Subject: mini-proposal on layerability

From: Tony Skjellum <[email-masked]>
Date: Sun, 21 Mar 93 12:16:22 CST

Dear Context subcommittee,

This is something we should share with pt2pt, but I put it before you
first.... (and to topologies, for their interest).

To build effective layers in MPI, a reasonable receipt selectivity must
be provided.  As suggested by Jim, this can be accomplished by two
integers as selectors for message wildcarding...

		dont_care 
		care

So, each receipt function uses the algorithm (received_rag & ~dont_care)&
	care

With such a feature, layerability (eg, virtual topologies) could be much
more easily added into MPI.  Without this, (ie, accept or reject
tag as ANY or only one specific), it is very difficult to layer.

It is useful that wildcarding of sender is provided, and all-or-nothing
wildcarding is fine for sender, since sender is specified by opaque TID.

Example, wildcard receipt in a 3D grid usage of tags to implement
	a virtual topology.
If a topology is embedded in a tag (p,q,r), then it is conveniently
possible to do wildcarding on any dimension, given the care/dont_care
bits.  

Another option would always be to multiple out the (p,q,r) triplet
to give the rank-in-group.  This could be stored in the tag instead,
but severe multiplications/division needs would occur to determine a
match (provided a general matching function were also contemplated).

In short, a lot of flexiblity occurs with the care/dont_care bits, and
I will ask for this feature, so that layerability can reasonably be
claimed in MPI1.

---

From: [email-masked] (James Cownie)
Date: Mon, 22 Mar 93 09:48:03 GMT

There is a typo in Tony's message :

> So, each receipt function uses the algorithm 
>   (received_rag & ~dont_care) & care

This should read
    (received_tag & ~dontcare) == care

i.e. don't care specifies a set of bits to be ignored in the comparison, 
     care       specifies the precise value of the other bits

Note that you can do complete wild-carding like this by setting
	dont_care = -1;
	care      =  0;

-- Jim
James Cownie 
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