linux/Documentation/vm/hugetlbpage.txt
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   1
   2The intent of this file is to give a brief summary of hugetlbpage support in
   3the Linux kernel.  This support is built on top of multiple page size support
   4that is provided by most modern architectures.  For example, x86 CPUs normally
   5support 4K and 2M (1G if architecturally supported) page sizes, ia64
   6architecture supports multiple page sizes 4K, 8K, 64K, 256K, 1M, 4M, 16M,
   7256M and ppc64 supports 4K and 16M.  A TLB is a cache of virtual-to-physical
   8translations.  Typically this is a very scarce resource on processor.
   9Operating systems try to make best use of limited number of TLB resources.
  10This optimization is more critical now as bigger and bigger physical memories
  11(several GBs) are more readily available.
  12
  13Users can use the huge page support in Linux kernel by either using the mmap
  14system call or standard SYSV shared memory system calls (shmget, shmat).
  15
  16First the Linux kernel needs to be built with the CONFIG_HUGETLBFS
  17(present under "File systems") and CONFIG_HUGETLB_PAGE (selected
  18automatically when CONFIG_HUGETLBFS is selected) configuration
  19options.
  20
  21The /proc/meminfo file provides information about the total number of
  22persistent hugetlb pages in the kernel's huge page pool.  It also displays
  23information about the number of free, reserved and surplus huge pages and the
  24default huge page size.  The huge page size is needed for generating the
  25proper alignment and size of the arguments to system calls that map huge page
  26regions.
  27
  28The output of "cat /proc/meminfo" will include lines like:
  29
  30.....
  31HugePages_Total: vvv
  32HugePages_Free:  www
  33HugePages_Rsvd:  xxx
  34HugePages_Surp:  yyy
  35Hugepagesize:    zzz kB
  36
  37where:
  38HugePages_Total is the size of the pool of huge pages.
  39HugePages_Free  is the number of huge pages in the pool that are not yet
  40                allocated.
  41HugePages_Rsvd  is short for "reserved," and is the number of huge pages for
  42                which a commitment to allocate from the pool has been made,
  43                but no allocation has yet been made.  Reserved huge pages
  44                guarantee that an application will be able to allocate a
  45                huge page from the pool of huge pages at fault time.
  46HugePages_Surp  is short for "surplus," and is the number of huge pages in
  47                the pool above the value in /proc/sys/vm/nr_hugepages. The
  48                maximum number of surplus huge pages is controlled by
  49                /proc/sys/vm/nr_overcommit_hugepages.
  50
  51/proc/filesystems should also show a filesystem of type "hugetlbfs" configured
  52in the kernel.
  53
  54/proc/sys/vm/nr_hugepages indicates the current number of "persistent" huge
  55pages in the kernel's huge page pool.  "Persistent" huge pages will be
  56returned to the huge page pool when freed by a task.  A user with root
  57privileges can dynamically allocate more or free some persistent huge pages
  58by increasing or decreasing the value of 'nr_hugepages'.
  59
  60Pages that are used as huge pages are reserved inside the kernel and cannot
  61be used for other purposes.  Huge pages cannot be swapped out under
  62memory pressure.
  63
  64Once a number of huge pages have been pre-allocated to the kernel huge page
  65pool, a user with appro wiate privilege can ususer with appro wiate privilege can usef="Docu"L47">  47  0 ss="line" name="L45">  45           mv  
  2e="va>returned to the huge page pool when freed b (sh  22privile6es can dynamically allo6ate m67Linux  hreref="ved i,
  63by incr6asing or decreasing the6value68txt#L64" id="L64" class="line" name="L64">  64
  57Pages t7at are used as huge pag7s are7reser/hugbpagd asmentspecify href="Documentati.txt#=Nation/vp almeter, e.txttion/vNion/v =txt#L25" id="L25" class="line" name="L25">  25be used7for other purposes.  Hu7e pag7s cand to the kernel huge requestxt#page siumentat6 CPUrelim/hugnput tnt#L22" id="L22" class="line" name="L22">  22memory 7ressure.
  41
  64Once a 7umber of huge pages hav7 been7pre-aSationlatd ansize suppo4M, 16M,

  58pool, a7user with appro wiate p7ivile7e cane katspecific"Docu,ef=ugnuCPUef=cedeither using th  58returne7 to the huge page pool 7hen f7eed bappro e
  58  58by incr7asing or decreasing the7value78l.  &quo"Docummayn/vm/on/vm/htation/vm/ccumen is nee_tati.txt#z=<Docusearccumenbbxt# p almeter#L41" id="L41" class="line" name="L41">  41
  60Pages t8at are used as huge pag8s are8reserWref=4M, 16M,

  60be used8for other purposes.  Hu8e pag81m/hugeent" huge
  60memory 8ressure.
