To: J3                                                     J3/18-117
From: Bill Long
Subject: Put with Notify
Date: 2018-February-07


Discussion:
-----------

A capability called "put with notify" is popular in the SHMEM
community as a very efficient, narrowly focused, synchronization
technique for data transfers between images. The basic idea is to
combine a "put" (definition of a variable on a different image) with a
notification mechanism that allows the receiving image to know that
the data has arrived.  While long requested for coarrays in Fortran,
there was previously no simple syntax to allow for two things on the
left side of an assignment.  Fortran 2018 introduced events along with
specifiers as part of an image selector, which could be extended to
overcome this problem.

Feature outline:
----------------

Extend the image selector syntax to allow a NOTIFY = <event-variable>
specifier in the variable of an assignment. NOTIFY could alternatively
be spelled POST.  For example,

   x[10, notify=ex] = y

where ex is a coarray declared type(event_type). The effective event
post is to the ex variable on the same image as specified by the image
selector.  On image 10,

   event wait(ex)
   [here it is safe to reference x and get the new value]

If X was an array "waiting" for data to be transferred to separate
elements from multiple images, the event wait could include a count.

The feature differs from the sequence of

    x[10] = y
    event post (ex)

in that the event post operation in the new feature can be
incorporated into the same data packet as the value of y, and also
that the synchronization is only on this particular transfer, and not
all outstanding memory operations on the image. In other words,

    x[10, notiry=ex] = y

does not constitute an image control statement.