Let's say the following is being returned when attempting to start one or more Apache web server instances.

No space left on device AH00023 Couldn't create the mutex

This typically implies that the Operating System has reached it's semaphore limit. The ipcs command with the -s or --semaphores flag can be used to display the semaphores currently in used. Notice the output includes semid (semaphore ID).

[root@server1 ~]# ipcs --semaphores

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x00000000 131073     nobody     600        1         
0x00000000 196610     nobody     600        1         
0x00000000 131076     nobody     600        1         
0x00000000 5          nobody     600        1         
0x00000000 131078     nobody     600        1         
0x00000000 7          nobody     600        1

The ipcs command with the -s or --semaphores flag and -i or --id option can be used to get the process ID (PID) associated with the semaphore. In this example, semaphore ID 131073 is associated with PID 129086.

[root@server1 ~]# ipcs -s -i 131073

Semaphore Array semid=131073
uid=99   gid=99  cuid=0  cgid=0
mode=0600, access_perms=0600
nsems = 1
otime = Tue Aug  3 00:00:12 2021  
ctime = Mon Aug  2 23:56:16 2021  
semnum     value      ncount     zcount     pid       
0          1          0          0          129086

The ps command can be used to display details on the process. 

[root@server1 ~]# ps -ef | grep 129086
nobody   129086 126409  0 00:00 ?        00:00:00 /path/to/bin/httpd -d /path/to -f /path/to/conf/httpd90.conf -k start

The kill command can be used to kill the PIDs that are being used.

kill -9 129086

Sometimes, "no such process" may be returned.

[root@server1 ~]# kill -9 12345
bash: kill: (12345) - No such process

The ipcrm command with the -s or --semaphore-id option can be used to remove semaphores.

[root@server1 ~]# ipcrm -s 131073