Introducing semaphores in Linux

The translation of the article was prepared on the eve of the start of the course "Administrator Linux.Basic" .

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A semaphore is a mechanism that allows competing processes and threads to share resources and helps in solving various synchronization problems such as races, deadlocks (deadlocks), and misbehaving threads.



To solve these problems, the kernel provides tools such as mutexes, semaphores, signals, and barriers.



There are three types of semaphores:

1. Binary semaphore
2. Counting semaphore
3. Semaphore set

Viewing IPC Status

The commands below can be used to obtain information about the current state of inter-process communication (IPC) facilities.

# ipcs
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 65536 root 600 393216 2 dest
0x00000000 98305 root 600 393216 2 dest
0x00000000 131074 root 600 393216 2 dest
0x00000000 163843 root 600 393216 2 dest
0x00000000 196612 root 600 393216 2 dest
0x00000000 229381 root 600 393216 2 dest
0x00000000 262150 root 600 393216 2 dest
0x00000000 294919 root 600 393216 2 dest
0x00000000 327688 root 600 393216 2 dest
------ Semaphore Arrays --------

key semid owner perms nsems

------ Message Queues --------
key msqid owner perms used-bytes messages

Active semaphore arrays

Displays information about active semaphore arrays.

# ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems

Shared memory segments

View information about active segments of shared memory.

# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 65536 root 600 393216 2 dest
0x00000000 98305 root 600 393216 2 dest

Limits

The command ipcs -ldisplays the limits of shared memory, semaphores and messages.

# ipcs -l
------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 4194303
max total shared memory (kbytes) = 1073741824
min seg size (bytes) = 1

------ Semaphore Limits --------
max number of arrays = 128
max semaphores per array = 250
max semaphores system wide = 32000
max ops per semop call = 32
semaphore max value = 32767

------ Messages: Limits --------
max queues system wide = 16
max size of message (bytes) = 65536
default max size of queue (bytes) = 65536

Shared memory

The command below displays shared memory.

# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 65536 root 600 393216 2 dest
0x00000000 98305 root 600 393216 2 dest
0x00000000 131074 root 600 393216 2 dest
0x00000000 163843 root 600 393216 2 dest
0x00000000 196612 root 600 393216 2 dest
0x00000000 229381 root 600 393216 2 dest
0x00000000 262150 root 600 393216 2 dest
0x00000000 294919 root 600 393216 2 dest
0x00000000 327688 root 600 393216 2 dest

Resource creators

The command displays the user and group of the owner and creator of the resource.

# ipcs -m -c

------ Shared Memory Segment Creators/Owners --------
shmid perms cuid cgid uid gid
65536 600 root root root root
98305 600 root root root root
131074 600 root root root root
163843 600 root root root root
196612 600 root root root root
229381 600 root root root root
262150 600 root root root root
294919 600 root root root root
327688 600 root root root root

Using IPC Tools

In the example below, the parameter -udisplays a summary of the use of all IPC tools.

# ipcs -u

------ Shared Memory Status --------
segments allocated 9
pages allocated 864
pages resident 477
pages swapped 0
Swap performance: 0 attempts 0 successes

------ Semaphore Status --------
used arrays = 0
allocated semaphores = 0

------ Messages: Status --------
allocated queues = 0
used headers = 0
used space = 0 bytes

When services are stopped, semaphores and shared memory segments must also be deleted. If they are not removed, then this can be done using the ipcrm command, passing the identifier of the IPC object.

# ipcs -a
# ipcrm -s < sem id>

You can also change the semaphore limits using sysctl.

# /sbin/sysctl -w kernel.sem=250