Replicating memes

Before

For returning the results of a SPARQL SELECT query we used to have a callback like this. I removed error handling, you can find the original here.

We need to marshal a database result_set to a GPtrArray because dbus-glib fancies that. This is a lot of boxing the strings into GValue and GStrv. It does allocations, so not good.

static void
query_callback(TrackerDBResultSet *result_set,GError *error,gpointer user_data)
{
  TrackerDBusMethodInfo *info = user_data;
  GPtrArray *values = tracker_dbus_query_result_to_ptr_array (result_set);
  dbus_g_method_return (info->context, values);
  tracker_dbus_results_ptr_array_free (&values);
}

void
tracker_resources_sparql_query (TrackerResources *self, const gchar *query,
                                DBusGMethodInvocation *context, GError **error)
{
  TrackerDBusMethodInfo *info = ...; guint request_id;
  TrackerResourcesPrivate *priv= ...; gchar *sender;
  info->context = context;
  tracker_store_sparql_query (query, TRACKER_STORE_PRIORITY_HIGH,
                              query_callback, ...,
                              info, destroy_method_info);
}

After

Last week I changed the asynchronous callback to return a database cursor. In SQLite that means an sqlite3_step(). SQLite returns const pointers to the data in the cell with its sqlite3_column_* APIs.

This means that now we’re not even copying the strings out of SQLite. Instead, we’re using them as const to fill in a raw DBusMessage:

static void
query_callback(TrackerDBCursor *cursor,GError *error,gpointer user_data)
{
  TrackerDBusMethodInfo *info = user_data;
  DBusMessage *reply; DBusMessageIter iter, rows_iter;
  guint cols; guint length = 0;
  reply = dbus_g_method_get_reply (info->context);
  dbus_message_iter_init_append (reply, &iter);
  cols = tracker_db_cursor_get_n_columns (cursor);
  dbus_message_iter_open_container (&iter, DBUS_TYPE_ARRAY,
                                    "as", &rows_iter);
  while (tracker_db_cursor_iter_next (cursor, NULL)) {
    DBusMessageIter cols_iter; guint i;
    dbus_message_iter_open_container (&rows_iter, DBUS_TYPE_ARRAY,
                                      "s", &cols_iter);
    for (i = 0; i < cols; i++, length++) {
      const gchar *result_str = tracker_db_cursor_get_string (cursor, i);
      dbus_message_iter_append_basic (&cols_iter,
                                      DBUS_TYPE_STRING,
                                      &result_str);
    }
    dbus_message_iter_close_container (&rows_iter, &cols_iter);
  }
  dbus_message_iter_close_container (&iter, &rows_iter);
  dbus_g_method_send_reply (info->context, reply);
}

Results

The test is a query on 13500 resources where we ask for two strings, repeated eleven times. I removed a first repeat from each round, because the first time the sqlite3_stmt still has to be created. This means that our measurement would get a few more milliseconds. I also directed the standard out to /dev/null to avoid the overhead created by the terminal. The results you see below are the value for “real”.

There is of course an overhead created by the “tracker-sparql” program. It does demarshaling using normal dbus-glib. If your application uses DBusMessage directly, then it can avoid the same overhead. But since for both rounds I used the same “tracker-sparql” it doesn’t matter for the measurement.

$ time tracker-sparql -q "SELECT ?u  ?m { ?u a rdfs:Resource ;
          tracker:modified ?m }" > /dev/null

Without the optimization:

0.361s, 0.399s, 0.327s, 0.355s, 0.340s, 0.377s, 0.346s, 0.380s, 0.381s, 0.393s, 0.345s

With the optimization:

0.279s, 0.271s, 0.305s, 0.296s, 0.295s, 0.294s, 0.295s, 0.244s, 0.289s, 0.237s, 0.307s

The improvement ranges between 7% and 40% with average improvement of 22%.

This entry was posted on Sunday, April 25th, 2010 at 4:39 pm and is filed under Informatics and programming, Tracker, condescending, controversial, english, extremely condescending, extremely controversial, maemo, meego, very condescending. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.