Finalizing commit taking very long

Hello,

I am having a strange latency problem on my instance of Postgres that
I don't know how to investigate.

I am accessing the db instance using a Java application and the
Cayenne mapping framework. Everything works fine, except when it is
time to delete a user account (that is a user of the application, not
of Postgres).

Deleting an account trigger a (sort-of) cascade delete to remove also
all the dependent records stored on the db. The cascade constraints
are managed by the Cayenne library, and the db receives a list of
delete statements for all the rows of the different tables that should
be deleted; this is probably not optimal from a db point of view, but
is extremely convenient from an application point of view.

Anyway, the deletion of all the records is executed without much
problems (I had a huge slowdown here a few weeks ago, but I was
missing an index on a constraint).

Obviously, all the delete statements are grouped into a single
transaction; and when it is time to commit this transaction, the db
instance takes "forever".

Here are some logs taken from the db server:

[ lot of delete statements skipped]
2007-10-24 12:13:17 CEST LOG:  statement: EXECUTE <unnamed>  [PREPARE:
 DELETE FROM connection.USRCNN WHERE ID_USRCNN = $1]
2007-10-24 12:13:17 CEST LOG:  duration: 0.206 ms
2007-10-24 12:13:17 CEST LOG:  duration: 0.206 ms  statement: EXECUTE
<unnamed>  [PREPARE:  DELETE FROM connection.USRCNN WHERE ID_USRCNN =
$1]
2007-10-24 12:13:17 CEST LOG:  statement: PREPARE <unnamed> AS DELETE
FROM clipperz.USR WHERE ID_USR = $1
2007-10-24 12:13:17 CEST LOG:  statement: <BIND>
2007-10-24 12:13:17 CEST LOG:  statement: EXECUTE <unnamed>  [PREPARE:
 DELETE FROM clipperz.USR WHERE ID_USR = $1]
2007-10-24 12:13:17 CEST LOG:  duration: 0.761 ms
2007-10-24 12:13:17 CEST LOG:  duration: 0.761 ms  statement: EXECUTE
<unnamed>  [PREPARE:  DELETE FROM clipperz.USR WHERE ID_USR = $1]
2007-10-24 12:13:17 CEST LOG:  statement: <BIND>
2007-10-24 12:13:17 CEST LOG:  statement: EXECUTE <unnamed>  [PREPARE:  COMMIT]
2007-10-24 12:13:51 CEST LOG:  autovacuum: processing database "clipperz_beta"
2007-10-24 12:14:51 CEST LOG:  autovacuum: processing database "clipperz_beta"
2007-10-24 12:15:10 CEST LOG:  duration: 113300.147 ms
2007-10-24 12:15:10 CEST LOG:  duration: 113300.147 ms  statement:
EXECUTE <unnamed>  [PREPARE:  COMMIT]


As you may notice, the commit phase takes almost 2 full minutes. :-(

How can I understand what is going on on that timeframe in order to
try to fix it?

Thanks for your attention.

Best regards,

Giulio Cesare

PS: I run my development machine on a MacOS, with Postgres running on
a Parallels virtual machine. I don't think this really matters for the
above problem, but in case ...