ovsdb: Strong references cascade performance fix.

Improves the performance of OVSDB avoiding the chain
reaction produced when modifing rows with a strong
reference and the pointed rows have more strong
references.

The approach taken was using the change bitmap to avoid
triggering a change count when the column hasn't changed.

One way to trigger the issue is emulating a simple linked list
with strong references within a table, where each new row
points to the previous.

Without the fix OVSDB creates a ovsdb_txn_row (and a copy
of the row) for each row in the table.
With the fix it only creates two ovsdb_txn_row: the new row and
the directly pointed row.

Signed-off-by: Esteban Rodriguez Betancourt <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

ovsdb/transaction.c

@@ -271,16 +271,18 @@ update_row_ref_count(struct ovsdb_txn *txn, struct ovsdb_txn_row *r)
         const struct ovsdb_column *column = node->data;
         struct ovsdb_error *error;
 
-        if (r->old) {
-            error = ovsdb_txn_adjust_row_refs(txn, r->old, column, -1);
-            if (error) {
-                return OVSDB_WRAP_BUG("error decreasing refcount", error);
+        if (bitmap_is_set(r->changed, column->index)) {
+            if (r->old) {
+                error = ovsdb_txn_adjust_row_refs(txn, r->old, column, -1);
+                if (error) {
+                    return OVSDB_WRAP_BUG("error decreasing refcount", error);
+                }
             }
-        }
-        if (r->new) {
-            error = ovsdb_txn_adjust_row_refs(txn, r->new, column, 1);
-            if (error) {
-                return error;
+            if (r->new) {
+                error = ovsdb_txn_adjust_row_refs(txn, r->new, column, 1);
+                if (error) {
+                    return error;
+                }
             }
         }
     }