What's canceling autovacuum tasks?

Hi there.

During the weekend I've worked for hours on recovering table bloat. Now I was hoping that after the tables are properly trimmed, then after the next delete operation which created dead tuples, autovacuum will go into effect and do its job properly, and prevent the situation from recurring.

Indeed autovacuum started working on some of the tables. At least one of these tables was one that I have trimmed up using CLUSTER. So I was watching that autovacuum process carefully. And then suddenly it was gone, after working for 20-odd hours. And I had even more dead tuples in pg_stat_user_tables for that table than before.

Looking at the log, I get this:

2011-02-06 15:21:42 IST ERROR: canceling autovacuum task
2011-02-06 15:21:42 IST CONTEXT: automatic vacuum of table "reports.alerts.smsq__archive"

Why?

Postgres version is 8.3.11

Thank you,
Herouth