10-25-2009
about shared memory space in Linux !
Dear Friends ,
I am confused about shared memory in Linux . plz look @ the below output :
[oracle@report ~]$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda1 126G 61G 59G 52% /
tmpfs 1.7G 0 1.7G 0% /dev/shm
Here , I see that every time Redhat Linux occupied some space (here 1.7 GB) for shared memory . Would u plz tell me , why this shared memory is used .
1) Is this area only for oracle instance or others ?
2) If I dont need this space , can I manually remove this space ?
3) Is this shared memory resides only Linux o/s or other also other unix o/s version ?
Plz explain ... ...
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LEARN ABOUT CENTOS
shm_overview
SHM_OVERVIEW(7) Linux Programmer's Manual SHM_OVERVIEW(7)
NAME
shm_overview - overview of POSIX shared memory
DESCRIPTION
The POSIX shared memory API allows processes to communicate information by sharing a region of memory.
The interfaces employed in the API are:
shm_open(3) Create and open a new object, or open an existing object. This is analogous to open(2). The call returns a file descriptor
for use by the other interfaces listed below.
ftruncate(2) Set the size of the shared memory object. (A newly created shared memory object has a length of zero.)
mmap(2) Map the shared memory object into the virtual address space of the calling process.
munmap(2) Unmap the shared memory object from the virtual address space of the calling process.
shm_unlink(3) Remove a shared memory object name.
close(2) Close the file descriptor allocated by shm_open(3) when it is no longer needed.
fstat(2) Obtain a stat structure that describes the shared memory object. Among the information returned by this call are the
object's size (st_size), permissions (st_mode), owner (st_uid), and group (st_gid).
fchown(2) To change the ownership of a shared memory object.
fchmod(2) To change the permissions of a shared memory object.
Versions
POSIX shared memory is supported since Linux 2.4 and glibc 2.2.
Persistence
POSIX shared memory objects have kernel persistence: a shared memory object will exist until the system is shut down, or until all pro-
cesses have unmapped the object and it has been deleted with shm_unlink(3)
Linking
Programs using the POSIX shared memory API must be compiled with cc -lrt to link against the real-time library, librt.
Accessing shared memory objects via the file system
On Linux, shared memory objects are created in a (tmpfs) virtual file system, normally mounted under /dev/shm. Since kernel 2.6.19, Linux
supports the use of access control lists (ACLs) to control the permissions of objects in the virtual file system.
CONFORMING TO
POSIX.1-2001.
NOTES
Typically, processes must synchronize their access to a shared memory object, using, for example, POSIX semaphores.
System V shared memory (shmget(2), shmop(2), etc.) is an older shared memory API. POSIX shared memory provides a simpler, and better
designed interface; on the other hand POSIX shared memory is somewhat less widely available (especially on older systems) than System V
shared memory.
SEE ALSO
fchmod(2), fchown(2), fstat(2), ftruncate(2), mmap(2), mprotect(2), munmap(2), shmget(2), shmop(2), shm_open(3), shm_unlink(3), sem_over-
view(7)
COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-10 SHM_OVERVIEW(7)