Remove unnecessary edge list scan during insert.

When a thread reaches the end of the edge list without finding a
spot for the edge it is trying to insert/update, it tries to atomically
swap in a new edge block on the end of the list. If this fails, the
existing code would return to the beginning of the list and re-scan
the entire thing before trying to swap in a new block. With this patch,
the thread just keeps searching from its current position.

lib/stinger_core/inc/stinger_batch_insert.h

@@ -273,7 +273,6 @@ class BatchInserter
         }
 
         while (next_update() != updates_end) {
-            eb_index_t * block_ptr = curs.loc;
             curs.eb = readff(curs.loc);
             /* 2: The edge isn't already there.  Check for an empty slot. */
             for (stinger_eb *tmp = ebpool_priv + curs.eb; tmp != ebpool_priv; tmp = ebpool_priv + readff(&tmp->next)) {
@@ -320,19 +319,20 @@ class BatchInserter
                         if (next_update() == updates_end) { return; }
                     }
                 }
-
-                block_ptr = &(tmp->next);
+                // Move the cursor to the next edge block
+                curs.loc = &(tmp->next);
+                curs.eb = readff(curs.loc);
             }
 
             /* 3: Needs a new block to be inserted at end of list. */
             // Try to lock the tail pointer of the last block
-            eb_index_t old_eb = readfe (block_ptr);
+            eb_index_t old_eb = readfe (curs.loc);
             if (!old_eb) {
                 // Create a new edge block
                 eb_index_t newBlock = new_eb (G, type, src);
                 if (newBlock == 0) {
                     // Ran out of edge blocks!
-                    writeef (block_ptr, (uint64_t)old_eb);
+                    writeef (curs.loc, (uint64_t)old_eb);
                     while(next_update() != updates_end)
                     {
                         adapter::set_result(*next_update(), EDGE_NOT_ADDED);
@@ -346,11 +346,11 @@ class BatchInserter
                     ebpool_priv[newBlock].next = 0;
                     push_ebs (G, 1, &newBlock);
                     // Unlock the tail pointer
-                    writeef (block_ptr, (uint64_t)newBlock);
+                    writeef (curs.loc, (uint64_t)newBlock);
                 }
             } else {
-                // Another thread already added a block, unlock and try again
-                writeef (block_ptr, (uint64_t)old_eb);
+                // Another thread already added a block, unlock and keep searching
+                writeef (curs.loc, (uint64_t)old_eb);
             }
         }
     }

lib/stinger_core/src/stinger.c

@@ -1169,7 +1169,6 @@ stinger_update_directed_edge(struct stinger *G,
   }
 
   while (1) {
-    eb_index_t * block_ptr = curs.loc;
     curs.eb = readff((uint64_t *)curs.loc);
     /* 2: The edge isn't already there.  Check for an empty slot. */
     for (tmp = ebpool_priv + curs.eb; tmp != ebpool_priv; tmp = ebpool_priv + readff((uint64_t *)&tmp->next)) {
@@ -1214,25 +1213,26 @@ stinger_update_directed_edge(struct stinger *G,
           }
         }
       }
-      block_ptr = &(tmp->next);
+      curs.loc = &(tmp->next);
+      curs.eb = readff(curs.loc);
     }
 
     /* 3: Needs a new block to be inserted at end of list. */
-    eb_index_t old_eb = readfe ((uint64_t *)block_ptr );
+    eb_index_t old_eb = readfe (curs.loc);
     if (!old_eb) {
       eb_index_t newBlock = new_eb (G, type, src);
       if (newBlock == 0) {
-        writeef ((uint64_t *)block_ptr, (uint64_t)old_eb);
+        writeef (curs.loc, (uint64_t)old_eb);
         return -1;
       } else {
         update_edge_data_and_direction (G, ebpool_priv + newBlock, 0, dest, weight, timestamp, direction, EDGE_WEIGHT_SET);
         ebpool_priv[newBlock].next = 0;
         push_ebs (G, 1, &newBlock);
       }
-      writeef ((uint64_t *)block_ptr, (uint64_t)newBlock);
+      writeef (curs.loc, (uint64_t)newBlock);
       return 1;
     }
-    writeef ((uint64_t *)block_ptr, (uint64_t)old_eb);
+    writeef (curs.loc, (uint64_t)old_eb);
   }