(j3.2006) Comments to 10-166 (early coarray TR draft)

Tobias Burnus burnus
Fri Apr 1 12:58:57 EDT 2011


Hi all,

admittedly, it is probably a bad timing as everyone is interested in TR 
29113 and not other work items. However, I happened to have time to 
glance at 10-166 (draft of coarray TR, dated 2010/02/18).

First, I spotted an "ALL STOP" which should be an ERROR STOP (in A.1.1).

Secondly, I miss a possibility to broadcast values to all (or to a 
team); unless I have missed something even with TR one has still to do do:

   if (this_image()==1) then
     ! READ input file
     ! Distribute values:
     do image = 2, num_images()
       z[image] = z
     end do
   end do
   SYNC ALL

(Or in the "IF" a "SYNC IMAGES(*)" and in ELSE a "SYNC IMAGES(1)"). I 
think sending the value to each other image, image by image, is rather 
slow if many images are involved. (Assume a calculation on 6k Blue Gene 
processors or using the full 294,912 processors of the HPC system 600 
metres from here.) On such systems, sending the configuration can then 
take a significant amount of the total computation time. That time is 
wasted especially as there is a dedicated collective network with 
one-to-all broadcast functionality.

For reductions, the draft provides the most important ones. However, I 
see again some unneeded communication as: "A collective subroutine is 
one that is invoked on a team of images to perform a calculation on 
those images and which assigns the value of the result on all of them" 
(4.1.1). While that is often the desired result, one frequently needs 
the result only at one image. Coming again back to calculation on a 
many-processor system: Doing the collective operations in a tree-like 
manner and sending it to a single reduction-master image is faster than 
collecting it on all systems - especially since there is a barrier (team 
synchronization) after the reduction, which could be avoided on all but 
the one image which is interested in the reduction.

Tobias

PS: It would be nice if someone could save the three comments such that 
they can be discussed, when the topic comes up again after TR 29113.

PPS: I hope I have found the latest draft.



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