Configuring HugePages on Linux

HugePages is a feature integrated into the Linux kernel. For Oracle Database, using HugePages reduces the operating system maintenance of page states and increases Translation Lookaside Buffer (TLB) hit ratio.

Note:

HugePages support is available in kernels built with the CONFIG_HUGETLBFS and CONFIG_HUGETLB_PAGE configuration options.

Oracle recommends that you reserve a minimum of 30 percent of the total memory for standard pages and not more than 70 percent of the total memory for HugePages.

Ensure that you have created the Oracle Database software owner (oracle) user accounts and all its groups. For more information see, Automatically Configuring Oracle Linux with Oracle Database Preinstallation RPM or Creating an Oracle Software Owner User.

Enabling Using HugePages

Perform these steps to enable HugePages on Linux:

Check if the kernel supports HugePages:
```
$ grep Huge /proc/meminfo
```
Edit the memlock setting in the /etc/security/limits.conf file.

Specify the memlock setting in KB. Set the maximum locked memory limit to at least 90 percent of the current RAM when you enable HugePages memory and at least to 3145728 KB (3 GB) if you disable HugePages. For example, if you have 64 GB RAM, then add the following entries to increase the maximum locked-in-memory address space:
```
*   soft   memlock    60397977
*   hard   memlock    60397977
```
You can also set the memlock value higher than your SGA requirements.
Log in as the oracle user again and run the ulimit -l command to verify the new memlock setting:
```
$ ulimit -l
60397977
```
Run the following command to display the value of the Hugepagesize variable:
```
$ grep Hugepagesize /proc/meminfo
```

Determine the gid or oracle user group ID value:

# id oracle

The output is similar to the following:

uid=54321(oracle) gid=54321(oinstall) groups=54321(oinstall),54322(dba), 
54323(oper),54324(backupdba),54325(dgdba),54326(kmdba),54327(asmdba),54330(racdba)

Note the value of gid.

Set the vm.hugetlb_shm_group kernel parameter value to the gid value:
```
# sysctl -w vm.hugetlb_shm_group=gid
```
where gid is the oracle user group ID. For example, value of gid=54321.

To ensure that this value is available even after the system restarts, add the following entry to the /etc/sysctl.d/99-hugetlb-shm-group.conf file that contains this setting.
vm.hugetlb_shm_group=gid.

Configuring the Capacity for HugePages

To configure the required number of HugePages for your databases, perform this step after you complete the Oracle Database installation and create your databases.

Complete the following procedure to create a script that computes recommended values for HugePages configuration for the current shared memory segments:
1. Create a text file hugepages_settings.sh
  
  See My Oracle Support note 401749.1 to create the hugepages_settings.sh script.
2. Change the permission of the hugepages_settings.sh file:
```
$ chmod +x hugepages_settings.sh
```
Run the hugepages_settings.sh script to compute values for the HugePages configuration.
```
$ ./hugepages_settings.sh
```
Note:
Before you run this script, ensure that all the databases that use hugepages run.
Set the following kernel parameter, where value is the HugePages value that you determined in Step 2:
```
# sysctl -w vm.nr_hugepages=value
```
To ensure that HugePages is allocated after the system restarts, add the following entry to the /etc/sysctl.d/99-nr_hugepages.conf file, where value is the HugePages value that you determined in Step 2:
```
vm.nr_hugepages=value
```
Note:
If you cannot set your HugePages allocation using nr_hugepages, then your available memory may be fragmented. Restart your server for the Hugepages allocation to take effect.
Run the following command to check the available HugePages:
```
$ grep Huge /proc/meminfo
```
Restart the instances.
Run the following command to check the available HugePages (1 or 2 pages free):
```
$ grep Huge /proc/meminfo
```

Parent topic: Guidelines for Linux Operating System Installation