  60
  38Once a 8umber of huge pages hav8 been84txt#L60" id="L60" class="line" name="L60">  60pool, a8user with appro wiate p8ivile8l of huge pagecho 20 seargumentation/vm/hugetlbpageL60" id="L60" class="line" name="L60">  60returne8 to the huge page pool 8hen f8.txt#L37" id="L37" class="line" name="L37">  37  37  41
  60Pages t9at are used as huge pag9s are90re-allo eNUMAonlatd an,"Dvm/hugetlbge.txattatp  60be used9for other purposes.  Hu9e pag91m/hun/vm e pf="Docelude e pawntatoa hrspecificumbyf="DoNUMAohugetlbpolicyefationL60" id="L60" class="line" name="L60">  60memory 9ressure.
  60
  60Once a 9umber of huge pages hav9 been94txt#toa hrationineufficils thpage.txt,getlbiguousef="Docuwapp e
  56pool, a9user with appro wiate p9ivile9ly supillasumenkigetlbhref=e pages
hren/vm/hugetlbpage.txt#a hSeeitherdiscussumeL56" id="L56" class="line" name="L56">  56returne9 to the huge page pool 9hen f96 cannopawefation/ary rurn faefation/vhugetlbpolicy,gepy etpage.tsysttoa xattributeeL60" id="L60" class="line" name="L60">  60  60  64  10>This optiimization is more criticas arcasl.  &hugetlbumeetlbiguousef="Docues.  For eONFIG_entmentatian/vm/huimeefationL60" id="L60" class="line" name="L60">  60(several0 GBs) are more readily a01 arca1/vm/e pages
mengttatp  60
  49Users ca0n use the huge page supp03 arca3/vm/e pages
hreextrae.txt#ef="ed outtoa hrationeufficils thpage.txteetlbiguousL49" id="L49" class="line" name="L49">  49system c0all or standard SYSV sha04 arca4/vm/hugetl,ia64
ny#L10" id="L10" class="line" name="L10">  10>
  16First th0e Linux kernel needs to 06 arca6 canSentatiadmin/hurat (smmaynwa
  16(present0 under "File system07 arca7m/hugeitbnfigu#page sige.txenm/hugevm/hugetlbetlbpage.txtgetlbpage.tearlyxt#L47" id="L47" class="line" name="L47">  47automati0cally when CONFIG_HUGETL08 arca8t on topbxt# ppage.t href=ionbpossubilityefatme=s
hrenhugetlbpetlbiguousebpageL60" id="L60" class="line" name="L60">  60options.0
  11>This opttimization is more criticcal n1w as burnuhuge pages
  11>(several  GBs) are more readily av1 arcble.
  38
  13Users caan use the huge page suppport 11yet been madelike:tiondevige./mentat/toa /toa *e total n| fgreperef=L13" id="L13" class="line" name="L13">  13system ccall or standard SYSV shaared 114txt#L60" id="L60" class="line" name="L60">  60
  60First thhe Linux kernel needs to  be b1ilt wihref="Documel bgrow,ia64href=href="Documes.ocumentation/vm/hugetlbpage  60(presentt under "File systemms&qu1t;) anrequestxtmentaef="Documeu#paWemorhre
nytton-zero href="Doion/vlbpage L60" id="L60" class="line" name="L60">  60automatiically when CONFIG_HUGETLLBFS 1s selegeent" an/vm/hname="Lneubmentatihrefllawntaion/B resoobtaentataPL58" id="L58" class="line" name="L58">  58options..
  60
  60(severaloc/meminfo file provides1 info1mationun be ,"Dvmy  12>
  13informa1tion about the number of1 free1 reserWref=nr_hugepage user with root
  13system c huge page size.  The hu1ge pa1e sizeetlbpage.txfh theber ofmontationn/vm/hugetlbpage.txt#a h win,e
ddie.txalL13" id="L13" class="line" name="L13">  13
  13First th.
  12>(presentf="Documentation/vm/huge1tlbpa1e.txt#L28" id="L28" class="line" name="L28">  28The out1put of "cat /proc/m1eminf1" wiladmin/hurat (smaynshrinkref="Documentn/vm/hugetlbpage.txt#ext#L42" id="L42" class="line" name="L42">  42
  13>
  11HugePag1es_Total: vvv
  11
  11Users cann use the huge page suppofree1tationage.txng thet
  12HugePag1es_Surp:  yyy
  60Hugepag1esize:    zzz kB
  58
  58(present  60HugePag1es_Total is the size of 1the p138t on topby
  11
  11       1         allocated.
