| Infodoc ID |
|
Synopsis |
|
Date |
| 2270 |
|
Understanding semaphores, seminfo_ semaphore info |
|
9 Jul 1998 |
The Solaris 2.X kernel comes configured with defaults for semaphore parameters
which may not be large enough for certain applications. Care is warranted in
altering them, as these changes will effect any program which makes semaphore
system service calls.
This document provides the default and maximum values for the semaphore
tunable parameters, and supplies a description of each of them. In most
cases, the listed maxima is the largest value which will fit in the data type
of the parameter; they are not intended as suggested settings. Specific
maxima can not be provided in a general document, as they are dependant upon
the hardware resources available on the system being tuned, and on the
application(s) making use of them.
Some parameters list more than one value in the 'Max' column. This indicates
that the maximum value for the tunable parameter does not match the maximum
value which is supported by other elements of the system. In these instances,
the first number is the limit imposed by the parameter data type, while the
second number is the limit imposed by the parameters usage; where supplied,
the usage value must be considered the upper limit for tuning. Unless
otherwise noted, setting a parameter to a value larger than the specified
usage limit will not return an error, but will result in indeterministic
behavior.
Name Default Max Brief Description
------ ------- -------------- -------------------------------------
semmap 10 2147483647 Number of entries in semaphore map
semmni 10 65535 Number of semaphore sets (identifiers)
semmns 60 2147483647 Number of semaphores in the system
65535 (usage)
semmnu 30 2147483647 Number of "undo" structures in the system
semmsl 25 2147483647 Max number of semaphores, per semaphore id
65535 (usage)
semopm 10 2147483647 Max number of operations, per semaphore call
semume 10 2147483647 Max number of "undo" entries, per process
semusz 96 *see below* Size in bytes of "undo" structure
semvmx 32767 2147483647 Semaphore maximum value
65535 (usage)
semaem 16384 2147483647 Adjust on exit maximum value
32767 (usage)
Detailed Descriptions
---------------------
semmap
Defines the size of the semaphore resource map; each block of available,
contiguous semaphores requires one entry in this map. This is the pool from
which semget(2) acquires semaphore sets.
When a semaphore set is removed (deleted), if the block of semaphores to be
freed is adjacent to a block of semaphores already in the resource map, the
semaphores in the set being removed are added to the existing map entry;
no new map entry is required. If the semaphores in the removed set are not
adjacent to those in an existing map entry, then a new map entry is required
to track these semaphores; if there are no more map entries available, the
system has to discard an entry, 'permanently' losing a block of semaphores
(permanence is relative; a reboot fixes the problem). If this should occur,
a WARNING will be generated, the text of which will be something like
"rmallocmap: rmap overflow, lost ...". The end result is that a user could
later get ENOSPC errors from semget(2) even though it doesn't look like all
the semaphores are allocated.
semmni
Defines the number of semaphore sets (identifiers), system wide. Every
semaphore set in the system has a unique indentifier and control structure.
The system pre-allocates kernel memory for semmni control structures; each
control structure is 84 bytes. If no more identifiers are available,
semget(2) returns ENOSPC.
Attempting to set semmni to a value greater than 65535 will result in
generation of a WARNING, and the value will be set to 65535.
semmns
Defines the number of semaphores in the system; 16 bytes of kernel memory is
pre-allocated for each semaphores. If there is not a large enough block of
contiguous semaphores in the resource map (see semmap) to satisfy the request,
semget(2) returns ENOSPC.
Fragmentation of the semaphore map will result in ENOSPC errors, even though
there may appear to be ample free semaphores. Despite attempts by the system
to merge free sets (see semmap), the size of the clusters of free semaphores
generally decreases over time. For this reason, semmns frequently must be set
higher than the actual number of semaphores required.
semmnu
Defines the number of semaphore undo structures in the system. semusz (see
below) bytes of kernel memory are pre-allocated for each undo structure; one
undo structure is required for every process for which undo information must
be recorded. semop() will return ENOSPC if it is requested to record undo
information and there are no undo structures available.
semmsl
Limits the number of semaphores that can be created for a single semaphore id.
If semget(2) returns EINVAL, this limit should be increased. This parameter
is only used to validate the argument passed to semget(2). Logically, it
should be less than or equal to semmns (see above). Setting semmsl too high
might allow a few identifiers to hog all the semaphores in the system.
semopm
Limits the number of operations that are allowed in a single semop(2) call.
If semop(2) returns E2BIG, this limit should be increased. This parameter is
only used to validate the argument passed to semop(2).
semume
Limits the number of undo records that can exist for a process. If semop(2)
returns EINVAL, this limit should be increased. In addition to its use in
validating arguments to semop(2), this parameter is used to calculate the
value of semusz (see below).
semusz
Defines the size of the semaphore undo structure. Any attempt to modify this
parameter directly will be ignored; semusz is always calculated based upon
the value of semume (see above); semusz = 8 * (semume + 2).
semvmx
Limits the maximum value of a semaphore. Due to the interaction with undo
structures and semaem (see below), this tuneable should not be increased
beyond its default value of 32767, unless you can guarantee that SEM_UNDO is
never and will never be used. It can be safely reduced, but doing so provides
no savings.
semaem
Limits the maximum value of an adjust-on-exit undo element. No system
resources are allocated based on this value.
Setting Semaphore Parameters
----------------------------
Semaphore parameters are changed by editing the /etc/system file and adding
lines of the form:
set semsys:seminfo_variable = value
where 'variable' is one of the entries from the name column, above. For
example, the entry:
set semsys:seminfo_semmap = 15
would increase the size of the semaphore resource map to 15.
After modifying the /etc/system file, the system must be rebooted in order to
implement the new values.
Checking Semaphore Parameters
-----------------------------
InterProcess Communication (IPC) parameters can be checked via the sysdef
command:
$ sysdef -i
Toward the bottom of the output, there will a section which should look
something like the following:
*
* IPC Semaphores
*
10 entries in semaphore map (SEMMAP)
10 semaphore identifiers (SEMMNI)
60 semaphores in system (SEMMNS)
30 undo structures in system (SEMMNU)
25 max semaphores per id (SEMMSL)
10 max operations per semop call (SEMOPM)
10 max undo entries per process (SEMUME)
32767 semaphore maximum value (SEMVMX)
16384 adjust on exit max value (SEMAEM)
The values at the left will reflect either the default value, or the value
specified in the /etc/system file, if that parameter has been modified.
If the values displayed are all zero, load the semaphore module:
# modload -p sys/semsys
and run sysdef -i again.
Kernel Memory Limitation
------------------------
In release 2.5 and higher, the semaphores structures are prevented from using
more than 1/4 of all kernel memory; if the specified tuning values would
surpass this limit, a WARNING is generated, and the semaphore module will not
be loaded.
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