What does pg_stat_activity.xact_start mean for autovacuum threads? If I see something like:

db=&gt; SELECT datname,pid,state, (now() - query_start)::interval, query FROM pg_stat_activity where state != 'idle' and query like 'autovacuum%';
 datname |  pid  | state  |    interval     |                        query                        
---------+-------+--------+-----------------+-----------------------------------------------------
 theseus | 21936 | active | 00:00:00.940774 | autovacuum: VACUUM ANALYZE pg_catalog.pg_constraint
 theseus | 16775 | active | 00:33:57.896069 | autovacuum: VACUUM pg_toast.pg_toast_1234


How should I interpret xact_start vis a vis the vacuum_cost_limit and autovacuum_vacuum_cost_delay? In other words is xact_start the time at which the thread woke up again to do another chunk of vacuum_cost_limit work, or does it represent many chunks?

And a follow-up: my vacuum_cost_limit setting is the default of 200 (very conservative); if we're seeing a very long "transaction" interval and that interval should be measuring a very small amount of work, what could explain the long runtime? It's my understanding that autovacuum doesn't block on other activity.
The xact_start field should be before or equal to query_start, covering the entire duration of the vacuuming of the table.  

Your vacuum is slow because your setting of vacuum_cost_limit is very conservative.  It is throttling itself, not blocking on other things.