Understanding pg_autovacuum CPU Usage

I've been using pg_autovacuum on a largish postgres installation
successfully for several weeks, now. I know that pg_autovacuum has no
facility for monitoring the system to know whether any particular time
is a good one for it to do it's thing, but I'm wondering what the
occasional episodes of 99.9% CPU usage (as reported by top) are.

I keep a pg_autovacuum.log, and when the spikes start, often nothing is
written to the log. And what ends up being written has been of varying
amounts of potential strain. Frequently, the tables involved are
smallish. So I'm wondering: does the CPU usage correspond to any time
pg_autovacuum arouses from its sleep state and begins looking for what
it ought to do?

Any thoughts?

-tfo