{"id":26,"date":"2009-04-06T15:13:40","date_gmt":"2009-04-06T13:13:40","guid":{"rendered":"http:\/\/blog.websupport.sk\/?p=26"},"modified":"2025-03-11T21:06:03","modified_gmt":"2025-03-11T20:06:03","slug":"linux-page-cache-a-control-groups","status":"publish","type":"post","link":"https:\/\/www.websupport.sk\/blog\/2009\/04\/linux-page-cache-a-control-groups\/","title":{"rendered":"Linux page cache a control groups"},"content":{"rendered":"Doba \u010d\u00edtania:<\/b><\/span> 5<\/span> min.<\/span><\/span>

Teraz v\u00e1m prin\u00e1\u0161ame tro\u0161ku technicky n\u00e1ro\u010dnej\u0161\u00ed \u010dl\u00e1nok o problematike s ktorou sa museli na\u0161i admini vysporiada\u0165.<\/p>\n

Ve\u013emi zjednodu\u0161ene m\u00f4\u017eeme rozdeli\u0165 pam\u00e4\u0165 v Linuxe na 2 typy:<\/p>\n

    \n
  • mapped – aplik\u00e1cie – stack, data, loadnut\u00e9 kni\u017enice, alokovan\u00e1 pam\u00e4\u0165 za behu app, …<\/li>\n
  • unmapped – page cache (diskov\u00e1 cache) – cacheuj\u00fa sa tam s\u00fabory pri \u010d\u00edtan\u00ed\/z\u00e1pise na disky, rie\u0161i to kernel nez\u00e1visle od aplik\u00e1ci\u00ed<\/li>\n
  • (a e\u0161te m\u00e1me nejak\u00e9 \u010fal\u0161ie ako slab, kde s\u00fa TCP\/IP stack buffre, directory entries, at\u010f.)<\/li>\n<\/ul>\n

    Page cache je v Linuxe nie najlep\u0161ie implementovan\u00e1.
    \nJeden z probl\u00e9mov, ktor\u00e9 ma tr\u00e1pia, je napr\u00edklad to, \u017ee jeden task pri z\u00e1pise alebo na\u010d\u00edtan\u00ed ve\u013ek\u00e9ho mno\u017estva d\u00e1t odstr\u00e1ni z page cache v\u0161etko ostatn\u00e9 a nahrad\u00ed to t\u00fdm, \u010do \u010d\u00edta\/zapisuje.<\/p>\n

    Na desktope to tak nevad\u00ed.
    \nPredstavme si ale vy\u0165a\u017een\u00fd server, kde pristupuj\u00fa stovky klientov a ka\u017ed\u00fd jeden d\u00e9mon, na ktor\u00fd je pripojen\u00fd klient, \u010d\u00edta in\u00e9 d\u00e1ta z disku a mas\u00edvne sa tam vyu\u017e\u00edvaj\u00fa nacachovan\u00e9 data.<\/p>\n

    Zrazu sa spust\u00ed nejak\u00fd backup alebo in\u00e1 aplik\u00e1cia a za\u010dne robi\u0165 s objemn\u00fdm mno\u017estvom d\u00e1t.
    \nStane sa to, \u017ee sa d\u00e1ta potrebn\u00e9 pre tie d\u00e9mony z page cache odstr\u00e1nia, n\u00e1sledne d\u00e9mony \u010dakaj\u00fa na IO z diskov, lebo nemaj\u00fa d\u00e1ta nacachovan\u00e9, server laguje a je to pekne v ****<\/span> \ud83d\ude42<\/p>\n

    Uk\u00e1\u017eka, server zo 16 GB RAM, vytvor\u00edme 6 GB file, zma\u017eeme ho a pozrieme sa na pam\u00e4\u0165:<\/p>\n

