[mpiwg-hybridpm] Changing MPI_GROUP|COMM_COMPARE for endpoints

[Jim Dinan]

Hi Dan,

Thanks for doing such a nice job of capturing the new text for group/comm
compare.  I updated the draft proposal with the new text and uploaded it to
the ticket at https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/380.

I agree on the inadequacy of the interface comments.  I think the question
we should raise first is whether we need to solve this problem as part of
endpoints, or whether what we have is sufficient and this problem should be
addressed separately.

Cheers,
 ~Jim.

On Thu, Jan 29, 2015 at 1:22 PM, Daniel Holmes <dholmes at epcc.ed.ac.uk>
wrote:

>  Hi Jim/all,
>
> I won't be able to attend the next Hybrid WG teleconference (I'm on
> holiday from tomorrow until 10th Feb) but that will be the last opportunity
> to discuss changes to the endpoints text before it needs to be sent to the
> forum mailing list if it is to be in time to be formally read.
>
> At the last teleconference, we decided that we should look at a "minimum
> change" option and a "maximum readability" or "maximum de-duplication"
> option.
>
> --- exec summary ---
>
> This is a tricky change with unexpected repercussions to other areas that
> needs a lot of careful thought.
> It would be unfortunate if that extended effort delayed/derailed the main
> endpoints proposal.
> We have a very good reason to go for the minimum change option described
> below.
>
> --- option 1 - minimum change ---
>
> I think that the obvious "minimum change" option is that we propose not to
> change MPI_COMM_COMPARE or MPI_GROUP_COMPARE at all and not add the new
> result of MPI_ALIASED either. This could be moved to a separate proposal
> that would be dependent on #380 passing. Some would claim the new
> comparison ticket would be essential if endpoints passes - this is the only
> good reason to combine the two changes.
>
> --- option 2 - maximum readability/de-duplication ---
>
> This is the current text for MPI_GROUP_COMPARE:
>
> MPI_IDENT results if the group members and group order is exactly the same
> in both groups.
> This happens for instance if group1 and group2 are the same handle.
> MPI_SIMILAR results if
> the group members are the same but the order is different. MPI_UNEQUAL
> results otherwise.
>
> Here's suggested new text for MPI_GROUP_COMPARE:
>
> Groups are identical if they contain the same group members in the same
> order.
> Groups are similar if they contain the same group members but not in the
> same order.
> MPI_IDENT results if the group handles refer to the same group member in
> identical groups.
> MPI_ALIASED results if the group handles refer to different group members
> in identical groups.
> MPI_SIMILAR results if the group handles refer to the same group member in
> similar groups.
> MPI_UNEQUAL results otherwise.
>
> This is the current text for MPI_COMM_COMPARE:
>
> MPI_IDENT results if and only if comm1 and comm2 are handles for the same
> object (identical
> groups and same contexts). MPI_CONGRUENT results if the underlying groups
> are identical
> in constituents and rank order; these communicators differ only by
> context. MPI_SIMILAR
> results if the group members of both communicators are the same but the
> rank order differs.
> MPI_UNEQUAL results otherwise.
>
> Here's suggested new text for MPI_COMM_COMPARE:
>
> Communicators are identical if they have identical communication contexts;
> this implies that their underlying groups are also identical.
> MPI_IDENT results if the communicator handles refer to the same rank in
> identical communicators.
> MPI_ALIASED results if the communicator handles refer to different ranks
> in identical communicators.
> MPI_CONGRUENT results if the two communicators have identical underlying
> groups but different communication contexts.
> MPI_SIMILAR results if the two communicators have similar underlying
> groups.
> MPI_UNEQUAL results otherwise.
>
> --- notes ---
>
> Talking about same|different *group members* in identical|similar groups
> is preferred over talking about same|different *ranks* because
> "same|different rank in similar groups" does not guarantee "same|different
> endpoint in similar groups"
> which is what we want to say without including the word "endpoint".
>
> Talking about ranks in communicators is preferred over group members
> because communicators do not have group members even though their
> underlying groups do.
> They perhaps have members, i.e. without the "group" qualifier.
>
> The phrases "same rank" and "different ranks" are only ever applied to
> identical communicators
> because they are only guaranteed to mean "same endpoint" and "different
> endpoint" for identical communicators.
>
> --- issues ---
>
> This range of responses is incomplete and therefore inadequate.
> Incomplete because, for example in the group comparison:
> MPI_SIMILAR_ALIASED results if the group handles refer to different group
> members in similar groups.
>
> Inadequate because it seems that the interesting definition for the
> "aliased" property of handles is
> whether or not they refer to the same endpoint or different endpoints. To
> be able to determinable that
> for *any* two group/comm handles, "aliased" must be completely orthogonal
> to all the other criteria.
> We would need an additional MPI_UNEQUAL_ALIASED response for both
> comparison functions
> as well as MPI_CONGRUENT_ALIASED for communicator comparison.
> This implies that MPI_GROUP_ALIASED(group1, group2) and
> MPI_COMM_ALIASED(comm1, comm2)
> is a better approach.
>
> One of the main reasons for needing to know whether or not two handles
> refer to the same endpoint
> is the proposed restriction on usage of group manipulation functions, i.e.
> not allowing aliased handles.
> I believe we should define a fix for each of these functions so that this
> restriction is not needed.
>
> --- conclusion ---
>
> This is a tricky change with unexpected repercussions to other areas that
> needs a lot of careful thought.
> It would be unfortunate if that extended effort delayed/derailed the main
> endpoints proposal.
> We have a very good reason to go for the minimum change option.
>
> Cheers,
> Dan.
>
> On 26/01/2015 21:42, Jim Dinan wrote:
>
> All,
>
>  Here is the diff of changes from the December meeting.  There's one spot
> where a few options for the text are included and the MPI_ALIASED changes
> are still pending (thanks to Dan for leading this tricky task).
>
>  Thanks,
>  ~Jim.
>
> On Mon, Jan 26, 2015 at 10:53 AM, Jim Dinan <james.dinan at gmail.com> wrote:
>
>> Hi All,
>>
>>  Reminder that there will be a meeting at 11am CT today.
>>
>>   ~Jim.
>>
>>  =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
>>
>>  Meeting Info:
>>
>>
>> https://cisco.webex.com/ciscosales/j.php?ED=236535652&UID=0&PW=NOGE0NDk5MmVh&RT=MiMxMQ%3D%3D
>>
>> +1-866-432-9903
>> Meeting ID: 206095536
>>
>>  https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/MPI3Hybrid
>>
>>  =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=
>>
>
>
>
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>
> --
> Dan Holmes
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>
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