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>

lib/ovsdb-data.c

@@ -2253,6 +2253,106 @@ ovsdb_datum_diff(struct ovsdb_datum *diff,
     }
 }
 
+/* Apply 'diff' to 'a'.
+ *
+ * Return NULL if the 'a' is successfully updated, otherwise, return
+ * ovsdb_error. */
+struct ovsdb_error *
+ovsdb_datum_apply_diff_in_place(struct ovsdb_datum *a,
+                                const struct ovsdb_datum *diff,
+                                const struct ovsdb_type *type)
+{
+    struct ovsdb_error *error = NULL;
+    struct ovsdb_datum result;
+    size_t i, new_size;
+    unsigned int *idx, pos;
+    enum {
+        DIFF_OP_ADD,
+        DIFF_OP_REMOVE,
+        DIFF_OP_UPDATE,
+    } *operation;
+
+    if (!ovsdb_type_is_composite(type)) {
+        ovsdb_datum_destroy(a, type);
+        ovsdb_datum_clone(a, diff, type);
+        return NULL;
+    }
+
+    operation = xmalloc(diff->n * sizeof *operation);
+    idx = xmalloc(diff->n * sizeof *idx);
+    new_size = a->n;
+    for (i = 0; i < diff->n; i++) {
+        if (!ovsdb_datum_find_key(a, &diff->keys[i], type->key.type, &pos)) {
+            operation[i] = DIFF_OP_ADD;
+            new_size++;
+        } else if (type->value.type != OVSDB_TYPE_VOID
+                   && !ovsdb_atom_equals(&diff->values[i], &a->values[pos],
+                                         type->value.type)) {
+            operation[i] = DIFF_OP_UPDATE;
+        } else {
+            operation[i] = DIFF_OP_REMOVE;
+            new_size--;
+        }
+        idx[i] = pos;
+    }
+
+    /* Make sure member size of 'new' conforms to type. */
+    if (new_size < type->n_min || new_size > type->n_max) {
+        error = ovsdb_error(NULL, "Datum crated by diff has size error");
+        goto exit;
+    }
+
+    ovsdb_datum_init_empty(&result);
+    ovsdb_datum_reallocate(&result, type, new_size);
+
+    unsigned int copied = 0;
+    for (i = 0; i < diff->n; i++) {
+        pos = idx[i];
+
+        if (copied < pos) {
+            /* Copying all atoms that should go before the current one. */
+            ovsdb_datum_push_unsafe(&result, a, copied, pos - copied, type);
+            copied = pos;
+        }
+
+        switch (operation[i]) {
+        case DIFF_OP_UPDATE:
+        case DIFF_OP_ADD:
+            /* Inserting new atom from 'diff'. */
+            ovsdb_atom_clone(&result.keys[result.n],
+                             &diff->keys[i], type->key.type);
+            if (type->value.type != OVSDB_TYPE_VOID) {
+                ovsdb_atom_clone(&result.values[result.n],
+                                 &diff->values[i], type->value.type);
+            }
+            result.n++;
+            if (operation[i] != DIFF_OP_UPDATE) {
+                break;
+            }
+            /* fall through */
+
+        case DIFF_OP_REMOVE:
+            /* Destroying atom. */
+            ovsdb_atom_destroy(&a->keys[pos], type->key.type);
+            if (type->value.type != OVSDB_TYPE_VOID) {
+                ovsdb_atom_destroy(&a->values[pos], type->value.type);
+            }
+            copied++; /* Skipping removed atom. */
+            break;
+        }
+    }
+    /* Copying remaining atoms. */
+    ovsdb_datum_push_unsafe(&result, a, copied, a->n - copied, type);
+    a->n = 0;
+
+    ovsdb_datum_swap(&result, a);
+    ovsdb_datum_destroy(&result, type);
+exit:
+    free(operation);
+    free(idx);
+    return error;
+}
+
 /* Apply 'diff' to 'old' to regenerate 'new'.
  *
  * Return NULL if the 'new' is successfully generated, otherwise, return

lib/ovsdb-data.h

@@ -252,6 +252,12 @@ struct ovsdb_error *ovsdb_datum_apply_diff(struct ovsdb_datum *new_datum,
                                            const struct ovsdb_type *type)
 OVS_WARN_UNUSED_RESULT;
 
