[Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)

Fri Oct 22 07:33:36 CDT 2021

Dan,

Thank you for your encouraging response.

Firstly, let me state that I have no strong feelings for either
MPI_ADDRESS_NULL nor MPI_COMM_NULL other than they would satisfy my sense
of "symmetry" or "closure". In my own programming I rarely find myself
unclear what buffer address or communicator a specific code section is
targeting. Most often it is the pt2pt matching triad (proc,tag,comm) that
needs to be tailored to express the "intent" of the code so that an
unimplemented fringe case does not pass by unnoticed. (Merely causes a
dead-lock - as if that is any help - I know...)

My view of the MPI_*_NULL entities in the standard is that they are
comparable to NaN in floating point. MPI_TAG_NULL is not a member of the
set of valid tags, and can not be matched in any tag comparison operation,
not even itself or as "not a member of". MPI_TAG_ANY should not match a
message with MPI_TAG_NULL, and there should indeed be no way to create a
"real" message with MPI_TAG_NULL. So a statement as you suggested

"When the tag argument is MPI_TAG_NULL the only allowed processor argument
> is MPI_PROC_NULL"

preferably in section 3.2.4 just before the last sentence describing the
use of dest=MPI_PROC_NULL would be all that is needed. A probe for such a
message (that can never be created as a "real" message) will thus return a
status object with a count of 0 and a tag of MPI_TAG_ANY and no change in
behaviour to existing implementations.

So the updated suggested changes to the standards are:

   1. Insert into appendix "A1 Assorted constants":    MPI_TAG_NULL
   2. Insert into section "9.1.2 Environmental Inquiries"

   (Line 34, end of first paragraph)  [... is also a valid value for
   MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the
   0...MPI_TAG_UB range.

   3. Insert into section 3.2.4 before the last sentence (line 42.5):

   "When the tag argument is MPI_TAG_NULL the only allowed processor
   argument is MPI_PROC_NULL"

   4. Add to section 3.10 "Null processes", last in the section:

   "*Advice to users:* Note that MPI_TAG_NULL is not a valid tag in the
   0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the associated
   processor argument is MPI_PROC_NULL"

To avoid a lengthy discussion into what probes of non-existing messages
should return as a result, I suggest that the committee sees any triad
(proc,tag,comm) where either component has a "NULL" value as defined in the
standard (currently only MPI_PROC_NULL exists) is in essence a "NaN"-triad,
and returns the already defined result of 0 elements of
(MPI_PROC_NULL,MPI_ANY_TAG) as of section 3.10. Also, just by chance, this
would not conflict with an introduction of an MPI_COMM_NULL item in the
future.

On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes at chi.scot> wrote:

> Hi Nils,
>
> In general, I like the idea of having default/invalid values for these
> MPI-related user variables.
>
> Would MPI_TAG_NULL be valid for a point-to-point send operation that does
> NOT use MPI_PROC_NULL but is matched with receive operation at the
> destination MPI process that uses MPI_ANY_TAG? Or should this be an
> MPI_ERR_ARG/MPI_ERR_TAG error condition? Commonly, codes that use
> MPI_ANY_TAG in receive operations must choose an arbitrary valid tag value
> to use in the send operations at the source MPI process(es). Permitting the
> use of MPI_TAG_NULL in combination with MPI_ANY_TAG might aid debugging
> when the matching does not go the way the user expected. However, it would
> add complexity to the matching rules/implementation. There is an
> interaction here with INFO key assertions such as `mpi_assert_no_any_tag` —
> which means the error could be discovered at the sender process -- and
> `mpi_assert_only_any_tag` (if that one ever gets accepted into MPI).
>
> There is already an MPI constant that is commonly set to `(void*)0` (in
> C), which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is
> not a good idea because MPI_BOTTOM is a valid address; although attempting
> to read/write at that address is likely to cause trouble, using it as a
> base for a user-defined datatype or a window offset is common/accepted
> practice. Would it be sufficient for your purposes to use MPI_BOTTOM as
> your “not valid” value?
>
> IMHO, there would need to be a statement that, “when the comm argument is
> MPI_COMM_NULL, the only acceptable value for the source/dest argument is
> MPI_PROC_NULL” because asking for the 3rd item from an empty set (for
> example) is clearly an error. Your proposal tries to treat MPI_COMM_NULL as
> if it is a valid MPI intra-communicator that has size zero, or a valid
> inter-communicator that has a remote group of size zero. I think I would
> prefer to create MPI_COMM_EMPTY for that purpose, and continue to use
> MPI_COMM_NULL for situations that call for an invalid communicator, i.e.
> something that can never be used as a communicator without causing an error
> condition.
>
> The phrase used elsewhere in the MPI Standard is “XYZ parameter is
> significant only <when condition>”
>
> Cheers,
> Dan.
> —
> Dr Daniel Holmes PhD
> Executive Director
> Chief Technology Officer
> CHI Ltd
> danholmes at chi.scot
>
>
>
> On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments <
> mpi-comments at lists.mpi-forum.org> wrote:
>
> *Summary:*
> - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously
> illegal value (i.e. <0)
> - Explicitly allow Point-to-Point messages to have one of more of: tag
> MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses
> (including MPI_ADDRESS_NULL)  and communicator MPI_COMM_NULL,  if (and only
> if) the corresponding send or receive argument is MPI_PROC_NULL.
>
> *Argument:*
> When writing code it is useful if it goes off with a bang when you do
> something wrong. To catch matching errors in sends and receives I looked
> for a MPI_TAG_NULL to use as a default value to catch cases where the tag
> was not set correctly. I found that there was no such item. Referring to
> the standard I found that tag values are restricted to the values
> 0...MPI_TAG_UB, where MPI_TAG_UB (>=32767) can be found using the
> MPI_Comm_get_attr() function.
>
> So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the
> MPI implementation I was using blew up when I used it in a MPI_Sendrecv
> with the remote task being MPI_PROC_NULL throwing an error message "illegal
> tag value", which was no surprise to me since I had set it to this value
> explicitly.
>
> The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) {
> ... }   around every point-to-point call, but I find it unnecessary and
> makes the code look more complex. I think that MPI_PROC_NULL was invented
> for exactly this purpose, to be a transparent if(...) clause for the
> operation. Is there really a need to be restrictive on datat ype, tag or
> any other argument when the P2P operation is with MPI_PROC_NULL?
>
> I would expect implementations to choose  MPI_ADDRESS_NULL be  equal to
> (*) 0 in C  and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be
> default integer kind -1.
>
> *Implementation:*
> Append to section "3.10 Null processes" text along the lines of:
>
> A call to any point-to-point messaging function with a process argument of
> MPI_PROC_NULL is allowed to have any value for buffer address, data type,
> tag and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL,
> MPI_TAG_NULL and MPI_COMM_NULL without causing an error condition in the
> execution.
>
>
> Insert into appendix "A1 Assorted constants"
>
> MPI_TAG_NULL and MPI_ADDRESS_NULL
>
>
> Insert into section "9.1.2 Environmental Inquiries"  (which is an odd
> place where Tag values are specified in the standard):
>
> (Line 34, end of first paragraph)  [... is also a valid value for
> MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the
> 0...MPI_TAG_UB range. The value MPI_TAG_NULL is erroneous to use in any
> communication unless the associated process argument is MPI_PROC_NULL.
>
>
> Add to section "15.3.3 Convention for Returning Strings"
>
> The constant MPI_ADDRESS_NULL can be used as an equivalent to the null
> pointer.
>
>
>
>
> /Nils Smeds
>
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