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Subject: [OMPI devel] RFC: Use of ompi_proc_t flags field
From: Ralph Castain (rhc_at_[hidden])
Date: 2009-01-21 09:58:12

What: Extend the current use of the ompi_proc_t flags field (without
changing the field itself)

Why: Provide more atomistic sense of locality to support new
collective/BTL components

Where: Add macros to define and check the various flag fields in ompi/
proc.h. Revise the orte_ess.proc_is_local API to return a uint8_t
instead of bool.

When: For OMPI v1.4

Timeout: COB Fri, Feb 6, 2009


The current ompi_proc_t structure has a uint8_t flags field in it.
Only one bit of this field is currently used to flag that a proc is
"local". In the current context, "local" is constrained to mean "local
to this node".

New collectives and BTL components under development by LANL (in
partnership with others) require a greater degree of granularity on
the term "local". For our work, we need to know if the proc is on the
same socket, PC board, node, switch, and CU (computing unit). We
therefore propose to define some of the unused bits to flag these
"local" conditions. This will not extend the field's size, nor impact
any other current use of the field.

Our intent is to add #define's to designate which bits stand for which
local condition. To make it easier to use, we will add a set of macros
that test the specific bit - e.g., OMPI_PROC_ON_LOCAL_SOCKET. These
can be used in the code base to clearly indicate which sense of
locality is being considered.

We would also modify the orte_ess modules so that each returns a
uint8_t (to match the ompi_proc_t field) that contains a complete
description of the locality of this proc. Obviously, not all
environments will be capable of providing such detailed info. Thus,
getting a "false" from a test for "on_local_socket" may simply
indicate a lack of knowledge. This is acceptable for our purposes as
the algorithm will simply perform sub-optimally, but will still work.

Please feel free to comment and/or request more information.