    #<\/span><\/strong><\/span> dd if=\/dev\/zero of=foo bs=1M count=6144\n6144+0 records in\n6144+0 records out\n6442450944 bytes (6.4 GB) copied, 67.489 seconds, 95.5 MB\/s<\/span><\/strong>\n# free -m\n\u00a0\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0total \u00a0 \u00a0 \u00a0used \u00a0 \u00a0 \u00a0free \u00a0 \u00a0shared \u00a0 \u00a0buffers \u00a0 \u00a0cached\nMem: \u00a0 \u00a0 \u00a0 \u00a015928 \u00a0 \u00a0 \u00a015872 \u00a0 \u00a0 \u00a0 \u00a055 \u00a0 \u00a0 \u00a0 \u00a0 \u00a00 \u00a0 \u00a0 \u00a0 \u00a0 \u00a00 \u00a0 \u00a0 \u00a012946\n-\/+ buffers\/cache: \u00a0 \u00a0 \u00a02926 \u00a0 \u00a0 \u00a013001\nSwap: \u00a0 \u00a0 \u00a0 \u00a0 \u00a0949 \u00a0 \u00a0 \u00a0 \u00a0520 \u00a0 \u00a0 \u00a0 \u00a0428<\/span><\/strong>\n# rm foo\n# free -m\n\u00a0\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0total \u00a0 \u00a0 \u00a0used \u00a0 \u00a0 \u00a0free \u00a0 \u00a0shared \u00a0 \u00a0buffers \u00a0 \u00a0cached\nMem: \u00a0 \u00a0 \u00a0 \u00a015928 \u00a0 \u00a0 \u00a09630 \u00a0 \u00a0 \u00a06297 \u00a0 \u00a0 \u00a0 \u00a0 \u00a00 \u00a0 \u00a0 \u00a0 \u00a0 \u00a00 \u00a0 \u00a0 \u00a06865<\/span><\/strong>\n-\/+ buffers\/cache: \u00a0 \u00a0 \u00a02765 \u00a0 \u00a0 \u00a013163\nSwap: \u00a0 \u00a0 \u00a0 \u00a0 \u00a0949 \u00a0 \u00a0 \u00a0 \u00a0520 \u00a0 \u00a0 \u00a0 \u00a0428<\/span><\/strong><\/strong><\/strong><\/strong><\/strong><\/span><\/span><\/strong><\/pre>\n
    Ako m\u00f4\u017eete vidie\u0165 – 6GB n\u00e1m zmizlo z cached a je free.
    \nVytvorenie 6GB s\u00faboru n\u00e1m odstr\u00e1nilo z cache 6GB nacachovan\u00fdch d\u00e1t a n\u00e1sledne zmazanie s\u00faboru uvo\u013enilo pam\u00e4\u0165 z page cache.<\/p>\n
    V Linuxe u\u017e dlho existuje nejak\u00e9 limitovanie pam\u00e4te pre procesy (syscall setrlimit()), napr. builtin pr\u00edkaz ulimit v shelloch, alebo pam v \/etc\/security\/limits.conf (hlavne pre u\u017e\u00edvate\u013eov, ktor\u00ed sa prihl\u00e1sia).
    \nTo n\u00e1m ale limituje iba mapped pam\u00e4\u0165, nie page cache. M\u00f4\u017eete aplik\u00e1ciu olimitova\u0165 povedzme na 100 MB, ale aj tak pri \u010d\u00edtan\u00ed\/z\u00e1pise ve\u013ek\u00e9ho mno\u017estva d\u00e1t v\u00e1m odcachuje potrebn\u00e9 d\u00e1ta z page cache.<\/p>\n
    V nov\u00fdch kerneloch n\u00e1m pribudla pekn\u00e1 featura tzv. control groups, pomocou nej m\u00f4\u017eeme nastavi\u0165 pre r\u00f4zne procesy limity na celkov\u00fa pou\u017eit\u00fa pam\u00e4\u0165.
    \nTeda mapped aj unmapped, taktie\u017e nastavi\u0165 iba konkr\u00e9tne CPU pri smp syst\u00e9moch, kde maj\u00fa dan\u00e9 procesy be\u017ea\u0165.
    \nKernel v Ubuntu m\u00e1 v\u0161etko \u010do je treba, tak si to po\u010fme rovno uk\u00e1za\u0165, ako sa to konfiguruje.<\/p>\n
    \n
    # mkdir \/cgroup\n# mount -t cgroup -o cpu<\/strong><\/span>set,memory none \/cgroup<\/strong><\/span><\/span><\/pre>\n
