You should manage shared memory carefully if you are running multiple oracle instances on the same server

The ipcs command on LINUX/UNIX systems can help you to monitor the ORACLE SGA by displaying the size of each shared memory segment of the SGA.
If an oracle instance crashes abruptly, it may be possible that some shared memory segments are not released, if this happen you can use the ipcrm command to remove these segments.
The SHMMAX system setting allows you to increase the maximum size of a single shared memory segment.

To see how many oracle shared memory segments are used on your system

[oracle@dbbckp ~]$ ipcs

------ Shared Memory Segments --------

key        shmid      owner      perms      bytes      nattch     status      

0x00000000 77266945   oracle     640        4096       0                       

0x00000000 77299714   oracle     640        4096       0                       

0xc8f5d818 77332483   oracle     640        4096       0                       

0x00000000 77004804   oracle     640        4096       0                       

0x00000000 77037573   oracle     640        4096       0                       

0x4975d098 77070342   oracle     640        4096       0                       

------ Semaphore Arrays --------

key        semid      owner      perms      nsems     

0x56565656 688142     root       666        3         

0x0000041e 786449     root       644        1         

0x000003d4 819218     root       644        1         

0x0000033d 851987     root       644        1         

0xe5873228 233832468  oracle     640        125       

0xe5873229 233865237  oracle     640        125       

0xe587322a 233898006  oracle     640        125       

0xe587322b 233930775  oracle     640        125       

0xe587322c 233963544  oracle     640        125       

0xe587322d 233996313  oracle     640        125       

0xe587322e 234029082  oracle     640        125       

0xe587322f 234061851  oracle     640        125       

0xe5873230 234094620  oracle     640        125       

0x66d899d4 233046045  oracle     640        125       

------ Message Queues --------

key        msqid      owner      perms      used-bytes   messages    

0x000004d2 65537      root       666        0            0 

But if your are running multiple oracle instances on the same server, this output doesn't tell you which  instance is using which segments.

You can use the oracle sysresv utility to identify segments used by an instance (ORACLE_SID = DRTEST)

[oracle@dbbckp ~]$ sysresv

IPC Resources for ORACLE_SID "DRTEST" :

Shared Memory:

ID              KEY

77004804        0x00000000

77037573        0x00000000

77070342        0x4975d098

Semaphores:

ID              KEY

233046045       0x66d899d4

233078814       0x66d899d5

233111583       0x66d899d6

233144352       0x66d899d7

233177121       0x66d899d8

233209890       0x66d899d9

233242659       0x66d899da

233275428       0x66d899db

233308197       0x66d899dc

Oracle Instance alive for sid "DRTEST"

Now you can use the ipcrm command to remove these segments

ipcrm -s <ID> for Semaphores

ipcrm -m <ID> for Shared Memory