一、概述
HugePages是通過使用大頁內存來取代傳統的4kb內存頁面，使得管理虛擬地址數變少，加快了從虛擬地址到物理地址的映射以及通過摒棄內存頁面的換入換出以提高內存的整體性能。尤其是對於8GB以上的內存以及較大的Oracle SGA size，建議配值並使用HugePage特性。同時， hugepage是作為一個優化項，而不是必須設置項。如果系統性能穩定無異常，則大頁內存不是必須設置的。
二、實施過程
注：由於各系統的參數設置各異，所以以下只描述整體實施步驟和方法，具體參數在實施操作時根據系統參數情況再做具體安排。
1.查看當前系統是否配值HugePages
下面的查詢中HugePages相關的幾個值都為0，表明當前未配值HugePages，其次可以看到Hugepagesize為2MB。
$ grep Huge /proc/meminfo
HugePages_Total: 0
HugePages_Free: 0
HugePages_Rsvd: 0
Hugepagesize: 2048 kB
2.修改用戶的memlock限制
通過修改/etc/security/limits.conf 配值文件來實現
該參數的值通常配值位略小於當前的已安裝系統內存，如當前你的系統內存為64GB，可以做如下設置
* soft memlock 60397977
* hard memlock 60397977
上述的設置單位為kb，不會降低系統性能。至少也要配值為略大於系統上所有SGA的總和。
使用ulimit -l 來校驗該設置
3.禁用AMM(Oracle 11g)
如果當前的Oracle 版本為10g，可以跳過此步驟。
如果當前的Oracle 版本為11g，由於AMM(Automatic Memory Management)特性與Hugepages不兼容，需要禁用AMM。
SQL>ALTER SYSTEM RESET memory_target SCOPE=SPFILE;
SQL>ALTER SYSTEM RESET memory_max_target SCOPE=SPFILE;
SQL>ALTER SYSTEM SET sga_target=<n>g SCOPE=SPFILE;
SQL>ALTER SYSTEM SET pga_aggregate_target=<n>g SCOPE=SPFILE;
SQL>SHUTDOWN IMMEDIATE;
SQL>STARTUP;
4.計算vm.nr_hugepages 的值
使用Oracle 提供的腳本hugepages_settings.sh的腳本來計算vm.nr_hugepages的值（腳本見附錄）
在執行腳本之前確保所有的Oracle 實例已啟動以及ASM也啟動(存在的情形下)
$ ./hugepages_settings.sh
...
Recommended setting: vm.nr_hugepages = 1496
5.編輯/etc/sysctl.conf 來設置vm.nr_hugepages參數
$ sysctl -w vm.nr_hugepages = 1496
$ sysctl -p
6.停止所有的Instance並重啟server
上述的所有步驟已經實現了動態修改，但對於HugePages的分配需要重新啟動server才能生效。
7.驗證配值
HugePages相關參數的值會隨着當前服務器上的實例的停止與啟動而動態發生變化
通常情況下，HugePages_Free的值應當小於HugePages_Total的值，在HugePages被使用時HugePages_Rsvd值應當為非零值。
$ grep Huge /proc/meminfo
HugePages_Total: 131
HugePages_Free: 20
HugePages_Rsvd: 20
Hugepagesize: 2048 kB
如下面的情形，當服務器上僅有的一個實例被關閉后，HugePages_Rsvd的值為零。且HugePages_Free等於HugePages_Total
$ grep Huge /proc/meminfo
HugePages_Total: 131
HugePages_Free: 131
HugePages_Rsvd: 0
Hugepagesize: 2048 kB
三、使用大頁內存注意事項
下面的三種情形應當重新配置HugePages
a、物理內存的增減或減少
b、在當前服務器上新增或移出Instance
c、Instance的SGA大小增加或減少
如果未能調整HugePages，可能會引發下面的問題
a、數據庫性能地下
b、出現內存不足或者過度使用交換空間
c、數據庫實例不能被啟動
d、關鍵性系統服務故障
四、回退方案
取消大頁內存，恢復AMM內存管理機制即可。
四、附錄
計算vm.nr_hugepages 值的腳本：hugepages_settings.sh
#!/bin/bash
#
# hugepages_settings.sh
#
# Linux bash script to compute values for the
# recommended HugePages/HugeTLB configuration
#
# Note: This script does calculation for all shared memory
# segments available when the script is run, no matter it
# is an Oracle RDBMS shared memory segment or not.
#
# This script is provided by Doc ID 401749.1 from My Oracle Support
# http://support.oracle.com
# Welcome text
echo "
This script is provided by Doc ID 401749.1 from My Oracle Support
(http://support.oracle.com) where it is intended to compute values for
the recommended HugePages/HugeTLB configuration for the current shared
memory segments. Before proceeding with the execution please note following:
* For ASM instance, it needs to configure ASMM instead of AMM.
* The 'pga_aggregate_target' is outside the SGA and
you should accommodate this while calculating SGA size.
* In case you changes the DB SGA size,
as the new SGA will not fit in the previous HugePages configuration,
it had better disable the whole HugePages,
start the DB with new SGA size and run the script again.
And make sure that:
* Oracle Database instance(s) are up and running
* Oracle Database 11g Automatic Memory Management (AMM) is not setup
(See Doc ID 749851.1)
* The shared memory segments can be listed by command:
# ipcs -m
Press Enter to proceed..."
read
# Check for the kernel version
KERN=`uname -r | awk -F. '{ printf("%d.%d\n",$1,$2); }'`
# Find out the HugePage size
HPG_SZ=`grep Hugepagesize /proc/meminfo | awk '{print $2}'`
if [ -z "$HPG_SZ" ];then
echo "The hugepages may not be supported in the system where the script is being executed."
exit 1
fi
# Initialize the counter
NUM_PG=0
# Cumulative number of pages required to handle the running shared memory segments
for SEG_BYTES in `ipcs -m | cut -c44-300 | awk '{print $1}' | grep "[0-9][0-9]*"`
do
MIN_PG=`echo "$SEG_BYTES/($HPG_SZ*1024)" | bc -q`
if [ $MIN_PG -gt 0 ]; then
NUM_PG=`echo "$NUM_PG+$MIN_PG+1" | bc -q`
fi
done
RES_BYTES=`echo "$NUM_PG * $HPG_SZ * 1024" | bc -q`
# An SGA less than 100MB does not make sense
# Bail out if that is the case
if [ $RES_BYTES -lt 100000000 ]; then
echo "***********"
echo "** ERROR **"
echo "***********"
echo "Sorry! There are not enough total of shared memory segments allocated for
HugePages configuration. HugePages can only be used for shared memory segments
that you can list by command:
# ipcs -m
of a size that can match an Oracle Database SGA. Please make sure that:
* Oracle Database instance is up and running
* Oracle Database 11g Automatic Memory Management (AMM) is not configured"
exit 1
fi
# Finish with results
case $KERN in
'2.4') HUGETLB_POOL=`echo "$NUM_PG*$HPG_SZ/1024" | bc -q`;
echo "Recommended setting: vm.hugetlb_pool = $HUGETLB_POOL" ;;
'2.6') echo "Recommended setting: vm.nr_hugepages = $NUM_PG" ;;
*) echo "Unrecognized kernel version $KERN. Exiting." ;;
esac
# End