J3/17-122r1
To: J3
From: Malcolm Cohen & Bill Long
Subject: SYNC TEAM semantics and requirements
Date: 2017 February 13


1. Introduction

SYNC TEAM allows sync-ing with an immediate subteam of the current team,
but there is no requirement for the team-variable to specify a team that
contains the executing image.  However, the semantics seem to assume the
team does contain the executing image.

Note that it is only the "team whose parent is the current team" option
that causes a problem, the current team or an ancestor team are both fine.


2. Discussion

Suppose we have a team ODD consisting of images 1, 3 and 5 in the parent
team ALL, and a team EVEN consisting of images 2, 4 and 6 in the parent
team ALL.  That is, each image does

   TYPE(TEAM_TYPE),SAVE :: tmp[*]
   TYPE(TEAM_TYPE) odd,even,all

   all = GET_TEAM()
   FORM TEAM(MOD(THIS_IMAGE(),2),tmp)
   odd = tmp[1]
   even = tmp[2]
   SYNC ALL

Then sometime later, still in team ALL, images 1, 2, 3 and 5 of ALL do
  SYNC TEAM (ODD)

According to 11.6.6p2, this (if successful)
  "performs a synchronization of [ ODD ]",
i.e. images 1, 3, 5 of ALL will sync, nothing is said re image 2 of ALL.

The next sentence of p2 redundantly describes this in gory detail:
  "Execution on an image, M, of the segment following the SYNC TEAM
   statement is delayed until each other image of the specified team has
   executed a SYNC TEAM statement specifying the same team as many times
   as has image M in this team."
i.e. images 1, 3 and 5 of ALL (images 1,2,3 of ODD) will sync, again,
nothing is said about image 2 of ALL.

However, the final sentence of p2 says
  "The segments that executed before the SYNC TEAM statement on an image
   precede the segments that execute after the corresponding SYNC TEAM
   statement on another image."
i.e. this says that image 2 of ALL does participate in the sync to the
extent of segment ordering (but not delaying? - inconceivable!).

This would appear to be an oversight.  Perhaps an earlier form of the
team facility did not permit assignments which could produce this.

Clearly TEAM+other syncing can never work, since if all the team
members arrived at the SYNC TEAM before the other, they will have
synced and gone, not waiting for the other.


3. Edit to 17-007

[213:4] 11.6.6 SYNC TEAM statement, p1, append new sentence
  "The executing image shall be a member of the specified team."
{Requirement.}


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