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Don Burleson Blog

Using Linux Hugepages with Oracle

Oracle Database Tips by Donald BurlesonNovember 10, 2015

The Linux operating system has some features that are far superior to Windows, and Linux is now being used on larger servers, many of which are supporting Oracle databases.

Important note: Some experts DO NOT recommend using Automatic Shared Memory Management (AMM, e.g. setting memory_target) with Linux hugepages. See MOSC note 749851.1 "HugePages and Oracle Database 11g Automatic Memory Management (AMM) on Linux". Also see MOSC notes 361323.1 and 361468.1.

In sum, AMM is not compatible with Linux Hugepages. Also, beware that AMM re-size operations can cripple Oracle performance in some cases.

Everyone is aware of the issues with 32-bit servers running Linux, and the fact that 64-bit Linux allows large RAM regions that are not possible in 32-bit. However, there is one more little tweak that can be applied to either 32-bit or 64-bit Oracle servers and that is the use of HugePages.

There are couple of important benefits of HugePages have some very real benefits for Oracle systems.

First, the "real" page size increases to 2 meg, 50 times larger than the old 4k page sizes.
RAM allocated for Hugepages is fenced. Unlike standard RAM allocated to the Oracle SGA and PGA, Hugepages RAM is resident and it cannot be paged out, and it's ineligible for swapping.

This Linux 2.6 kernel feature simply utilizes hugepages to reduce virtual memory I/O operations when working with lots of memory. Here are some documented limits for hugepages:

Hardware Platform	Kernel 2.4	Kernel 2.6
Linux x86 (IA32)	4MB	4MB
Linux x86-64 (AMD64, EM64T)	2MB	2MB
Linux Itanium (IA64)	256MB	256MB
IBM Power Based Linux (PPC64)	NA	16MB
IBM zSeries Based Linux	NA	NA
IBM S/390 Based Linux	NA	NA

The process to enable huge pages is as follows:

X = grep Hugepagesize /proc/meminfo
Y = Largest (MB of all client SGA's) * 1024
Z = # Huge Pages needed = Y / X
Set Huge Page Pool size

edit /etc/sysctl.con
vm.nr_hugepages = Z

Increase ulimit parameter memlock for oracle user
- edit /etc/security/limits.conf
- oracle soft memlock Y
- oracle hard memlock Y
reboot

Many people with huge 64-bit Oracle servers increase their SGA size without implementing HugePages. The results have been well documented (see Oracle's MOSC document id = 361670.1) where SGA sizes greater than 10GB have displayed decreases in performance. So as a general practice, always implement Huge Pages.

Riyaj Shamsudeen notes these steps for using hugepages in Oracle:

To setup HugePages, the following changes must be completed:
Set the vm.nr_hugepages kernel parameter to a suitable value. In this case, we decided to use 12GB and set the parameter to 6144 (6144*2M=12GB). You can run:
echo 6144 > /proc/sys/vm/nr_hugepages
or
sysctl -w vm.nr_hugepages=6144
Of course, you must make sure this set across reboots too.
The oracle userid needs to be able to lock a greater amount of memory. So, /etc/securities/limits.conf must be updated to increase soft and hard memlock values for oracle userid.
oracle          soft    memlock        12582912
oracle          hard   memlock        12582912
After setting this up, we need to make sure that SGA is indeed using HugePages. The value, (HugePages_Total- HugePages_Free)*2MB will be the approximate size of SGA (or it will equal the shared memory segment shown in the output of ipcs -ma).
cat /proc/meminfo |grep HugePages
HugePages_Total:  6144
HugePages_Free:   1655 <-- Free pages are less than total pages.
Hugepagesize:     2048 kB

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