Red Hat ENTERPRISE LINUX 5.1 - LINUX ORACLE Tuning Manual page 50

Chapter 14. Large Memory Optimization, Big Pages, and Huge Pages
In Red Hat Enterprise Linux 3 the desired size of the Huge Pages pool is specified in megabytes. The
size of the pool should be configured by the incremental size of the Huge Page size. To obtain the size
of Huge Pages, execute the following command:
$ grep Hugepagesize /proc/meminfo
Hugepagesize: 2048 kB
$
The number of Huge Pages can be configured and activated by setting hugetlb_pool in the proc
file system. For example, to allocate a 1GB Huge Page pool, execute:
# echo 1024 > /proc/sys/vm/hugetlb_pool
Alternatively, you can use sysctl(8) to change it:
# sysctl -w vm.hugetlb_pool=1024
To make the change permanent, add the following line to the file /etc/sysctl.conf. This file is
used during the boot process. The Huge Pages pool is usually guaranteed if requested at boot time:
# echo "vm.hugetlb_pool=1024" >> /etc/sysctl.conf
If you allocate a large number of Huge Pages, the execution of the above commands can take a while.
To verify whether the kernel was able to allocate the requested number of Huge Pages, execute:
$ grep HugePages_Total /proc/meminfo
HugePages_Total: 512
$
The output shows that 512 Huge Pages have been allocated. Since the size of Huge Pages on this
system is 2048 KB, a Huge Page pool of 1GB has been allocated and pinned in physical memory.
If HugePages_Total is lower than what was requested with hugetlb_pool, then the system does
either not have enough memory or there are not enough physically contiguous free pages. In the latter
case the system needs to be rebooted which should give you a better chance of getting the memory.
To get the number of free Huge Pages on the system, execute:
$ grep HugePages_Free /proc/meminfo
Free system memory will automatically be decreased by the size of the Huge Pages pool allocation
regardless whether the pool is being used by an application like Oracle database or not being used:
$ grep MemFree /proc/meminfo
After an Oracle database starts up, verify the Huge Pages usage. The number of free Huge Pages
should decrease.
$ grep HugePages_Free /proc/meminfo
To free the Huge Pages pool, you can execute:
# echo 0 > /proc/sys/vm/hugetlb_pool
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