How to identify which query is running - reg.

Dear Group,
We have a java front-end for postgresql 3.4. When
we monitor the system usage using top we find couple of postmasters
taking up close to 90% of the CPU time in total. I would like to know
which are the queries that are currently running at this point of time.
How do we do this?? Another thing that I have noticed is that when I
give df -h, I find there is a partition /dev/shm which is somewhere in
the range of about 128MB and never used. What is this supposed to be.
Is it being not used a good sign or am I loosing on my performance? How
do we get it to be used by the database if it will help improve the
performance. Thanks in advance.

Yours sincerely,
Shan.