Brendan Gregg, en af udviklerne af DTrace, der nu arbejder med v\u00e6rkt\u00f8jer til ydeevneanalyse baseret p\u00e5 BPF i Linux-kernen, har opsummeret sine erfaringer fra probleml\u00f8sningen med ydeevne, som Netflix st\u00f8dte p\u00e5 ved migrationen af databasesystemet Cassandra fra CentOS til Ubuntu i Amazon EC2-milj\u00f8er baseret p\u00e5 Xen. Efter migreringen steg CPU-belastningen med 30 %, og forsinkelserne ved udf\u00f8relse af skriveoperationer steg omtrent tilsvarende. Det viste sig, at ydeevnen af applikationer, der intensivt foresp\u00f8rger tidsinformation, er meget afh\u00e6ngig af den valgte tidskilde i systemet. <\/p>\n
I starten var \u00e5rsagen til den nedsatte ydeevne ikke \u00e5benlys, og diagnosticeringen begyndte med at spore den mulige indvirkning fra konstant k\u00f8rende eller periodisk startede ressourcetunge systemprocesser ved hj\u00e6lp af v\u00e6rkt\u00f8jerne top og execsnoop. Men alt pegede p\u00e5, at ressourceforbruget var steget netop i databasesystemet Cassandra, skrevet i Java. Sammenligningen af profilresultaterne fra to Cassandra-processer, der k\u00f8rte parallelt i CentOS og Ubuntu og behandlede de samme foresp\u00f8rgsler, viste, at ca. 32 % af al den tid, der blev brugt, gik til kaldet os::javaTimeMillis(), som anvendes til at f\u00e5 information om den aktuelle tid. <\/p>\n
Efter dette blev der udf\u00f8rt et eksperiment, hvor der blev skrevet en simpel applikation i Java, der kaldte metoden System.currentTimeMillis() en hundred millioner gange i en l\u00f8kke. K\u00f8rsel af applikationen viste, at det tog 13 sekunder p\u00e5 CentOS, mens det p\u00e5 Ubuntu tog omkring 68 sekunder, alts\u00e5 5 gange langsommere. En lignende program blev skrevet i C, der kaldte funktionen gettimeofday() en hundred millioner gange, men liknende resultater blev opn\u00e5et. <\/p>\n
Da det blev klart, at \u00e5rsagen til problemet var funktionen til at returnere den aktuelle tid, blev opm\u00e6rksomheden rettet mod at \u00e6ndre m\u00e5linger ved at v\u00e6lge forskellige kilder til pr\u00e6cise tid. If\u00f8lge indholdet af \"\/sys\/devices\/system\/clocksource\/clocksource0\/current_clocksource\" blev timeren \"xen\" som standard anvendt ved opstart af Linux i g\u00e6stesystemet. Efter \u00e6ndring af tidskilden til \"tsc\" blev k\u00f8retiden for testapplikationen p\u00e5 Ubuntu reduceret fra 68 til 3,3 sekunder, alts\u00e5 hurtigere med 20 gange. Der blev desuden udf\u00f8rt en pr\u00e6stationstest af tidskilden kvm-clock, som viste en stigning i forsinkelserne p\u00e5 20 % sammenlignet med TSC. $ cat \/sys\/devices\/system\/clocksource\/clocksource0\/available_clocksource xen tsc hpet acpi_pm $ cat \/sys\/devices\/system\/clocksource\/clocksource0\/current_clocksource xen $ time java TimeBench real 1m8.300s user 0m38.337s sys 0m29.875s $ echo tsc > \/sys\/devices\/system\/clocksource\/clocksource0\/current_clocksource $ time java TimeBench real 0m3.370s user 0m3.353s sys 0m0.026s <\/p>\n
For at f\u00e5 tiden ved valg af tidskilden TSC anvendes CPU-instruktionen RDTSC, hvis udf\u00f8relse ikke kr\u00e6ver et systemopkald (instruktionen kr\u00e6ver ikke forh\u00f8jede privilegier og afgiver v\u00e6rdien fra den indbyggede tids t\u00e6ller i CPU'en). Som standard aktiveres TSC ikke, da denne kilde i gamle dage ikke udelukkede gradvis tid drift, som i andre h\u00e5ndtere korrigeres programm\u00e6ssigt for at opn\u00e5 mere pr\u00e6cise m\u00e5linger. If\u00f8lge en ingeni\u00f8r, der specialiserer sig i processorudvikling, er bekymringerne om tidsforskydninger ved brug af TSC for l\u00e6ngst ikke l\u00e6ngere relevante og i moderne processorer<\/a> kan denne kilde give stabile m\u00e5linger i \u00e5revis. <\/p>\n Overs\u00e6ttelse af arbejdsmilj\u00f8er servere<\/a> I Netflix f\u00f8rte skiftet til TSC kilden til en reduktion af forsinkelserne ved skrivning p\u00e5 43% og opn\u00e5ede resultater med Ubuntu, der var 4 gange bedre end konfigurationen med CentOS med tidskilden \"xen\". Resultaterne af den udf\u00f8rte unders\u00f8gelse blev sendt til Amazon, som officielt anbefalede at bruge TSC tidskilden som standard i AWS EC2-milj\u00f8er baseret p\u00e5 Xen-hypervisoren (i milj\u00f8er baseret p\u00e5 Nitro-hypervisoren forbliver anbefalingen kvm-clock). \u0411\u0440\u0435\u043d\u0434\u0430\u043d \u0413\u0440\u0435\u0433 (Brendan Gregg), \u043e\u0434\u0438\u043d \u0438\u0437 \u0440\u0430\u0437\u0440\u0430\u0431\u043e\u0442\u0447\u0438\u043a\u043e\u0432 DTrace, \u043d\u044b\u043d\u0435 \u0440\u0430\u0437\u0432\u0438\u0432\u0430\u044e\u0449\u0438\u0439 \u0441\u0440\u0435\u0434\u0441\u0442\u0432\u0430 \u0434\u043b\u044f \u0430\u043d\u0430\u043b\u0438\u0437\u0430 \u043f\u0440\u043e\u0438\u0437\u0432\u043e\u0434\u0438\u0442\u0435\u043b\u044c\u043d\u043e\u0441\u0442\u0438 \u043d\u0430 \u0431\u0430\u0437\u0435 BPF \u0432 \u044f\u0434\u0440\u0435 Linux, \u043e\u0431\u043e\u0431\u0449\u0438\u043b \u043e\u043f\u044b\u0442, 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Kilde: opennet.ru<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"