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Operating Systems Linux Red Hat Does it make sense to reduce the total shared memory Post 302992198 by gandolf989 on Wednesday 22nd of February 2017 03:19:09 PM
Old 02-22-2017
Does it make sense to reduce the total shared memory

We have several dozen Redhat 5, 6 and 7 servers that are running Oracle databases. On some databases we are using automatic memory management, which uses shared memory. On other databases we are use manual memory management, which does not use shared memory.

When I see that a server is swapping and not using that much shared memory, does it make sense to shrink the amount of shared memory allocated to reduce the swap usage? I have read that setting /dev/shm does not allocate memory. But I wonder if it is still preventing non shared memory from staying in physical memory.

Thanks.

Code:
$ ~ > df -h /dev/shm
Filesystem            Size  Used Avail Use% Mounted on
tmpfs                  26G  5.4G   20G  22% /dev/shm
$ ~ > free -m
             total       used       free     shared    buffers     cached
Mem:         32186      27556       4629          0         30      23724
-/+ buffers/cache:       3801      28384
Swap:        20474       4375      16098

 

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shmget(2)							System Calls Manual							 shmget(2)

Name
       shmget - get shared memory segment

Syntax
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/shm.h>

       int shmget (key, size, shmflg)
       key_t key;
       int size, shmflg;

Description
       The system call returns the shared memory identifier associated with key.

       A  shared  memory  identifier and associated data structure and shared memory segment of size size bytes are created for key, if one of the
       following is true:

	    The key is equal to IPC_PRIVATE.  For further information, see

	    The key does not already have a shared memory identifier associated with it, and (shmflg & IPC_CREAT ) is true.

       Upon creation, the data structure associated with the new shared memory identifier is initialized as follows:

	    The and are set equal to the effective user ID and effective group ID of the calling process.

	    The low-order nine bits of are set equal to the low-order nine bits of shmflg.  The is set equal to the value of size.

	    The and are set equal to zero (0).	The shm_ctime is set equal to the current time.

Return Values
       Upon successful completion, a non-negative integer, namely, a shared memory identifier is returned.  Otherwise, a value of -1  is  returned
       and errno is set to indicated the error.

Diagnostics
       The system call fails if any of the following is true:

       [EINVAL]       The size is less than the system-imposed minimum or greater than the system-imposed maximum.

       [EACCES]       A  shared  memory  identifier  exists for key, but operations permission, as specified by the low-order nine bits of shmflg,
		      would not be granted.  For further information, see

       [EINVAL]       A shared memory identifier exists for key, but the size of the segment associated with it is less than size and size is  not
		      equal to zero.

       [ENOENT]       A shared memory identifier does not exist for key, and (shmflg & IPC_CREAT ) is false.

       [ENOSPC]       A  shared  memory  identifier  is to be created, but the system-imposed limit on the maximum number of allowed shared memory
		      identifiers would be exceeded.

       [ENOMEM]       A shared memory identifier and the associated shared memory segment are to be created, but the amount of available  physical
		      memory is not sufficient to fill the request.

       [EEXIST]       A shared memory identifier exists for key, but ((shmflg & IPC_CREAT ) and (shmflg & IPC_EXCL )) is true.

See Also
       shmctl(2), shmop(2), ftok(3)

																	 shmget(2)
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