question about measuring shared_buffers usage

Hi ,

I have following setting in my postgresql.conf file.

shared_buffers = 10000 (8k blocks) ; which is apprx 80MB

my question is how can i know how much is the usage
of the shared_buffers so that it can be increased or
decreased. below is the output of ipcs , I am wondering
is postgres using 420MB shared memory (but shared_buffers
is only 80MB)

$ ipcs -m

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 3637248 postgres 600 52228 0
0x0052e6a9 7798785 postgres 600 40034304 42
0x00000000 4489218 postgres 600 52228 0
0x00000000 4521987 postgres 600 52228 0
0x00000000 4554756 postgres 600 52228 0
0x00000000 4587525 postgres 600 52228 0
0x00000000 4620294 postgres 600 52228 0
0x00000000 2883591 postgres 600 52228 0
0x00000000 3670024 postgres 600 74240 0
0x00000000 3702793 postgres 600 768 0
0x00000000 3735562 postgres 600 524 0
0x00000000 3768331 postgres 600 4 0
0x0052e2c1 7831564 postgres 600 420610048 121

regds
mallah.