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Re: [mpich-devel] [mpich-intel] Sessions Considering Changing MPI_COMM_WORLD
by Dinan, James 31 Oct '17

31 Oct '17

IIUC, this would break code that uses MPI_COMM_WORLD as a static initializer, e.g., to initialize a global variable like "MPI_Comm mycomm = MPI_COMM_WORLD". Why can't the handles themselves be compile-time constants corresponding to objects that are not created until the library is initialized (how it works now)? Note that such handles are defined to be compile-time constants. I don't know that they necessarily correspond to symbols that you can take an address of. ~Jim. From: intel <intel-bounces(a)lists.mpich.org<mailto:[email protected]>> on behalf of "Bland, Wesley" <wesley.bland(a)intel.com<mailto:[email protected]>> Date: Tuesday, October 31, 2017 at 10:16 AM To: "devel(a)mpich.org<mailto:[email protected]>" <devel(a)mpich.org<mailto:[email protected]>> Cc: "intel(a)mpich.org<mailto:[email protected]>" <intel(a)mpich.org<mailto:[email protected]>> Subject: Re: [mpich-intel] [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD I misrepresented what I was trying to say here I think. I don't believe it's necessary to remove the symbol MPI_COMM_WORLD. What it does change (at least for MPICH-derived implementations) is that before MPI_INIT(), MPI_COMM_WORLD might be MPI_COMM_NULL (or something else with no meaning), and after MPI_INIT(), the handle would be changed to something more useful. The fact that the handle would change midstream is what's different. On Oct 30, 2017, at 5:51 PM, Jeff Hammond <jeff.science(a)gmail.com<mailto:[email protected]>> wrote: Lots of MPI libraries create their own communicator from the compile-time constant rather than a communicator argument. Breaking even a few of these will ruin MPI’s exhalted reputation in the HPC world. This doesn't prevent the global variable from existing. In fact, if you were blindly using MPI_COMM_WORLD before without checking whether or not MPI was initialized, bad things were going to happen anyway. What this does do is say that if you haven't called MPI_INIT(), MPI_COMM_WORLD will probably point to MPI_COMM_NULL or something. Perhaps saying that MPI_COMM_WORLD won't be a compile time constant isn't correct. The better way of saying it is probably that MPI_COMM_WORLD won't evaluate to anything useful until it's filled in during MPI_INIT(). If you end up using the sessions interface instead, the MPI library never has to create MPI_COMM_WORLD and incur all of the associated startup cost. The better option is to provide new headers and libraries for sessions that break backwards compatibility. Or just have users opt-in to sessions by not using MPI_COMM_WORLD at all. That way you’re only breaking code that wants this feature rather than many that don’t. Completely breaking backward compatibility is one option that was considered, but the working group thought that if we did things this way, it would still be possible for legacy codes to work without moving to the new Sessions interface. What’s your Fortran story? If you don’t have a trivial solution for users of mpif.h, you’ve lost approximately half if MPI’s users. I think it's similar to what it is now. MPI_COMM_WORLD is a global variable that has no meaning until MPI_INIT() is called.

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Re: [mpich-devel] whitespace cleanup proposal
by Bland, Wesley 31 Oct '17

31 Oct '17

I'm fine with leaving CH3 alone (I don't do much development in there), but it would make this sort of thing a lot less useful if we can't run it on all of the other code (mpi, mpid/common, util, etc.). There's still plenty of work happening there and if we're picking and choosing where the code cleanup script is run, automated checking gets harder too. > On Oct 31, 2017, at 9:44 AM, Kenneth Raffenetti <raffenet(a)mcs.anl.gov> wrote: > > Hi folks, > > Some devs have expressed a desire to run the maint/code-cleanup.sh script across the whole MPICH repo. This would make all whitespace and style consistent, and give us the opportunity to use an automated checker for new code to ensure it matches the rest of the repo. > > There has been reasonable concern that we not run this on the src/mpid/ch3 directory, as it would be too disruptive to downstream repos. I think we are in agreement that we will leave ch3 alone, if we do this. ch4 is still early enough along that we hope it would be ok to clean up now. > > Please let us know if you have feedback. We'd like to make a decision regarding this prior to the 3.3b1 release at SC17. > > Thanks, > Ken > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel

