two memory-consuming postgres processes

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