Linux 关于Transparent Hugepages的介绍【转】

透明大页介绍

TransparentHugePages的一些官方介绍资料：

TransparentHugePages(THP)areenabledbydefaultinRHEL6forallapplications.ThekernelattemptstoallocatehugepageswheneverpossibleandanyLinuxprocesswillreceive2MBpagesifthemmapregionis2MBnaturallyaligned.Themainkerneladdressspaceitselfismappedwithhugepages,reducingTLBpressurefromkernelcode.ForgeneralinformationonHugepages,see:WhatareHugePagesandwhataretheadvantagesofusingthem?

Thekernelwillalwaysattempttosatisfyamemoryallocationusinghugepages.Ifnohugepagesareavailable(duetononavailabilityofphysicallycontinuousmemoryforexample)thekernelwillfallbacktotheregular4KBpages.THParealsoswappable(unlikehugetlbfs).Thisisachievedbybreakingthehugepagetosmaller4KBpages,whicharethenswappedoutnormally.

Buttousehugepageseffectively,thekernelmustfindphysicallycontinuousareasofmemorybigenoughtosatisfytherequest,andalsoproperlyaligned.Forthis,akhugepagedkernelthreadhasbeenadded.Thisthreadwilloccasionallyattempttosubstitutesmallerpagesbeingusedcurrentlywithahugepageallocation,thusmaximizingTHPusage.

Inuserland,nomodificationstotheapplicationsarenecessary(hencetransparent).Buttherearewaystooptimizeitsuse.Forapplicationsthatwanttousehugepages,useofposix_memalign()canalsohelpensurethatlargeallocationsarealignedtohugepage(2MB)boundaries.

Also,THPisonlyenabledforanonymousmemoryregions.Thereareplanstoaddsupportfortmpfsandpagecache.THPtunablesarefoundinthe/systreeunder/sys/kernel/mm/redhat_transparent_hugepage.

查看是否启用透明大页

1：命令cat/sys/kernel/mm/redhat_transparent_hugepage/enabled该命令适用于RedHatEnterpriseLinux系统

[code]
[code][root@getlnx06~]#more/etc/issue

RedHatEnterpriseLinuxServerrelease6.6(Santiago)

Kernel\ronan\m

[root@getlnx06~]#cat/sys/kernel/mm/redhat_transparent_hugepage/enabled

[always]madvisenever

[/code]
[/code]

2：命令cat/sys/kernel/mm/transparent_hugepage/enabled该命令适用于其它Linux系统

[code]
[code][root@getlnx06~]#cat/sys/kernel/mm/transparent_hugepage/enabled

alwaysmadvise[never]

[root@getlnx06~]#

[/code]
[/code]
使用命令查看时，如果输出结果为[always]表示透明大页启用了。[never]表示透明大页禁用、[madvise]表示

3：如何HugePages_Total返回0，也意味着透明大页禁用了

[code]
[code][root@getlnx06~]#grep-iHugePages_Total/proc/meminfo

HugePages_Total:0

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[/code]

4：cat/proc/sys/vm/nr_hugepages返回0也意味着透明大页禁用了。

[code]
[code][root@getlnx06~]#cat/proc/sys/vm/nr_hugepages

0

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[/code]

禁用、启用透明大页功能

方法1：设置/etc/grub.conf文件，在系统启动是禁用。

[code]
[code][root@getlnx06~]#vi/etc/grub.conf

#grub.confgeneratedbyanaconda

#

#Notethatyoudonothavetorerungrubaftermakingchangestothisfile

#NOTICE:Youhavea/bootpartition.Thismeansthat

#allkernelandinitrdpathsarerelativeto/boot/,eg.

#root(hd0,0)

#kernel/vmlinuz-versionroroot=/dev/mapper/VolGroup--LogVol0-LogVol01

#initrd/initrd-[generic-]version.img

#boot=/dev/sda

default=0

timeout=5

splashimage=(hd0,0)/grub/splash.xpm.gz

hiddenmenu

titleRedHatEnterpriseLinux6(2.6.32-504.el6.x86_64)

root(hd0,0)

kernel/vmlinuz-2.6.32-504.el6.x86_64roroot=/dev/mapper/VolGroup--LogVol0-LogVol01rd_NO_LUKSLANG=en_US.UTF-8rd_NO_MDSYSFONT=latarcyrheb-sun16crashkernel=autord_LVM_LV=VolGroup-LogVol0/LogVol01rd_LVM_LV=VolGroup-LogVol0/LogVol00KEYBOARDTYPE=pcKEYTABLE=usrd_NO_DMrhgbquiet

initrd/initramfs-2.6.32-504.el6.x86_64.img

transparent_hugepage=never

[/code]
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方法2：设置/etc/rc.local文件

[code]
[code][root@getlnx06~]#vi/etc/rc.local

#!/bin/sh

#

#Thisscriptwillbeexecuted*after*alltheotherinitscripts.