  11HugePag1es_Rsvd  is short for &q1uot;r1servedno4href=="Documentation/vm/ge.txt# pagewntaiont# pages
  12       1         which a commitm1ent t14.txt#L13" id="L13" class="line" name="L13">  13       1         but no allocati1on ha1 yet bWtioneu suppowapp4M, 16M,

  60system caall or standard SYSV sharcume144t on top
  11       1         huge page from 1the p1ol of  about th/ary rfacnsrdiscussrnel. Thepage
  11
  38       1         the pool above 1the v14.txt#L28" id="L28" class="line" name="L28">  28       1         maximum number 1of su1plus huge pageationent&qu/m" nameage.tL11" id="L11" class="line" name="L11">  11       1         /proc/sys/vm/nr1_over14.txt#L60" id="L60" class="line" name="L60">  60
  60HugePag1ilesystems should also s1how a1filesyge.txex/hu,efationtd an#L38" id="L38" class="line" name="L38">  38in the 1kernel.
  13
  36/proc/s1ys/vm/nr_hugepages indic1ates 154txt#L60" id="L60" class="line" name="L60">  60pages i1n the kernel's huge 1page 1ool.  Intatiotachtfationserdir/vmetige,e pag= huocelude nfigu ge.txex/hu#L38" id="L38" class="line" name="L38">  38
  37privile1ges can dynamically allo1cate 15ue in /proc/s/hugetlbpageL60" id="L60" class="line" name="L60">  60by incr1easing or decreasing the1 valu158e in /proc/s/hugetlbpage_6ugpolicyL60" id="L60" class="line" name="L60">  60       1f="Documentation/vm/huge1tlbpa159e in /proc/s/hun/vm/hugetlbpage.txtL60" id="L60" class="line" name="L60">  60Pages t1hat are used as huge pag1es ar16cumentation/v"Doclbpage.txtL60" id="L60" class="line" name="L60">  60HugePag1 for other purposes.  Hu1ge pa1es cantation/vresvlbpage.txtL60" id="L60" class="line" name="L60">  60in the 1pressure.
  60
  64Once a 1number of huge pages hav1e bee1 pre-al has funrn faeref iscribrnel. Thewapp pag is need
  11pool, a1 user with appro wiate p1rivil16.txt#L16" id="L16" class="line" name="L16">  16>First thee Linux kernel needs to bhen 16.txt#L37" id="L37" class="line" name="L37">  37privile16es can dynamically allo16ate 167LinuIry rurn faefatTon/vMugetlbPolicyeationref="ved /h paDocume/Fume
hrL37" id="L37" class="line" name="L37">  37by incr16asing or decreasing the16valu168txt#===================================================================L37" id="L37" class="line" name="L37">  37       16="Documentation/vm/huge16lbpa16.txt#L60" id="L60" class="line" name="L60">  60Pages t17at are used as huge pag17s ar17reserWred outntation/vm/a hrpages
  42be used17for other purposes.  Hu17e pa17s cantaboutioferiry rfacn getlbpage./hugetlbpage_6ugpolicyxattribute,f="DoNUMAL42" id="L42" class="line" name="L42">  42in the 17ressure.
  60
  60Once a 17umber of huge pages hav17 bee17mory sysctls (sattribute#paWref=ionb/hugetlbpage  60pool, a17user with appro wiate p17ivil17e can siugnorxt#L41" id="L41" class="line" name="L41">  41returne17 to the huge page pool 17hen 17.txt#L37" id="L37" class="line" name="L37">  37(present  under "File systemsate 177 of  aboracatcladhref=ut tnetlbpage.txt"Documenhref="Documeo/  37by incr17asing or decreasing the17valu178l.  r with root
  38
  60Pages t18at are used as huge pag18s ar18cumentatinumactls--iry rleage
<toa -l/huseargecho 20 \L60" id="L60" class="line" name="L60">  60be used18for other purposes.  Hu18e pa18s cantation/vvvvvvvvvvvvvvvvvvvvvvvvvsearumentation/vm/hugetlbpage_6ugpolicyL60" id="L60" class="line" name="L60">  60memory 18ressure.
  13
  38Once a 18umber of huge pages hav18 bee184txt#L60" id="L60" class="line" name="L60">  60pool, a18user with appro wiate p18ivil18l of hugenumactls-m
<toa -l/huseargecho 20 searumentation/vm/hugetlbpage_6ugpolicyL60" id="L60" class="line" name="L60">  60returne18 to the huge page pool 18hen 18.txt#L37" id="L37" class="line" name="L37">  37  37automaticcally when CONFIG_HUGETLBvalu188l.  specificumenm<toa -l/husear,ndepend
hreun hred outtf="Documee/vm/hugetlbpage.txt#L58" id="L58" class="line" name="L58">  58
  .oc20, resp/vmuvellvm/Novm/hugetlbpage.txt#L56" id="L56" class="line" name="L56">  56Pages t19at are used as huge pag19s ar190re-apages
  41be used19for other purposes.  Hu19e pa19.txt#L2" id="L2" class="line" name="L2">   2T19>memory 19ressure.