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whitespace cleanup proposal
by Kenneth Raffenetti 31 Oct '17

31 Oct '17

Hi folks, Some devs have expressed a desire to run the maint/code-cleanup.sh script across the whole MPICH repo. This would make all whitespace and style consistent, and give us the opportunity to use an automated checker for new code to ensure it matches the rest of the repo. There has been reasonable concern that we not run this on the src/mpid/ch3 directory, as it would be too disruptive to downstream repos. I think we are in agreement that we will leave ch3 alone, if we do this. ch4 is still early enough along that we hope it would be ok to clean up now. Please let us know if you have feedback. We'd like to make a decision regarding this prior to the 3.3b1 release at SC17. Thanks, Ken

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Bland, Wesley 31 Oct '17

31 Oct '17

I don't want to shut down a useful discussion, but this would probably have a more helpful audience on the Sessions Working Group mailing list (mpiwg-sessions(a)lists.mpi-forum.org<mailto:[email protected]>). There are plenty of folks there to continue this conversation, but I'd like to keep this thread on the original topic if possible since most of the people who could address concerns in the proposal aren't on this mailing list. :) On Oct 31, 2017, at 3:26 AM, Lisandro Dalcin <dalcinl(a)gmail.com<mailto:[email protected]>> wrote: Now I'll go off topic with some minor criticism to the proposal.

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Bland, Wesley 31 Oct '17

31 Oct '17

I misrepresented what I was trying to say here I think. I don't believe it's necessary to remove the symbol MPI_COMM_WORLD. What it does change (at least for MPICH-derived implementations) is that before MPI_INIT(), MPI_COMM_WORLD might be MPI_COMM_NULL (or something else with no meaning), and after MPI_INIT(), the handle would be changed to something more useful. The fact that the handle would change midstream is what's different. On Oct 30, 2017, at 5:51 PM, Jeff Hammond <jeff.science(a)gmail.com<mailto:[email protected]>> wrote: Lots of MPI libraries create their own communicator from the compile-time constant rather than a communicator argument. Breaking even a few of these will ruin MPI’s exhalted reputation in the HPC world. This doesn't prevent the global variable from existing. In fact, if you were blindly using MPI_COMM_WORLD before without checking whether or not MPI was initialized, bad things were going to happen anyway. What this does do is say that if you haven't called MPI_INIT(), MPI_COMM_WORLD will probably point to MPI_COMM_NULL or something. Perhaps saying that MPI_COMM_WORLD won't be a compile time constant isn't correct. The better way of saying it is probably that MPI_COMM_WORLD won't evaluate to anything useful until it's filled in during MPI_INIT(). If you end up using the sessions interface instead, the MPI library never has to create MPI_COMM_WORLD and incur all of the associated startup cost. The better option is to provide new headers and libraries for sessions that break backwards compatibility. Or just have users opt-in to sessions by not using MPI_COMM_WORLD at all. That way you’re only breaking code that wants this feature rather than many that don’t. Completely breaking backward compatibility is one option that was considered, but the working group thought that if we did things this way, it would still be possible for legacy codes to work without moving to the new Sessions interface. What’s your Fortran story? If you don’t have a trivial solution for users of mpif.h, you’ve lost approximately half if MPI’s users. I think it's similar to what it is now. MPI_COMM_WORLD is a global variable that has no meaning until MPI_INIT() is called.