    T\u00fdmto sme namountovali hlavn\u00fa cgroupu, kde s\u00fa v\u0161etky procesy.
    \nV \/cgroup\/tasks je zoznam pidov.<\/span><\/p>\n
    # cat \/cgroup\/cpuset.cpus<\/strong><\/span>\n<\/strong><\/span><\/span>0-3 <--- 2 CPU po 2 jadr\u00e1, teda 4 jadr\u00e1 dokopy.\n<\/strong><\/span><\/span># cat \/cgroup\/cpuset.mems<\/strong><\/span>\n<\/strong><\/span><\/span>0-1<--- iba pri numa syst\u00e9moch, ka\u017ed\u00e9 CPU m\u00e1 vlastn\u00fa pam\u00e4\u0165, maj\u00fa to hlavne Opterony,\nale s\u00fa aj Xeony z numa podporou.<\/strong><\/span><\/span><\/pre>\n
    Vytvor\u00edme si nov\u00fa cgroup pomocou mkdir<\/span><\/p>\n
    # mkdir \/cgroup\/test (tu sa nam objavia automaticky vsetky subory ako v \/cgroup\/)<\/span>\n<\/strong><\/span><\/span>\u00a0 Teraz mus\u00edme nastavi\u0165 CPU a pam\u00e4\u0165, lebo defaultne ma nov\u00e1 cgroupa v\u0161etko pr\u00e1zdne.\n<\/strong><\/span><\/span># cat \/cgroup\/cpuset.cpus >\/cgroup\/test\/cpuset.cpus\n# cat \/cgroup\/cpuset.mems >\/cgroup\/test\/cpuset.mems<\/span>\n<\/strong><\/span><\/span>\u00a0 Olimitujeme t\u00fato cgroupu na 128 MB RAM.\n<\/strong><\/span><\/span># echo 128M >\/cgroup\/test\/memory.limit_in_bytes<\/span>\n<\/strong><\/span><\/span>\u00a0 Hod\u00edme pid bashu, s ktor\u00fdm rob\u00edme do novej cgroupy, v\u0161etky novovytvoren\u00e9 child\nprocesy bud\u00fa automaticky v tejto cgroupe.\n<\/strong><\/span><\/span># echo $$ >\/cgroup\/test\/tasks<\/span>\n<\/strong><\/span><\/span>\u00a0 vytvor\u00edme 6GB file\n<\/strong><\/span><\/span># dd if=\/dev\/zero of=foo bs=1M count=6144<\/span>\n<\/strong><\/span><\/span>6144+0 records in\n6144+0 records out\n6442450944 bytes (6.4 GB) copied, 108.919 seconds, 59.1 MB\/s\n<\/strong><\/span><\/span># free -m<\/span>\n<\/strong><\/span><\/span>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 total\u00a0 \u00a0 \u00a0 used\u00a0 \u00a0 \u00a0 free\u00a0 \u00a0 shared\u00a0 \u00a0 buffers\u00a0 \u00a0 cached\nMem:\u00a0 \u00a0 \u00a0 \u00a0 15928\u00a0 \u00a0 \u00a0 15696\u00a0 \u00a0 \u00a0 \u00a0 231\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 0\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 0\u00a0 \u00a0 \u00a0 13417\n-\/+ buffers\/cache:\u00a0 \u00a0 \u00a0 2278\u00a0 \u00a0 \u00a0 13649\nSwap:\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 949\u00a0 \u00a0 \u00a0 \u00a0 556\u00a0 \u00a0 \u00a0 \u00a0 392\n<\/strong><\/span><\/span># rm foo\n# free -m<\/span>\n<\/strong><\/span><\/span>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 total\u00a0 \u00a0 \u00a0 used\u00a0 \u00a0 \u00a0 free\u00a0 \u00a0 shared\u00a0 \u00a0 buffers\u00a0 \u00a0 cached\nMem:\u00a0 \u00a0 \u00a0 \u00a0 15928\u00a0 \u00a0 \u00a0 15553\u00a0 \u00a0 \u00a0 \u00a0 374\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 0\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 0\u00a0 \u00a0 \u00a0 13291\n-\/+ buffers\/cache<\/strong><\/span><\/span>:\u00a0 \u00a0 \u00a0 2262\u00a0 \u00a0 \u00a0 13665\nSwap:\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 949\u00a0 \u00a0 \u00a0 \u00a0 556\u00a0 \u00a0 \u00a0 \u00a0 392<\/strong><\/span><\/span><\/pre>\n<\/div>\n
