not preclude pulling<BR>
> in other modules if necessary<BR>
> - Whatever we do, it should not conflict with the ABI efforts<BR>
> - Need to stay nice and be nicer wrt to the libraries (think<BR>
> threading) and keep things simple<BR>
> - The simplification argument, if put first, may not be liked by some<BR>
> <BR>
> - Next steps<BR>
> - Please comment on these minutes, and add/modify what I may have<BR>
> missed<BR>
> - I'll prepare a couple of slides by next week summarizing our<BR>
> discussion so far; again, your feedback will be most welcome<BR>
> - At the meeting, it may be great to meet F2F briefly and discuss any<BR>
> eventual loose ends before the presentation at the Forum; I'll see to<BR>
> this<BR>
> <BR>
> Best regards.<BR>
> <BR>
> Alexander<BR>
> <BR>
> --<BR>
> Dr Alexander Supalov<BR>
> Intel GmbH<BR>
> Hermuelheimer Strasse 8a<BR>
> 50321 Bruehl, Germany<BR>
> Phone: +49 2232 209034<BR>
> Mobile: +49 173 511 8735<BR>
> Fax: +49 2232 209029<BR>
> ---------------------------------------------------------------------<BR>
> Intel GmbH<BR>
> Dornacher Strasse 1<BR>
> 85622 Feldkirchen/Muenchen Germany<BR>
> Sitz der Gesellschaft: Feldkirchen bei Muenchen<BR>
> Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer<BR>
> Registergericht: Muenchen HRB 47456 Ust.-IdNr.<BR>
> VAT Registration No.: DE129385895<BR>
> Citibank Frankfurt (BLZ 502 109 00) 600119052<BR>
> <BR>
> This e-mail and any attachments may contain confidential material for<BR>
> the sole use of the intended recipient(s). Any review or distribution<BR>
> by others is strictly prohibited. If you are not the intended<BR>
> recipient, please contact the sender and delete all copies.<BR>
> -------------- next part --------------<BR>
> HTML attachment scrubbed and removed<BR>
> -------------- next part --------------<BR>
> An embedded message was scrubbed...<BR>
> From: "Supalov, Alexander" <alexander.supalov@intel.com><BR>
> Subject: Subsetting scope: a POV<BR>
> Date: Tue, 26 Feb 2008 11:10:15 -0000<BR>
> Size: 17674<BR>
> Url: <BR>
> <a href="http://lists.mpi-forum.org/MailArchives/mpi3-subsetting/attachments/20080228/6">http://lists.mpi-forum.org/MailArchives/mpi3-subsetting/attachments/20080228/6</a><BR>
> 73bb604/attachment.mht <BR>
> <BR>
> ------------------------------<BR>
> <BR>
> _______________________________________________<BR>
> Mpi3-subsetting mailing list<BR>
> Mpi3-subsetting@lists.mpi-forum.org<BR>
> <a href="http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-subsetting">http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-subsetting</a><BR>
> <BR>
> <BR>
> End of Mpi3-subsetting Digest, Vol 1, Issue 5<BR>
> *********************************************<BR>
</FONT></FONT></SPAN>
</BODY>
</HTML>