[J3] Questions about DO CONCURRENT and locality

[Bill Long]

> On Jul 3, 2020, at 2:59 PM, Daniel C Chen via J3 <j3 at mailman.j3-fortran.org> wrote:
> 
> This is a known issue that was first discussed in https://j3-fortran.org/doc/year/15/15-150.txt 
> The problem is that the wording in F2008 makes it almost impossible for the processor to 
> determine if an entity that appears in a DO CONCURRENT construct is shared or local. 
> I said "almost" because there is a way to implement it that requires a lot of 
> book keeping and copy-in/copy-out, which pretty much destroy the performance
> this feature is supposed to bring.

Right.  The main problem comes when a variable definition is protected by a condition, usually an IF statement.  It is difficult to tell whether this constitutes a previous definition before a subsequent reference outside the IF construct. But the 2008 rules, determination of local versus shared depended on such ordering of definition/reference.  This is part of why we have locality specs now.   The F2008 implementation was substantially complicated because of this. 
> 
> This finding is also the reason we added localities for DO CONCURRENT in F2018 to
> allow users to specify the locality by themselves.
> However, due to the concerns of backward compatibility, F2018 didn't add "default"
> locality even though it was requested at the time. 
> As the result, there is still a hole in the standard that makes 
> determination of a SHARED variable very difficult. It will completely kill the performance
> if a processor has to implement it). 

If I recall the discussion, allowing DEFAULT(SHARED) would basically force the user to declare all of the local  variables as LOCAL or LOCAL_INIT to have the loop make logical sense.   Similarly for DEFAULT(LOCAL). Providing only DEFAULT(NONE) as a mechanism for requiring declaration of ALL the variables seemed less error-prone.  (OpenMP does allow DEFAULT(SHARED) and includes a long list of reasons that cause a variable to be automatically private.  Which is probably broken.)

Cheers,
Bill



> 
> Daniel
> 
> XL Fortran Development, Fortran Standard Representative
> IBM Toronto Software Lab
> Phone: 905-413-3056 
> Tie: 969-3056 
> Email: cdchen at ca.ibm.com
> http://www.ibm.com/software/awdtools/fortran/xlfortran
> 
> <graycol.gif>Steve Lionel via J3 ---2020-07-03 03:31:48 PM---In https://j3-fortran.org/doc/year/19/19-134.txt
> 
> From: Steve Lionel via J3 <j3 at mailman.j3-fortran.org>
> To: fortran standards email list for J3 <j3 at mailman.j3-fortran.org>
> Cc: Steve Lionel <steve at stevelionel.com>
> Date: 2020-07-03 03:31 PM
> Subject: [EXTERNAL] [J3] Questions about DO CONCURRENT and locality
> Sent by: "J3" <j3-bounces at mailman.j3-fortran.org>
> 
> 
> 
> 
> In https://j3-fortran.org/doc/year/19/19-134.txt  Peter Klausler asked 
> whether the words in the standard actually enabled parallelization of DO 
> CONCURRENT. I see that the paper was not taken up at 218 but I wasn't 
> aware of any discussion of the topic then or at later meetings. It came 
> up again today in discussions at the (online) FortranCon hosted by the 
> University of Zurich (which has been great so far - almost over.)
> 
> As I don't pretend to be a parallelization expert, I'd be interested in 
> hearing others' thoughts on the issues Peter raised. Thanks.
> 
> Steve
> 
> 
> 
> 

Bill Long                                                                       longb at cray.com
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