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.