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Lisandro Dalcin 31 Oct '17

31 Oct '17

I concur with Jed Brown. However, I would stress that "link time constants" is handled differently on POSIX vs Windows dynamic loaders, in the second you cannot "static MPI_Comm world = MPI_COMM_WORLD;") Now I'll go off topic with some minor criticism to the proposal. @JeffSquyres I prefer to complain with patches rather than words, but in this case there is no code to patch :-) I really hope that at some point this MPI Sessions proposal is watered down. There are quite a bit of things in the proposed API that looks superfluous and maybe they are not really needed. * Look these ones: MPI_FLAG_THREAD_NONCONCURRENT_SINGLE MPI_FLAG_THREAD_NONCONCURRENT_FUNNELED MPI_FLAG_THREAD_NONCONCURRENT_SERIALIZED MPI_FLAG_THREAD_CONCURRENT Do we really really need these new constants? We already have 4 constants related to threads and we should need to keep them for compatibility with MPI_Init_thread(). * Look this one: MPI_Session_get_names( IN MPI_Session session, OUT char **set_names) Why not returning the number of entries in the output array as well? What about Fortran wrappers? "The caller is responsible for freeing the returned array of strings." How? C stdlib free()? MPI_Free_mem()? Just the array, or also the string entries within the array? Won't someone think of the Fortrans! * Look this one: MPI_Create_comm_from_group( IN MPI_Group group, IN const char *tag, // for matching (see next slide) IN MPI_Info info, IN MPI_Errhandler errhander, OUT MPI_Comm *comm) Maybe It should be called MPI_Intracomm_create_from_group(), also MPI_Intercomm_create_from_group(). Not a strong complaint but just a question/observation: Dow we really need tag to be "const char*" and not just "int"? We already have the precedent of MPI_Intercomm_create() using integer tags to disambiguate. What about Fortran wrappers? Do we really need to pass MPI_Errhandler? Why not use the one passed to the Session create call? Of course, this would complicate the implementation, groups created from sessions should keep track of the session errhandler. Or maybe we should also pass the MPI_Session handle to the {intra|inter}comm create calls, i.e. something like MPI_{Inter|Intra}comm_create_from_session(IN MPI_Session session, ...) ? * Look at all these ones: MPI_Create_cart_comm_from_group() MPI_Create_graph_comm_from_group(…) MPI_Create_dist_graph_comm_from_group(…) MPI_Create_dist_graph_adjacent_comm_from_group(…) MPI_Create_win_from_group() MPI_Create_file_from_group() Do we really need all this API bomb exploding on our face? It seems to me that all of them can be implemented by first creating creating an intracomm out of the session, then create the other with the calls we already have in MPI. Of course performance matters, but I would argue that the potential performance issues have to be demonstrated first, then outweighed with the extra API complexity. * Finally, in the "Additional notes" section at the end, we have +objects derived from Session A cannot be used to communicate with objects derived from Session B +MPI cannot communicate between Sessions (Send, receive, put, get, intercommunicator creation, connect/accept, etc.) +Cannot have requests from different Sessions in a single call to the array TEST/WAIT functions So it seems that Sessions are rather limited in comparison to MPI_Init()/MPI_Finalize(). I'm wondering what's the point of slide 12 here: https://blogs.cisco.com/performance/mpi-sessions-a-proposal-for-the-mpi-for… Given the proposed limitations, then it would be better to keep the API as small as possible, and extend it in a future revision of the standard if deemed crucial for performance. On 31 October 2017 at 07:00, Jed Brown <jed(a)jedbrown.org> wrote: > Note that MPI_COMM_WORLD isn't required to be a compile-time constant in > the current standard and Open MPI doesn't treat it as one. (It is a > link-time constant.) So regardless of potential performance impact, > standard-compliant user code cannot treat it as a compile-time constant > (e.g., by using it in a case statement). > > I'm