    Voila, dd n\u00e1m nezmazalo 6 GB z page cache, ale bolo limitovan\u00e9 na 128 MB RAM a v pr\u00edpade, \u017ee aplik\u00e1cia prekro\u010d\u00ed sama nastaven\u00fd limit pam\u00e4ti (nie page cache), tak ju killne OOM killer.<\/p>\n
    M\u00f4\u017eeme ju zru\u0161i\u0165<\/p>\n
    \n
    # rmdir \/cgroup\/test<\/span>\n<\/span><\/strong><\/span>rmdir: \/cgroup\/test: Device or resource busy\n\u00a0 Samozrejme sa ned\u00e1, lebo tam m\u00e1me bash,\n\u00a0 tak ho najsk\u00f4r prehod\u00edme do hlavnej cgroupy\n<\/span><\/strong><\/span># echo $$ >\/cgroup\/tasks\n# rmdir \/cgroup\/test<\/span><\/strong><\/span><\/pre>\n<\/div>\n
    Page cache je st\u00e1le zle vyrie\u0161en\u00e9, ale toto je aspo\u0148 nie\u010do, \u010d\u00edm m\u00f4\u017eeme olimitova\u0165 r\u00f4zne aplik\u00e1cie na serveri.
    \n\u010eal\u0161ou zauj\u00edmavou vecou v cgroups, ktor\u00e1 e\u0161te nie je priamo v kerneli, je bio-cgroup + dm-ioband – t\u00fdm sa d\u00e1 limitova\u0165 I\/O bandwith, ale netestoval som.<\/p>\n
    Existuj\u00fa aj nejak\u00e9 userspace tooly, ale nesk\u00fa\u0161al som ani jeden. Niekde na Fedora str\u00e1nke som videl, \u017ee maj\u00fa aj pam modul pre cgroups, netestoval som ho.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Teraz v\u00e1m prin\u00e1\u0161ame tro\u0161ku technicky n\u00e1ro\u010dnej\u0161\u00ed \u010dl\u00e1nok o problematike s ktorou sa museli na\u0161i admini vysporiada\u0165. Ve\u013emi zjednodu\u0161ene m\u00f4\u017eeme rozdeli\u0165 pam\u00e4\u0165 v Linuxe na 2 typy: mapped – aplik\u00e1cie – stack, data, loadnut\u00e9 kni\u017enice, alokovan\u00e1 pam\u00e4\u0165 za behu app, … unmapped – page cache (diskov\u00e1 cache) – cacheuj\u00fa sa tam s\u00fabory pri \u010d\u00edtan\u00ed\/z\u00e1pise na disky, […]<\/p>\n","protected":false},"author":84,"featured_media":5585,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[133,169,460,620,747,887],"class_list":["post-26","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-technologie","tag-cache","tag-control-groups","tag-linux","tag-page-file","tag-server","tag-unix"],"yoast_head":"\nLinux page cache a control groups - Websupport blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.websupport.sk\/blog\/2009\/04\/linux-page-cache-a-control-groups\/\" \/>\n<meta property=\"og:locale\" content=\"sk_SK\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Linux page cache a control groups - Websupport blog\" \/>\n<meta property=\"og:description\" content=\"Teraz v\u00e1m prin\u00e1\u0161ame tro\u0161ku technicky n\u00e1ro\u010dnej\u0161\u00ed \u010dl\u00e1nok o problematike s ktorou sa museli na\u0161i admini vysporiada\u0165. 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