sema… what ? interprocess communication (1)

April 9, 2012 — Leave a comment

if you followed the oracle installation guide there are some kernel parameters to be configured for the oracle database. one of them specified by four values is about semaphores:

  • semmsl: the maximum number of semaphores per semaphore set
  • semmns: the maximum number of semaphore of the entire system
  • semopm: number of maximum operations per semop call
  • semmni: the maximum number of semaphore sets of the entire system

the question is: what are these semaphores about and what are they for?


a semaphore is a counter associated with a data structure which provides locking and synchronization of critical regions. there is one semaphore ( initialized to 1 ) for each data structure to be protected. the atomic methods “down” and “up” are used to decrease and increase the counter. if the kernel wants access to a protected structure it executes the “down” method and if the result is not negative ( the counter is equal or greater than zero ) access to the resource is granted. if the counter is negative the process which wishes to access the resource is blocked and added to the sempahore list ( a kind of queue ). as time goes by some process finishes its work and executes the “up” method which allows one process in the semaphore list to proceed.

in linux there are two kinds of semaphores:

  • kernel semaphores ( for kernel control paths )
  • system V IPC semaphores ( for user mode processes ), IPC stands for “interprocess communication”

the IPC semaphores are the ones relevant to the oracle database. semaphores are created by the function semget() which returns the semaphore identifier. there are two other functions for creating ipc resources, which are:

  • msgget(): which is for message queues
  • shmget(): which is for shared memory

there must be at least one semaphore for each oracle process ( the processes parameter of the database ). as each session to the database needs to be synchronized with other sessions ( and sessions are memory structures ) oracle must request resources from the operation system to be able to handle concurrency in the sga to which all sessions are connected to.

the semaphores queue of pending requests is implemented as a double linked list. you remember? the same concepts over and over again. actually semaphores are sometimes called mutexes ( and there are internal functions like init_MUTEX )… surprised ?

to display the current limits the following command can be used:

ipcs -ls
------ Semaphore Limits --------
max number of arrays = 1024
max semaphores per array = 250
max semaphores system wide = 32000
max ops per semop call = 32
semaphore max value = 32767

or you can directly query the /proc filesystem:

cat /proc/sys/kernel/sem
250 32000 32 1024

to check the currently allocated semaphores:

ipcs -s
------ Semaphore Arrays --------
key semid owner perms nsems
0x127f81f8 163842 oracle 640 124
0x3d2c0d44 1933315 oracle 640 129
0x3d2c0d45 1966084 oracle 640 129
0x3d2c0d46 1998853 oracle 640 129
0x3d2c0d47 2031622 oracle 640 129
0x3d2c0d48 2064391 oracle 640 129

if you want to see some semaphore operations in action do, for example, a strace on the smon process and wait one or two seconds:

ps -ef | grep dbw
oracle 2723 1 0 08:31 ? 00:00:03 ora_smon_dbs300
root 3153 3111 2 09:04 pts/1 00:00:00 grep smon
strace -p 2723
semtimedop(819203, {{17, -1, 0}}, 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
gettimeofday({1333697562, 861932}, NULL) = 0
semtimedop(819203, {{17, -1, 0}}, 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
gettimeofday({1333697565, 871767}, NULL) = 0
semtimedop(819203, {{17, -1, 0}}, 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
gettimeofday({1333697568, 893455}, NULL) = 0
semtimedop(819203, {{17, -1, 0}}, 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
gettimeofday({1333697571, 905050}, NULL) = 0
semtimedop(819203, {{17, -1, 0}}, 1, {3, 0}) = -1 EAGAIN (Resource temporarily unavailable)
gettimeofday({1333697574, 920094}, NULL) = 0

here you can clearly see calls to semtimedop which return with -1 ( EAGAIN, the call expired ).

if you followed the series on how to setup a test infrastructure or you have a test infrastructure available to play with here is a little demonstration:

be sure to save your current kernel semaphore settings:

cat /proc/sys/kernel/sem
250 32000 32 1024

minimize the settings to very low values and try to restart ( or start ) the oracle instance:

echo "2 10 1 2" > /proc/sys/kernel/sem

if you write the values to the /proc/sys/kernel/…-parameter files the values will be in effect immediately. be sure what you are doing if you’re touching these parameters.

startup or restart the oracle instance:

su - oracle
sqlplus / as sysdba
startup force
ORA-27154: post/wait create failed
ORA-27300: OS system dependent operation:semget failed with status: 28
ORA-27301: OS failure message: No space left on device
ORA-27302: failure occurred at: sskgpbitsper

what happened ? the database was not able to allocate the resources ( sempahores ) it needs, thus can not start up. the message “No space left on device” indicates some issue with disk space, but it’s actually the memory in question here. fyi: the allocation of semaphores ( and shared memory ) does only occur during the “startup nomount” phase. this is the only time oracle requests this resources.

fix it, and try again:

su -
echo "250 32000 32 1024" > /proc/sys/kernel/sem
su - oracle
startup force

conclusion: semaphores are one critical part for interprocess communication and provide a way for locking and synchronization. if the database can not allocate the resources it needs from the operating system it will fail to start up.

the values recommended in the installation guides are minimum values. if your session count to the database grows you may need to adjust the semaphore parameters.

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