#Youcanputyourowninitializationstuffinhereifyoudon't

#wanttodothefullSysVstyleinitstuff.

touch/var/lock/subsys/local

iftest-f/sys/kernel/mm/redhat_transparent_hugepage/enabled;then

echonever>/sys/kernel/mm/redhat_transparent_hugepage/enabled

fi

[/code]
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使用上面的配置后必须重启操作系统才能生效，你也可以运行下面命令不用重启操作系统。

Youmustrebootyoursystemforthesettingtotakeeffect,orrunthefollowingtwoecholinestoproceedwiththeinstallwithoutrebooting:

[code]
[code][root@getlnx06~]#echonever>/sys/kernel/mm/redhat_transparent_hugepage/enabled

[root@getlnx06~]#cat/sys/kernel/mm/redhat_transparent_hugepage/enabled

alwaysmadvise[never]

[root@getlnx06~]#

[/code]
[/code]

小知识点：

1：从RedHat6,OEL6,SLES11andUEK2kernels开始，系统缺省会启用TransparentHugePages：用来提高内存管理的性能透明大页（TransparentHugePages）和之前版本中的大页功能上类似。主要的区别是：TransparentHugePages可以实时配置，不需要重启才能生效配置；

2：TransparentHugePages在32位的RHEL6中是不支持的。

TransparentHugePagesarenotavailableonthe32-bitversionofRHEL6.

3：ORACLE官方不建议我们使用RedHat6,OEL6,SLES11andUEK2kernels时的开启透明大页（TransparentHugePages），因为透明大页（TransparentHugePages）存在一些问题：

1.在RAC环境下透明大页（TransparentHugePages）会导致异常节点重启，和性能问题；

2.在单机环境中，透明大页（TransparentHugePages）也会导致一些异常的性能问题；

TransparentHugePagesmemoryisenabledbydefaultwithRedHatEnterpriseLinux6,SUSELinuxEnterpriseServer11,andOracleLinux6withearlierreleasesofOracleLinuxUnbreakableEnterpriseKernel2(UEK2)kernels.TransparentHugePagesmemoryisdisabledinlaterreleasesofOracleLinuxUEK2kernels.TransparentHugePagescancausememoryallocationdelaysduringruntime.Toavoidperformanceissues,OraclerecommendsthatyoudisableTransparentHugePagesonallOracleDatabaseservers.OraclerecommendsthatyouinsteadusestandardHugePagesforenhancedperformance.TransparentHugePagesmemorydiffersfromstandardHugePagesmemorybecausethekernelkhugepagedthreadallocatesmemorydynamicallyduringruntime.StandardHugePagesmemoryispre-allocatedatstartup,anddoesnotchangeduringruntime.

StartingwithRedHat6,OEL6,SLES11andUEK2kernels,TransparentHugePagesareimplementedandenabled(default)inanattempttoimprovethememorymanagement.TransparentHugePagesaresimilartotheHugePagesthathavebeenavailableinpreviousLinuxreleases.ThemaindifferenceisthattheTransparentHugePagesaresetupdynamicallyatruntimebythekhugepagedthreadinkernelwhiletheregularHugePageshadtobepreallocatedatthebootuptime.BecauseTransparentHugePagesareknowntocauseunexpectednoderebootsandperformanceproblemswithRAC,OraclestronglyadvisestodisabletheuseofTransparentHugePages.Inaddition,TransparentHugepagesmaycauseproblemseveninasingle-instancedatabaseenvironmentwithunexpectedperformanceproblemsordelays.Assuch,OraclerecommendsdisablingTransparentHugePagesonallDatabaseserversrunningOracle.

4：安装VerticaAnalyticDatabase时也必须关闭透明大页功能。