also skeptical about ability to measure an impact on user code and > thought your SC paper was about compile-time specialization in the MPI > implementation. > > > As one data point, PETSc does not reference MPI_COMM_WORLD except for > some optional diagnostics and it would be pretty easy to remove. > > Kenneth Raffenetti <raffenet(a)mcs.anl.gov> writes: > >> Strictly speaking, compile-time objects like MPI_COMM_WORLD impose less >> overhead than dynamically created ones. See our SC paper for more >> details :). >> >> Ken >> >> On 10/30/2017 05:51 PM, Jeff Hammond wrote: >>> This is going to cause horrible problems. I hate to say it, but the time >>> for MPI to have avoided hidden state was in the 1990s. >>> >>> Lots of MPI libraries create their own communicator from the >>> compile-time constant rather than a communicator argument. Breaking even >>> a few of these will ruin MPI’s exhalted reputation in the HPC world. >>> >>> The better option is to provide new headers and libraries for sessions >>> that break backwards compatibility. Or just have users opt-in to >>> sessions by not using MPI_COMM_WORLD at all. That way you’re only >>> breaking code that wants this feature rather than many that don’t. >>> >>> What’s your Fortran story? If you don’t have a trivial solution for >>> users of mpif.h, you’ve lost approximately half if MPI’s users. >>> >>> Jeff >>> >>> On Mon, Oct 30, 2017 at 11:33 AM Bland, Wesley <wesley.bland(a)intel.com >>> <mailto:[email protected]>> wrote: >>> >>> The sessions working group is working on a proposal that, among >>> other things, would change the way MPI_COMM_WORLD works. As much as >>> I don't want to take things out of context for those who aren't >>> familiar with the proposal, I'm not going to summarize the whole >>> thing here. If you need that, look here: >>> https://github.com/mpiwg-sessions/sessions-issues/wiki/sessions_cheat_sheet >>> >>> >>> On today's call, we were trying to decide the result of a backward >>> compatibility break for the MPI_COMM_WORLD symbol. We want to keep >>> the MPI_COMM_WORLD symbol to allow legacy codes to work, but we >>> don't want to make it a compiler symbol anymore. It would only exist >>> if you call MPI_INIT. To that end, MPI_INIT would look something >>> like this: >>> >>> MPI_Init(...) { >>> MPI_Comm *comm; >>> MPI_Session *session; >>> >>> MPI_Session_init(..., session); >>> MPI_Group_create_from_session_name(&session, "mpi://WOLRD", comm); >>> MPI_COMM_WORLD = *comm; >>> } >>> >>> The problem here is that MPI_COMM_WORLD is no longer the >>> compile-time constant that it was before. For example, if you're >>> using MPI_COMM_WORLD in a library, this would cause problems. >>> >>> The working group is trying to figure out the results of this to >>> decide whether this would cause horrible problems. It seems that as >>> long as applications are reasonably well behaved and check if the >>> library is initialized first, they should work correctly. >>> >>> What is this community's opinion of this? >>> >>> Thanks, >>> Wesley >>> _______________________________________________ >>> To manage subscription options or unsubscribe: >>> https://lists.mpich.org/mailman/listinfo/devel >>> >>> -- >>> Jeff Hammond >>> jeff.science(a)gmail.com <mailto:[email protected]> >>> http://jeffhammond.github.io/ >>> >>> >>> _______________________________________________ >>> To manage subscription options or unsubscribe: >>> https://lists.mpich.org/mailman/listinfo/devel >>> >> _______________________________________________ >> To manage subscription options or unsubscribe: >> https://lists.mpich.org/mailman/listinfo/devel > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel -- Lisandro Dalcin ============ Research Scientist Computer, Electrical and Mathematical Sciences & Engineering (CEMSE) Extreme Computing Research Center (ECRC) King Abdullah University of Science and Technology (KAUST) http://ecrc.kaust.edu.sa/ 4700 King Abdullah University of Science and Technology al-Khawarizmi Bldg (Bldg 1), Office # 0109 Thuwal 23955-6900, Kingdom of Saudi Arabia http://www.kaust.edu.sa Office Phone: +966 12 808-0459

