json: Use reference counting in JSON objects

After profiling OVSDB insert performance it was found
that some significant portion of its time OVSDB is
calling the function json_clone.

Also, the current usages of json_clone never modify the json,
just keeps it to prevent it to be freed.

With that in mind the struct json, json_create, json_clone
and json_destroy were modified to keep a count of how many
references of the json struct are left. Only when that count
reaches zero the json struct is freed.

The old "json_clone" function was renamed as "json_deep_clone".

Some examples of the performance difference:

In these tests a test table with 4 columns (string, string,
bool, integer) was used. All the tests used "commit block".

*** 50 process each inserting 1000 rows ***

Master OVS
Test Duration                   131 seconds
Average Inserts Per second      746.2687 inserts/s
Average Insert Duration         134.1382 ms
Minimal Insert Duration           0.166202 ms
Maximum Insert Duration	        489.8593 ms

JSON GC Patch
Test Duration                   86 seconds
Average Inserts Per second    1176 inserts/s
Average Insert Duration         82.26761 ms
Minimal Insert Duration          0.165448 ms
Maximum Insert Duration        751.2111 ms

*** 5 process each inserting 10000 rows ***

Master OVS
Test Duration                      8 seconds
Average Inserts Per second      7142.857 inserts/s
Average Insert Duration            0.656431 ms
Minimal Insert Duration            0.125197 ms
Maximum Insert Duration           11.93203 ms

JSON GC Patch
Test Duration                      7 seconds
Average Inserts Per second      8333.333 inserts/s
Average Insert Duration            0.55688 ms
Minimal Insert Duration            0.143233 ms
Maximum Insert Duration           26.26319 ms

Signed-off-by: Esteban Rodriguez Betancourt <estebarb@hpe.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/openvswitch/json.h b/include/openvswitch/json.h
index 13f346cc2..83775ed0e 100644
--- a/include/openvswitch/json.h
+++ b/include/openvswitch/json.h
@@ -63,6 +63,7 @@ struct json_array {
 /* A JSON value. */
 struct json {
     enum json_type type;
+    size_t count;
     union {
         struct shash *object;   /* Contains "struct json *"s. */
         struct json_array array;
@@ -99,6 +100,7 @@ bool json_boolean(const struct json *);
 double json_real(const struct json *);
 int64_t json_integer(const struct json *);
 
+struct json *json_deep_clone(const struct json *);
 struct json *json_clone(const struct json *);
 void json_destroy(struct json *);