Persistent MySQL connection?

[djbaxter]

// ****** MASTER DATABASE PERSISTENT CONNECTIONS ******
// This option allows you to turn persistent connections to MySQL on or off.
// The difference in performance is negligible for all but the largest boards.
// If you are unsure what this should be, leave it off. (0 = off; 1 = on)
$config['MasterServer']['usepconnect'] = 1;

I have been experimenting with turning this ON or OFF since installing vBulletin a week ago.

What exactly does this do and under what circumstances will this result in performance improvements versus degradation?

[Steve Machol]

Honestly I've never been able to discern any difference in performance myself, other that turning this off will reduce the amount of MySQL connections being used.

[djbaxter]

OK, that's part of what confused me. On another forum (SMF support forum to be exact), one of the developers said that turning persistent connections ON would REDUCE the demands on the MySQL server because MySQL wouldn't be closing connections after each query. Then elsewhere in this forum someone stated that the loads would be reduced by turning peristent connections OFF.

I'm confused by the whole issue.

[Steve Machol]

They are right but again in my experience this does not make any discernable difference. Perhaps it would on a massive forum.

[Marsupilami]

If the MySQL-server is very remote to the webserver, or if the database connects through SSH encrypted tunnel, then from my findings the initial delays for serving a page to the user after some seconds of idle will be faster. Otherwise no difference.

So slight benefit on low traffic forums with a remote database server, and otherwize probably nothing to gain from persistent connections.

[djbaxter]

Are there circumstances where persistent connections could do any harm or degrade performance?