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Jed Brown 30 Oct '17

30 Oct '17

Note that MPI_COMM_WORLD isn't required to be a compile-time constant in the current standard and Open MPI doesn't treat it as one. (It is a link-time constant.) So regardless of potential performance impact, standard-compliant user code cannot treat it as a compile-time constant (e.g., by using it in a case statement). I'm also skeptical about ability to measure an impact on user code and thought your SC paper was about compile-time specialization in the MPI implementation. As one data point, PETSc does not reference MPI_COMM_WORLD except for some optional diagnostics and it would be pretty easy to remove. Kenneth Raffenetti <raffenet(a)mcs.anl.gov> writes: > Strictly speaking, compile-time objects like MPI_COMM_WORLD impose less > overhead than dynamically created ones. See our SC paper for more > details :). > > Ken > > On 10/30/2017 05:51 PM, Jeff Hammond wrote: >> This is going to cause horrible problems. I hate to say it, but the time >> for MPI to have avoided hidden state was in the 1990s. >> >> Lots of MPI libraries create their own communicator from the >> compile-time constant rather than a communicator argument. Breaking even >> a few of these will ruin MPI’s exhalted reputation in the HPC world. >> >> The better option is to provide new headers and libraries for sessions >> that break backwards compatibility. Or just have users opt-in to >> sessions by not using MPI_COMM_WORLD at all. That way you’re only >> breaking code that wants this feature rather than many that don’t. >> >> What’s your Fortran story? If you don’t have a trivial solution for >> users of mpif.h, you’ve lost approximately half if MPI’s users. >> >> Jeff >> >> On Mon, Oct 30, 2017 at 11:33 AM Bland, Wesley <wesley.bland(a)intel.com >> <mailto:[email protected]>> wrote: >> >> The sessions working group is working on a proposal that, among >> other things, would change the way MPI_COMM_WORLD works. As much as >> I don't want to take things out of context for those who aren't >> familiar with the proposal, I'm not going to summarize the whole >> thing here. If you need that, look here: >> https://github.com/mpiwg-sessions/sessions-issues/wiki/sessions_cheat_sheet >> >> >> On today's call, we were trying to decide the result of a backward >> compatibility break for the MPI_COMM_WORLD symbol. We want to keep >> the MPI_COMM_WORLD symbol to allow legacy codes to work, but we >> don't want to make it a compiler symbol anymore. It would only exist >> if you call MPI_INIT. To that end, MPI_INIT would look something >> like this: >> >> MPI_Init(...) { >> MPI_Comm *comm; >> MPI_Session *session; >> >> MPI_Session_init(..., session); >> MPI_Group_create_from_session_name(&session, "mpi://WOLRD", comm); >> MPI_COMM_WORLD = *comm; >> } >> >> The problem here is that MPI_COMM_WORLD is no longer the >> compile-time constant that it was before. For example, if you're >> using MPI_COMM_WORLD in a library, this would cause problems. >> >> The working group is trying to figure out the results of this to >> decide whether this would cause horrible problems. It seems that as >> long as applications are reasonably well behaved and check if the >> library is initialized first, they should work correctly. >> >> What is this community's opinion of this? >> >> Thanks, >> Wesley >> _______________________________________________ >> To manage subscription options or unsubscribe: >> https://lists.mpich.org/mailman/listinfo/devel >> >> -- >> Jeff Hammond >> jeff.science(a)gmail.com <mailto:[email protected]> >> http://jeffhammond.github.io/ >> >> >> _______________________________________________ >> To manage subscription options or unsubscribe: >> https://lists.mpich.org/mailman/listinfo/devel >> > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Kenneth Raffenetti 30 Oct '17

30 Oct '17

Strictly speaking, compile-time objects like MPI_COMM_WORLD impose less overhead than dynamically created ones. See our SC paper for more details :). Ken On 10/30/2017 05:51 PM, Jeff Hammond wrote: > This is going to cause horrible problems. I hate to say it, but the time > for MPI to have avoided hidden state was in the 1990s. > > Lots of MPI libraries create their own communicator from the > compile-time constant rather than a communicator argument. Breaking even > a few of these will ruin MPI’s exhalted reputation in the HPC world. > > The better option is to provide new headers and libraries for sessions > that break backwards compatibility. Or just have users opt-in to > sessions by not using MPI_COMM_WORLD at all. That way you’re only > breaking code that wants this feature rather than many that don’t. > > What’s your Fortran story? If you don’t have a trivial solution for > users of mpif.h, you’ve lost approximately half if MPI’s users. > > Jeff > > On Mon, Oct 30, 2017 at 11:33 AM Bland, Wesley <wesley.bland(a)intel.com > <mailto:[email protected]>> wrote: > > The sessions working group is working on a proposal that, among > other things, would change the way MPI_COMM_WORLD works. As much as > I don't want to take things out of context for those who aren't > familiar with the proposal, I'm not going to summarize the whole > thing here. If you need that, look here: > https://github.com/mpiwg-sessions/sessions-issues/wiki/sessions_cheat_sheet > > > On today's call, we were trying to decide the result of a backward > compatibility break for the MPI_COMM_WORLD symbol. We want to keep > the MPI_COMM_WORLD symbol to allow legacy codes to work, but we > don't want to make it a compiler symbol anymore. It would only exist > if you call MPI_INIT. To that end, MPI_INIT would look something > like this: > > MPI_Init(...) { > MPI_Comm *comm; > MPI_Session *session; > > MPI_Session_init(..., session); > MPI_Group_create_from_session_name(&session, "mpi://WOLRD", comm); > MPI_COMM_WORLD = *comm; > } > > The problem here is that MPI_COMM_WORLD is no longer the > compile-time constant that it was before. For example, if you're > using MPI_COMM_WORLD in a library, this would cause problems. > > The working group is trying to figure out the results of this to > decide whether this would cause horrible problems. It seems that as > long as applications are reasonably well behaved and check if the > library is initialized first, they should work correctly. > > What is this community's opinion of this? > > Thanks, > Wesley > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel > > -- > Jeff Hammond > jeff.science(a)gmail.com <mailto:[email protected]> > http://jeffhammond.github.io/ > > > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel >