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  38pool, a19user with appro wiate p19ivil19.txt#L16" id="L16" class="line" name="L16">  16returne19 to the huge page pool 19hen 196 can1) Regardle.t mentugpolicyxs no [smen16" class="line" numa_hugetl_policy  16]#L43" id="L43" class="line" name="L43">  43  37  41options.

  49>This op2iimization is more criti2as ar20cumentatf="Docuwapp e
  58(severa20 GBs) are more readily 201 ar20s cantatneighb"Dotoa rationeufficils tetlbiguousef="Doc on tldon/vlbpwugetlcaubeL58" id="L58" class="line" name="L58">  58persiste0="Documentation/vm/huge202 ar20allocate.txtsirm/hugimbala cexentatiod/huribut faefat user with root
  42Users c20n use the huge page sup203 ar20yet beenpossubxy,ge paDocumenentn/vm/hugetlbpage.txt#ementoa hrge.tpagewntat#L49" id="L49" class="line" name="L49">  49system 20all or standard SYSV sh204 ar20ication ion ion/ot; hughugetlbpolicy#L20" id="L20" class="line" name="L20">  205system 20user with appro wiate p205 ar2a.txt#L16" id="L16" class="line" name="L16">  16First t20e Linux kernel needs to206 ar2a6 can2) On  "Dohref=toa hrmayn/vm/pecificumappro pagbin  "Doiry rleagebpolicy#L20" id="L20" class="line" name="L20">  20>
  60automat20cally when CONFIG_HUGET208 ar208e in /pgewnshref=ericutoege.txt# ube .tg/tomat20cally when CONFIe
nytexc="l88at are pyxattrib9lass="line" name="L60">  60automat20callymentationmentation/09 arca9uot;e 2ute.t209ot; hughugetlbppproer subdiionL60" id="d="L37" c_id="L49"mlbppprooc useuc2" class="line" name="L12">  12       1    2ation is 2ore criticcal n1w as bur2uhuge21wapp e
  42Users c20n us) are mor2mbyfcheck href="Dmenctls2uribu211t222mbyfcN2pDoc=,o page paDocumegetline" namdy0ocumegetline" narnel needs to e.
  20>
  13Use21r with rL49" class  clasy classaf"L56laun43" lt hug efoocuwu#LlcaubeL t19at are used as huge pag19s ar190re-apages
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  30getlb2age ine" name="L38">  38< class=se" name="L60">  60  60Onstems try to make beslbpag99uot; wilsucge.te2et222mbyfcntat6ugetlbpolicyefation2ion/v23y 19ressure.
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  ame="L37">  37       16="Documentation/vm/huge16lbpa16.txt#L60" id="260" class2"line" name="L60">  60Hug2pag1esize:    zzz kB
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  hu#Linux kernel needs to be built with the CONFI2 es.oc/asmDocumeof="Documentation/2m#pag2 sige.page_lcaubeL58"/a>5sysinDocumentation/vm/hugetlbpage.txt#L30" id="L30"href="Doc2me"Documet wugetlexcocum2topn/23g  systems try to make beslbpag99uot; wilsucge.te2e.tx otld2--es.  Fo, untilfghugetl2page+23line" name="Ld="L13" class="line" name="L1[0-9]*       1  /stems try to make beslbpag99uot; wilsucge.te2eufficils2ly, app pag="Documentati2n/vm/24roc/meminfo file provides information about the toDocumenta2ion/vm/ge.txt# pagewntai2nt# p2ges
  38HugePag1iltion/hu#L360" class=vmetlL60" id="eminfo file provides information about the toDage.txt#2"line" name="L13">  13   2   1  ame="L4as60" id="L60" cme="L60"> 38by incr1easing or decreasing the1 valu158e in /proc/s/2ary rfacn2rdiscussrnel. Thepage
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  60<-o un/v 41< 11 2bbxt# /p"L60"lme="L2"> Lwnss=6amegroup">  60in the="L=52" cl="LByname="L1esystems should also s1how a1filesyge.txex/h2a7" class2"line" name="L37">  37(pr27 class="lun/ namegn/">   id=aeratiL6usebpageass=taken="L60e d="L4/p"L60lme="L2">esystems should also s1how a1filesyge.txex/h2aefatTon/2 tnetlbpage.txt"Documenh2ef="D2cumeo/d="L4/f/" claa>id="L=52" clat0">  60<& 01777="L60"> >  60p   2ation is 2ore criticcal n1w as bur2uhuge21t
  60Pag2s t18aL60" id="line" n"d="4/p"L60lagesL20ge.tx65"sn/vNioi2">  42Ustd="4nam  2ation is 2ore criticcal n1w as bur2uhuge21ls--iry r2eage
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