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14.8. Huge Pages and Shared Memory File System in Red Hat Enterprise Linux 3
The following example shows that the Huge Pages pool is not being used by the
ramfs shared memory file systems. The ramfs shared memory file systems can be used for Configuring Very Large Memory (VLM).
The
ipcs command shows only System V shared memory segments. It does not display shared memory of a shared memory file systems. The following command shows System V shared memory segments on a node running a database with an SGA of 2.6 GB:
ipcs -m
# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x98ab8248 1081344 oracle 600 77594624 0
0xe2e331e4 1245185 oracle 600 2736783360 0
The first shared memory segment of 74 MB was created by the ASM instance. The second shared memory segment of 2.6 GB was created by the database instance.
On this database system the size of the database buffer cache is 2 GB:
db_block_buffers = 262144 db_block_size = 8192
db_block_buffers = 262144
db_block_size = 8192
The following command shows that Oracle allocated a shared memory file of 2GB (262144*8192=2147483648) for the buffer cache on the
ramfs shared memory file system:
mount | grep ramfs ls -al /dev/shm
# mount | grep ramfs
ramfs on /dev/shm type ramfs (rw)
# ls -al /dev/shm
total 204
drwxr-xr-x 1 oracle dba 0 Oct 30 16:00 .
drwxr-xr-x 22 root root 204800 Oct 30 16:00 ..
-rw-r----- 1 oracle dba 2147483648 Nov 1 16:46 ora_orcl1_1277954
The next command shows how many Huge Pages are currently being used on this system:
grep Huge /proc/meminfo
$ grep Huge /proc/meminfo
HugePages_Total: 1536
HugePages_Free: 194
Hugepagesize: 2048 kB
$
The output shows that 1342 (1536-194) Huge Pages are being used. This translates into 2814377984 (1342*2048*1024) bytes being allocated in the Huge Pages pool. This number matches the size of both shared memory segments (2736783360+77594624=2814377984) displayed by the
ipcs command above.
This shows that the Huge Pages pool is not being used for the
ramfs shared memory file system. Hence, you do not need to increase the Huge Pages pool if you use the ramfs shared memory file system.
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