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Re: [mpich-devel] Sessions Considering Changing MPI_COMM_WORLD
by Jeff Hammond 30 Oct '17

30 Oct '17

This is going to cause horrible problems. I hate to say it, but the time for MPI to have avoided hidden state was in the 1990s. Lots of MPI libraries create their own communicator from the compile-time constant rather than a communicator argument. Breaking even a few of these will ruin MPI’s exhalted reputation in the HPC world. The better option is to provide new headers and libraries for sessions that break backwards compatibility. Or just have users opt-in to sessions by not using MPI_COMM_WORLD at all. That way you’re only breaking code that wants this feature rather than many that don’t. What’s your Fortran story? If you don’t have a trivial solution for users of mpif.h, you’ve lost approximately half if MPI’s users. Jeff On Mon, Oct 30, 2017 at 11:33 AM Bland, Wesley <wesley.bland(a)intel.com> wrote: > The sessions working group is working on a proposal that, among other > things, would change the way MPI_COMM_WORLD works. As much as I don't want > to take things out of context for those who aren't familiar with the > proposal, I'm not going to summarize the whole thing here. If you need > that, look here: > https://github.com/mpiwg-sessions/sessions-issues/wiki/sessions_cheat_sheet > > On today's call, we were trying to decide the result of a backward > compatibility break for the MPI_COMM_WORLD symbol. We want to keep the > MPI_COMM_WORLD symbol to allow legacy codes to work, but we don't want to > make it a compiler symbol anymore. It would only exist if you call > MPI_INIT. To that end, MPI_INIT would look something like this: > > MPI_Init(...) { > MPI_Comm *comm; > MPI_Session *session; > > MPI_Session_init(..., session); > MPI_Group_create_from_session_name(&session, "mpi://WOLRD", comm); > MPI_COMM_WORLD = *comm; > } > > The problem here is that MPI_COMM_WORLD is no longer the compile-time > constant that it was before. For example, if you're using MPI_COMM_WORLD in > a library, this would cause problems. > > The working group is trying to figure out the results of this to decide > whether this would cause horrible problems. It seems that as long as > applications are reasonably well behaved and check if the library is > initialized first, they should work correctly. > > What is this community's opinion of this? > > Thanks, > Wesley > _______________________________________________ > To manage subscription options or unsubscribe: > https://lists.mpich.org/mailman/listinfo/devel -- Jeff Hammond jeff.science(a)gmail.com http://jeffhammond.github.io/

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Sessions Considering Changing MPI_COMM_WORLD
by Bland, Wesley 30 Oct '17

30 Oct '17

The sessions working group is working on a proposal that, among other things, would change the way MPI_COMM_WORLD works. As much as I don't want to take things out of context for those who aren't familiar with the proposal, I'm not going to summarize the whole thing here. If you need that, look here: https://github.com/mpiwg-sessions/sessions-issues/wiki/sessions_cheat_sheet On today's call, we were trying to decide the result of a backward compatibility break for the MPI_COMM_WORLD symbol. We want to keep the MPI_COMM_WORLD symbol to allow legacy codes to work, but we don't want to make it a compiler symbol anymore. It would only exist if you call MPI_INIT. To that end, MPI_INIT would look something like this: MPI_Init(...) { MPI_Comm *comm; MPI_Session *session; MPI_Session_init(..., session); MPI_Group_create_from_session_name(&session, "mpi://WOLRD", comm); MPI_COMM_WORLD = *comm; } The problem here is that MPI_COMM_WORLD is no longer the compile-time constant that it was before. For example, if you're using MPI_COMM_WORLD in a library, this would cause problems. The working group is trying to figure out the results of this to decide whether this would cause horrible problems. It seems that as long as applications are reasonably well behaved and check if the library is initialized first, they should work correctly. What is this community's opinion of this? Thanks, Wesley

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