two memory-consuming postgres processes

[Alexy Khrabrov <deliverable@xxxxxxxxx>]

Greetings -- I have an UPDATE query updating a 100 million row table,  
and allocate enough memory via shared_buffers=1500MB.  However, I see  
two processes in top, the UPDATE process eating about 850 MB and the  
writer process eating about 750 MB.  The box starts paging.   Why is  
there the writer taking almost as much space as the UPDATE, and how  
can I shrink it?

Cheers,
Alexy