To: J3                                                          07-208
From: Bill Long and John Reid
Subject: UTI 86 (p199): SYNC TEAM unsafe and too slow?
Date: 2007 May 01
References: J3/07-007r1


Discussion

UTI 86 asks if it would be better to have the SYNC TEAM statement
verify that the executing image is part of the image-team, as opposed
to making the program non-conforming if that is not the case. We
believe the answer is no.  The point of teams is high performance.
Adding such a check into the execution of a SYNC TEAM statement
is counter to that goal. The editor asserts that it is O(1), but we
wonder if this will always be the case.

A programmer can extract the list of images from an image-team
variable with the TEAM_IMAGES intrinsic function and perform a check
if desired. In addition, an implementation is allowed to have a debug
mode that would link in slower versions of the sync statements that
could do checking of this sort.

We agree that it is desirable to check calls of the FORM_TEAM intrinsic
subroutine, where the check's affect on performance would be less
significant.  Assuming a team is cleanly formed, it should not later
become defective, so checks in later usage would be unnecessary.

The UTI recommends edits to both SYNC TEAM and FORM_TEAM.  We agree
that a change for FORM_TEAM is desirable and provide an edit below. We
do not recommend an edit for SYNC TEAM, but provide one below for
expediency in case J3 disagrees.


Edits to J3/07-007r1:

---------------------------------
Edits for FORM_TEAM (recommended):

[377:17-19] In 13.7.70 FORM_TEAM, para. 3, lines 3-5, replace
", and shall ... THIS_IMAGE()." by
". An error condition occurs unless:
   (1) IMAGES specifies the same set of image on all images of the team,
   (2) all the elements of IMAGES have values in the range
       1, ... , NUM_IMAGES() and there are no repeated values, and
   (3) one of the elements of IMAGES has the value THIS_IMAGE()."

[377:28-29] In 13.7.70 FORM_TEAM, replace para. 4 by
"If FORM_TEAM is invoked by an image, an error condition occurs unless
it is invoked by the same statement on all images specified by the
IMAGES argument. In the absence of an error condition, there is an
implicit team synchronization after the team is formed."

-------------------------------------
Edit for SYNC TEAM (not recommended):

[199:7-11] In 13.7.70 SYNC TEAM statement, para. 1, lines 2-6, replace
"The team ... invoked for the team." by
"The team is specified by the value of <image-team>. An error condition
occurs unless the team includes the executing image and all images of
the team execute a SYNC TEAM statement with a value of <image-team>
that was constructed by corresponding invocations of the intrinsic
function FORM_TEAM for the team. The images do not commence executing
subsequent statements until all images in the team have successfully
executed a SYNC TEAM statement for the team an equal number of times
since FORM TEAM was invoked for the team."