add code to help link reuse, start test numreplica

src/rabit_tracker.py

@@ -88,7 +88,6 @@ class SlaveEntry:
             self.sock.sendint(rnext)
         else:
             self.sock.sendint(-1)
-
         while True:
             ngood = self.sock.recvint()
             goodset = set([])
@@ -156,13 +155,37 @@ class Tracker:
             tree_map[r] = self.get_neighbor(r, nslave)
             parent_map[r] = (r + 1) / 2 - 1
         return tree_map, parent_map
+    def find_share_ring(self, tree_map, parent_map, r):
+        """
+        get a ring structure that tends to share nodes with the tree
+        return a list starting from r
+        """
+        nset = set(tree_map[r])
+        cset = nset - set([parent_map[r]])
+        if len(cset) == 0:
+            return [r]
+        rlst = [r]
+        cnt = 0
+        for v in cset:
+            vlst = self.find_share_ring(tree_map, parent_map, v)
+            cnt += 1
+            if cnt == len(cset):
+                vlst.reverse()
+            rlst += vlst
+        return rlst
     def get_ring(self, tree_map, parent_map):
+        """
+        get a ring connection used to recover local data
+        """
+        assert parent_map[0] == -1
+        rlst = self.find_share_ring(tree_map, parent_map, 0)
+        assert len(rlst) == len(tree_map)
         ring_map = {}
         nslave = len(tree_map)        
         for r in range(nslave):
             rprev = (r + nslave - 1) % nslave
             rnext = (r + 1) % nslave            
-            ring_map[r] = (rprev, rnext)
+            ring_map[rlst[r]] = (rlst[rprev], rlst[rnext])
         return ring_map
     def accept_slaves(self, nslave):
         tree_map, parent_map = self.get_tree(nslave)