Waiting on ExclusiveLock on extension

Hi,

After upgrading our database from 9.3.5 to 9.4.1 last night, the server
suffers from high CPU spikes. During these spikes, there are a lot of
these messages in the logs:

process X still waiting for ExclusiveLock on extension of relation
Y of database Z after 1036.234 ms

And:

process X acquired ExclusiveLock on extension of relation Y of
database Z after 2788.050 ms

What looks suspicious is that there are sometimes several "acquired"
messages for the exact same relation number in the exact same millisecond.

a) I'm assuming "extension" means growing the physical diskspace used by
a table-- is that correct?
b) How can you find the name of the relation being extended? based on
the relation number.
c) Why would Postgres grow a table twice in the same millisecond? Could
it be an index with a high fill factor?

Any suggestions on how to approach this issue are welcome.

Thanks for your time,
Andomar

P.S. The upgrade was done with pg_dump. So the database was converted
to SQL and then imported into 9.4.