The WaitForFirst command stops execution until one remote compute nodes is finished processing.  

The returned result is the node number of an idle worker node.

When more than one compute node finish at the same time (or when nodes are already idle prior to calling WaitForFirst), the lowest node number is returned.

The WaitForFirst command is only available in local Grid mode.

$\mathrm{Grid}:-\mathrm{Run}\left(0\,''for\; i\; from\; 1\; to\; 10^6\; do\; od:''\,'\mathrm{numnodes}'=2\right)$

$\mathrm{Grid}:-\mathrm{Run}\left(1\,''for\; i\; from\; 1\; to\; 10^5\; do\; od:''\right)$

$\mathrm{Grid}:-\mathrm{WaitForFirst}\left(\right)$

$1$

$\mathrm{Grid}:-\mathrm{Wait}\left(\right)$

runInParallel := proc( f, n )
  local i, result, freenode;
  result := Array(1..n);
  for i from 1 to min(Grid:-NumNodes(),n) do
      Grid:-Run(i-1,f,[i],assignto='result'[i]);
  end do;
  for i to n do
      freenode := Grid:-WaitForFirst();
      Grid:-Run(freenode,f,[i],assignto='result'[i]);
  end do;
  Grid:-Wait();
  return result;
end:

$\mathrm{runInParallel}\left(\mathrm{ithprime}\,7\right)$

$\left[\begin{array}{ccccccc}2& 3& 5& 7& 11& 13& 17\end{array}\right]$

$\mathrm{runInParallel}\left(i\mapsto x^i\,5\right)$

$\left[\begin{array}{ccccc}x& x^2& x^3& x^4& x^5\end{array}\right]$

$\mathrm{runInParallel}\left(i\mapsto \mathrm{combinat}:-\mathrm{unrankperm}\left(i\,4\right)\,\mathrm{combinat}:-\mathrm{numbperm}\left(4\right)\right)$

$\left[\begin{array}{cccccccccccccccccccccccc}\left[1\,2\,3\,4\right]& \left[1\,2\,4\,3\right]& \left[1\,3\,2\,4\right]& \left[1\,3\,4\,2\right]& \left[1\,4\,2\,3\right]& \left[1\,4\,3\,2\right]& \left[2\,1\,3\,4\right]& \left[2\,1\,4\,3\right]& \left[2\,3\,1\,4\right]& \left[2\,3\,4\,1\right]& \left[2\,4\,1\,3\right]& \left[2\,4\,3\,1\right]& \left[3\,1\,2\,4\right]& \left[3\,1\,4\,2\right]& \left[3\,2\,1\,4\right]& \left[3\,2\,4\,1\right]& \left[3\,4\,1\,2\right]& \left[3\,4\,2\,1\right]& \left[4\,1\,2\,3\right]& \left[4\,1\,3\,2\right]& \left[4\,2\,1\,3\right]& \left[4\,2\,3\,1\right]& \left[4\,3\,1\,2\right]& \left[4\,3\,2\,1\right]\end{array}\right]$

delay := proc( n, c )
  local i;
  for i from 1 to n do
      printf("%c",c);
      Threads:-Sleep(1);
  od;
  printf("\n");
  c;
end proc:

$\mathrm{Grid}:-\mathrm{Setup}\left('\mathrm{numnodes}'=4\right)$

$\left[4\,3\,1\,2\right]$

$\mathrm{Grid}:-\mathrm{Set}\left(\mathrm{delay}\right)$

$\mathrm{Grid}:-\mathrm{Run}\left(0\,\mathrm{delay}\,\left[15\,''x''\right]\right)$

$\mathrm{Grid}:-\mathrm{Run}\left(1\,\mathrm{delay}\,\left[3\,''+''\right]\right)$

$\mathrm{Grid}:-\mathrm{Run}\left(2\,\mathrm{delay}\,\left[16\,''!''\right]\right)$

$\mathrm{Grid}:-\mathrm{Run}\left(3\,\mathrm{delay}\,\left[9\,''z''\right]\right)$

$n≔\mathrm{Grid}:-\mathrm{WaitForFirst}\left(\right)$

$n≔0$

$\mathrm{Grid}:-\mathrm{Interrupt}\left(\right)$

$\mathrm{Grid}:-\mathrm{Wait}\left(\right)$

$\mathrm{Grid}:-\mathrm{GetLastResult}\left(n\right)$

The Grid[WaitForFirst] command was introduced in Maple 2015.

For more information on Maple 2015 changes, see Updates in Maple 2015.