autovacuum launcher eating too much CPU

Hi,

Darcy Buskermolen noticed that when one has many databases, the autovac
launcher starts eating too much CPU.

I tried it here with 200 databases and indeed it does seem to eat its
share.  Even with the default naptime, which I wouldn't have thought
that was too high (it does make the launcher wake up about three times a
second though).

I'm looking at a profile and I can't seem to make much sense out of it.
It seems to me like the problem is not autovac itself, but rather the
pgstat code that reads the stat file from disk.  Of course, autovac does
need to read the file fairly regularly.

Here is the top lines of gprof output.

Comments?  Is there something here that needs fixing?

--
Alvaro Herrera                        http://www.advogato.org/person/alvherre
"Investigación es lo que hago cuando no sé lo que estoy haciendo"
(Wernher von Braun)