lock contention, need profiling idea

[AI Rumman <rummandba@xxxxxxxxx>]

I see lots of similar log message at a certain time in a day on Postgresql 9,.1:

LOG:  process 18855 still waiting for ShareLock on transaction 2856146023 after 1001.209 ms

STATEMENT:  UPDATE table1 SET time = $1 WHERE id = $2

The table1 size is 17 G. 

What could be the reason for this lock contention?

autovacuum?

Please give some idea.

Thanks.