Brendan Gregg, um dos desenvolvedores do DTrace, agora desenvolve ferramentas para an\u00e1lise de desempenho baseadas em BPF no n\u00facleo do Linux. Ele resumiu a experi\u00eancia obtida ao analisar os problemas de desempenho que a Netflix enfrentou ao migrar seu banco de dados Cassandra do CentOS para o Ubuntu em ambientes executados na nuvem Amazon EC2 baseados em Xen. Ap\u00f3s a migra\u00e7\u00e3o, a carga na CPU aumentou em 30% e as lat\u00eancias ao executar opera\u00e7\u00f5es de grava\u00e7\u00e3o aumentaram aproximadamente na mesma propor\u00e7\u00e3o. Descobriu-se que o desempenho das aplica\u00e7\u00f5es que consultam intensivamente informa\u00e7\u00f5es de tempo depende muito da fonte de tempo exata escolhida pelo sistema. <\/p>\n
Inicialmente, a causa da redu\u00e7\u00e3o de desempenho n\u00e3o era \u00f3bvia e a investiga\u00e7\u00e3o come\u00e7ou com o rastreamento do poss\u00edvel impacto de processos do sistema que consomem muitos recursos, que est\u00e3o sempre em execu\u00e7\u00e3o ou s\u00e3o iniciados periodicamente, utilizando as ferramentas top e execsnoop. Mas tudo indicava que o consumo de recursos havia aumentado especificamente no banco de dados Cassandra, escrito em Java. A compara\u00e7\u00e3o das m\u00e9tricas de perfilamento de dois processos Cassandra executados paralelamente no CentOS e no Ubuntu, processando as mesmas consultas, mostrou que cerca de 32% do tempo total era gasto na chamada os::javaTimeMillis(), utilizada para obter informa\u00e7\u00f5es sobre a hora atual. <\/p>\n
Ap\u00f3s isso, foi realizado um experimento no qual um aplicativo simples foi escrito em Java, chamando o m\u00e9todo System.currentTimeMillis() cem milh\u00f5es de vezes em um loop. A execu\u00e7\u00e3o do aplicativo mostrou que no CentOS levou 13 segundos, enquanto no Ubuntu levou cerca de 68 segundos, ou seja, cinco vezes mais lento. Em C, foi escrita uma programa\u00e7\u00e3o semelhante que chamava a fun\u00e7\u00e3o gettimeofday() cem milh\u00f5es de vezes, mas os resultados foram an\u00e1logos. <\/p>\n
J\u00e1 que ficou claro que a fonte do problema era a fun\u00e7\u00e3o de retorno do tempo atual, a aten\u00e7\u00e3o se voltou para a varia\u00e7\u00e3o nos indicadores ao selecionar diferentes fontes de tempo preciso no sistema. De acordo com o conte\u00fado de '\/sys\/devices\/system\/clocksource\/clocksource0\/current_clocksource', por padr\u00e3o, ao iniciar o Linux em um sistema convidado, o timer 'xen' era utilizado. Ap\u00f3s mudar a fonte de tempo para 'tsc', o tempo de execu\u00e7\u00e3o do aplicativo de teste no Ubuntu diminuiu de 68 para 3,3 segundos, ou seja, ficou 20 vezes mais r\u00e1pido. Al\u00e9m disso, foi realizado um teste de desempenho da fonte de tempo kvm-clock, que mostrou um aumento de lat\u00eancias em 20%, em compara\u00e7\u00e3o com o 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
Para obter o tempo ao selecionar a fonte TSC, utiliza-se a instru\u00e7\u00e3o de processador RDTSC, cuja execu\u00e7\u00e3o n\u00e3o requer uma chamada de sistema (a instru\u00e7\u00e3o n\u00e3o exige privil\u00e9gios elevados e fornece um valor do contador de tempo embutido no CPU). Por padr\u00e3o, o TSC n\u00e3o \u00e9 ativado, pois nos tempos antigos, essa fonte n\u00e3o eliminava o drifts de tempo que, em outros manipuladores, eram corrigidos programaticamente para alcan\u00e7ar medi\u00e7\u00f5es mais precisas. Segundo um engenheiro especializado em desenvolvimento de processadores, as preocupa\u00e7\u00f5es sobre mudan\u00e7as de tempo ao usar o TSC n\u00e3o s\u00e3o mais v\u00e1lidas e processadores modernos<\/a> essa fonte pode fornecer medi\u00e7\u00f5es est\u00e1veis por anos. <\/p>\n A transi\u00e7\u00e3o para o TSC servidores<\/a> Na Netflix, a altera\u00e7\u00e3o para a fonte TSC levou a uma redu\u00e7\u00e3o de 43% nas lat\u00eancias de grava\u00e7\u00e3o e, ao usar o Ubuntu, os resultados obtidos superaram em 4 vezes as configura\u00e7\u00f5es com CentOS usando a fonte de tempo 'xen'. Os resultados do estudo foram encaminhados \u00e0 Amazon, que recomendou oficialmente que em ambientes AWS EC2 baseados no hipervisor Xen fosse utilizado por padr\u00e3o a fonte de tempo TSC (nos ambientes baseados no hipervisor Nitro, o kvm-clock continua sendo recomendado). \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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[…]<\/p>\n","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[702],"tags":[],"class_list":["post-101444","post","type-post","status-publish","format-standard","hentry","category-news"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\n","aioseo_head_json":{"title":"\ud83e\udd47An\u00e1lise do impacto no desempenho da fonte de tempo escolhida no sistema | ProHoster","description":"Brendan Gregg, um dos desenvolvedores do DTrace, atualmente desenvolvendo ferramentas de an\u00e1lise de desempenho baseadas em BPF no n\u00facleo do Linux, resumiu a experi\u00eancia adquirida ao analisar problemas de 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Fonte: opennet.ru<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"