+struct ovsdb_error * ovsdb_datum_apply_diff_in_place(
+                struct ovsdb_datum *a,
+                const struct ovsdb_datum *diff,
+                const struct ovsdb_type *type)
+OVS_WARN_UNUSED_RESULT;
+
 /* Raw operations that may not maintain the invariants. */
 void ovsdb_datum_remove_unsafe(struct ovsdb_datum *, size_t idx,
                                const struct ovsdb_type *);

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;

tests/ovsdb-data.at

@@ -846,18 +846,21 @@ OVSDB_CHECK_POSITIVE([generate and apply diff -- integer],
   [[diff-data '["integer"]' '[0]' '[2]']],
   [[diff: 2
 apply diff: 2
+apply diff in place: 2
 OK]])
 
 OVSDB_CHECK_POSITIVE([generate and apply diff -- boolean],
   [[diff-data '["boolean"]' '[true]' '[false]']],
   [[diff: false
 apply diff: false
+apply diff in place: false
 OK]])
 
 OVSDB_CHECK_POSITIVE([generate and apply diff -- string],
   [[diff-data '["string"]' '["AAA"]' '["BBB"]']],
   [[diff: "BBB"
 apply diff: "BBB"
+apply diff in place: "BBB"
 OK]])
 
 dnl Test set modifications.
@@ -870,15 +873,19 @@ OVSDB_CHECK_POSITIVE([generate and apply diff -- set],
   ]],
   [[diff: ["set",[0,2]]
 apply diff: ["set",[1,2]]
+apply diff in place: ["set",[1,2]]
 OK
 diff: 0
 apply diff: 1
+apply diff in place: 1
 OK
 diff: ["set",[0,1]]
 apply diff: ["set",[0,1]]
+apply diff in place: ["set",[0,1]]
 OK
 diff: ["set",[0,1]]
 apply diff: ["set",[]]
+apply diff in place: ["set",[]]
 OK]])
 
 dnl Test set modifications causes data to violate set size constrain.
@@ -898,18 +905,23 @@ OVSDB_CHECK_POSITIVE([generate and apply diff -- map],
   ]],
   [[diff: ["map",[["2 gills","1 chopin"],["2 pints","1 quart"]]]
 apply diff: ["map",[["2 pints","1 quart"]]]
+apply diff in place: ["map",[["2 pints","1 quart"]]]
 OK
 diff: ["map",[]]
 apply diff: ["map",[["2 gills","1 chopin"]]]
+apply diff in place: ["map",[["2 gills","1 chopin"]]]
 OK
 diff: ["map",[["2 gills","1 chopin"]]]
 apply diff: ["map",[]]
+apply diff in place: ["map",[]]
 OK
 diff: ["map",[["2 pints","1 quart"]]]
 apply diff: ["map",[["2 pints","1 quart"]]]
+apply diff in place: ["map",[["2 pints","1 quart"]]]
 OK
 diff: ["map",[["2 gills","1 gallon"]]]
 apply diff: ["map",[["2 gills","1 gallon"]]]
+apply diff in place: ["map",[["2 gills","1 gallon"]]]
 OK]])
 
 OVSDB_CHECK_NEGATIVE([generate and apply diff with map -- size error],

tests/test-ovsdb.c

@@ -512,6 +512,18 @@ do_diff_data(struct ovs_cmdl_context *ctx)
             ovs_fatal(0, "failed to apply diff");
         }
 
+        /* Apply diff to 'old' in place. */
+        error = ovsdb_datum_apply_diff_in_place(&old, &diff, &type);
+        if (error) {
+            char *string = ovsdb_error_to_string_free(error);
+            ovs_fatal(0, "%s", string);
+        }
+
+        /* Test to make sure 'old' equals 'new' now.  */
+        if (!ovsdb_datum_equals(&new, &old, &type)) {
+            ovs_fatal(0, "failed to apply diff in place");
+        }
+
         /* Print diff */
         json = ovsdb_datum_to_json(&diff, &type);
         printf ("diff: ");
@@ -522,6 +534,11 @@ do_diff_data(struct ovs_cmdl_context *ctx)
         printf ("apply diff: ");
         print_and_free_json(json);
 
+        /* Print updated 'old' */
+        json = ovsdb_datum_to_json(&old, &type);
+        printf ("apply diff in place: ");
+        print_and_free_json(json);
+
         ovsdb_datum_destroy(&new, &type);
         ovsdb_datum_destroy(&old, &type);
         ovsdb_datum_destroy(&diff, &type);