ovsdb-data: Add function to apply diff in-place.

ovsdb_datum_apply_diff() is heavily used in ovsdb transactions, but
it's linear in terms of number of comparisons.  And it also clones
all the atoms along the way.  In most cases size of a diff is much
smaller than the size of the original datum, this allows to perform
the same operation in-place with only O(diff->n * log2(old->n))
comparisons and O(old->n + diff->n) memory copies with memcpy.
Using this function while applying diffs read from the storage gives
a significant performance boost and allows to execute much more
transactions per second.

Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Acked-by: Mark D. Gray <mark.d.gray@redhat.com>

1 files changed, 4 insertions, 6 deletions

diff --git a/ovsdb/file.c b/ovsdb/file.c
index 59220824f..9f44007d9 100644
--- a/ovsdb/file.c
+++ b/ovsdb/file.c
@@ -113,19 +113,17 @@ ovsdb_file_update_row_from_json(struct ovsdb_row *row, bool converting,
         if (row_contains_diff
             && !ovsdb_datum_is_default(&row->fields[column->index],
                                        &column->type)) {
-            struct ovsdb_datum new_datum;
-
-            error = ovsdb_datum_apply_diff(&new_datum,
+            error = ovsdb_datum_apply_diff_in_place(
                                            &row->fields[column->index],
                                            &datum, &column->type);
             ovsdb_datum_destroy(&datum, &column->type);
             if (error) {
                 return error;
             }
-            ovsdb_datum_swap(&datum, &new_datum);
+        } else {
+            ovsdb_datum_swap(&row->fields[column->index], &datum);
+            ovsdb_datum_destroy(&datum, &column->type);
         }
-        ovsdb_datum_swap(&row->fields[column->index], &datum);
-        ovsdb_datum_destroy(&datum, &column->type);
     }
 
     return NULL;