autovaccum task got cancelled

Hello,

I'm running an application (with programs in Perl) through pgpool 3.1
with replication mode to two postgresql db servers (version 9.0.13).
Recently, I noticed that the following messages repeatedly showed in
postgres log files. As far as I know, the application programs do not
make any specific lock on the 'file' table. I'm not sure if it is
caused by the pgpool or something else.

Thanks for any help in advance.
Gary

2013-10-31 17:58:56 UTCDETAIL: Process 8580 waits for
ShareRowExclusiveLock on relation 11959608 of database 596746.
2013-10-31 17:58:56 UTCSTATEMENT: LOCK TABLE "file" IN SHARE ROW
EXCLUSIVE MODE
2013-10-31 17:58:56 UTCERROR: canceling autovacuum task
2013-10-31 17:58:56 UTCCONTEXT: automatic vacuum of table
"sd3ops1.public.file"
2013-10-31 18:01:30 UTCLOG: sending cancel to blocking autovacuum PID 8614
2013-10-31 18:01:30 UTCDETAIL: Process 8677 waits for
ShareRowExclusiveLock on relation 11959608 of database 596746.
2013-10-31 18:01:30 UTCSTATEMENT: LOCK TABLE "file" IN SHARE ROW
EXCLUSIVE MODE
2013-10-31 18:01:30 UTCERROR: canceling autovacuum task
2013-10-31 18:01:30 UTCCONTEXT: automatic vacuum of table
"sd3ops1.public.file"
2013-10-31 18:01:49 UTCLOG: could not receive data from client:
Connection reset by peer
2013-10-31 18:01:49 UTCLOG: unexpected EOF within message length word
2013-10-31 18:02:04 UTCLOG: sending cancel to blocking autovacuum PID 8753
2013-10-31 18:02:04 UTCDETAIL: Process 8777 waits for
ShareRowExclusiveLock on relation 11959608 of database 596746.
2013-10-31 18:02:04 UTCSTATEMENT: LOCK TABLE "file" IN SHARE ROW
EXCLUSIVE MODE
2013-10-31 18:02:04 UTCERROR: canceling autovacuum task
2013-10-31 18:02:04 UTCCONTEXT: automatic vacuum of table
"sd3ops1.public.file"
2013-10-31 18:03:09 UTCLOG: sending cancel to blocking autovacuum PID 8782
2013-10-31 18:03:09 UTCDETAIL: Process 8806 waits for
ShareRowExclusiveLock on relation 11959608 of database 596746.
2013-10-31 18:03:09 UTCSTATEMENT: LOCK TABLE "file" IN SHARE ROW
EXCLUSIVE MODE
2013-10-31 18:03:09 UTCERROR: canceling autovacuum task
2013-10-31 18:03:09 UTCCONTEXT: automatic vacuum of table
"sd3ops1.public.file"
2013-10-31 18:04:04 UTCLOG: sending cancel to blocking autovacuum PID 8810
2013-10-31 18:04:04 UTCDETAIL: Process 8395 waits for
ShareRowExclusiveLock on relation 11959608 of database 596746.
2013-10-31 18:04:04 UTCSTATEMENT: LOCK TABLE "file" IN SHARE ROW
EXCLUSIVE MODE