還不會JVM調優嗎？照着做就行

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"你好，我是看山。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這幾天壓測預生產環境，發現 TPS 各種不穩。因爲是重構的系統，據說原來的系統在高併發的時候一點問題沒有，結果重構的系統被幾十個併發壓一下就各種不穩定。雖然測試的同事沒有說啥，但自己感覺被啪啪的打臉。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"於是各種排查，最先想到的就是 JVM 參數，於是優化一番，希望能夠出一個好的結果。儘管後來證明不穩定的原因是安裝 LoadRunner 的壓測服務器不穩定，不關我的系統的事，不過也是記錄一下，一是做個備份，二是可以給別人做個參考。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"寫在前面的話","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"因爲 Hotspot JDK 提供的參數默認值，在小版本之間不斷變化，參數之間也會互相影響。而且，服務器配置不同，都可能影響最後的效果。所以千萬不要迷信網上的某篇文章（包括這篇）裏面的參數配置，一切的配置都需要自己親身測試一番才能用。針對於 JVM 參數默認值不斷變化，可以使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+PrintFlagsFinal","attrs":{}}],"attrs":{}},{"type":"text","text":"打印當前環境 JVM 參數默認值，比如：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"java -XX:PrintFlagsFinal -version","attrs":{}}],"attrs":{}},{"type":"text","text":"，也可以用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"java [生產環境參數] -XX:+PrintFlagsFinal –version | grep [待查證的參數]","attrs":{}}],"attrs":{}},{"type":"text","text":"查看具體的參數數據。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://www.howardliu.cn/files/java/global-flags.txt","title":"","type":null},"content":[{"type":"text","text":"這裏","attrs":{}}]},{"type":"text","text":" 是一個 8G 服務器的參數，JDK 版本信息如下：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"java version \"1.8.0_73\"\nJava(TM) SE Runtime Environment (build 1.8.0_73-b02)\nJava HotSpot(TM) 64-Bit Server VM (build 25.73-b02, mixed mode)\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"堆設置","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆內存設置應該算是一個 Java 程序猿的基本素養，最少也得修改過 Xms、Xmx、Xmn 這三個參數了。但是一個 2G 堆大小的 JVM，可能總共佔用多少內存的？","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆內存 ＋ 線程數 ＊ 線程棧 ＋ 永久代 ＋ 二進制代碼 ＋ 堆外內存","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2G + 1000 * 1M + 256M + 48/240M + (～2G) = 5.5G (3.5G)","attrs":{}}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆內存： 存儲 Java 對象，默認爲物理內存的 1/64","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"線程棧： 存儲局部變量（原子類型，引用）及其他，默認爲 1M","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"永久代： 存儲類定義及常量池，注意 JDK7/8 的區別","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"二進制代碼：JDK7 與 8，打開多層編譯時的默認值不一樣，從 48 到 240M","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"堆外內存： 被 Netty，堆外緩存等使用，默認最大值約爲堆內存大小","attrs":{}}]}]}],"attrs":{}}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"也就是說，堆內存設置爲 2G，那一個有 1000 個線程的 JVM 可能需要佔 5.5G，在考慮系統佔用、IO 佔用等等各種情況，一臺 8G 的服務器，也就啓動一個服務了。當然，如果線程數少、併發不高、壓力不大，還是可以啓動多個，而且也可以把堆內存降低。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"-Xms2g 與 -Xmx2g：堆內存大小，第一個是最小堆內存，第二個是最大堆內存，比較合適的數值是 2-4g，再大就得考慮 GC 時間","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"-Xmn1g 或 （-XX:NewSize=1g 和 -XX:MaxNewSize=1g） 或 -XX:NewRatio=1：設置新生代大小，JDK 默認新生代佔堆內存大小的 1/3，也就是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:NewRatio=2","attrs":{}}],"attrs":{}},{"type":"text","text":"。這裏是設置的 1g，也就是","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:NewRatio=1","attrs":{}}],"attrs":{}},{"type":"text","text":"。可以根據自己的需要設置。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:MetaspaceSize=128m 和 -XX:MaxMetaspaceSize=512m，JDK8 的永生代幾乎可用完機器的所有內存，爲了保護服務器不會因爲內存佔用過大無法連接，需要設置一個 128M 的初始值，512M 的最大值保護一下。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:SurvivorRatio：新生代中每個存活區的大小，默認爲 8，即 1/10 的新生代， 1/(SurvivorRatio+2)，有人喜歡設小點省點給新生代，但要避免太小使得存活區放不下臨時對象而要晉升到老生代，還是從 GC Log 裏看實際情況了。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"-Xss256k：在堆之外，線程佔用棧內存，默認每條線程爲 1M。存放方法調用出參入參的棧、局部變量、標量替換後的局部變量等，有人喜歡設小點節約內存開更多線程。但反正內存夠也就不必要設小，有人喜歡再設大點，特別是有 JSON 解析之類的遞歸調用時不能設太小。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:MaxDirectMemorySize：堆外內存/直接內存的大小，默認爲堆內存減去一個 Survivor 區的大小。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:ReservedCodeCacheSize：JIT 編譯後二進制代碼的存放區，滿了之後就不再編譯。默認開多層編譯 240M，可以在 JMX 裏看看 CodeCache 的大小。","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"GC 設置","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"目前比較主流的 GC 是 CMS 和 G1，有大神建議以 8G 爲界。（據說 JDK 9 默認的是 G1）。因爲應用設置的內存都比較小，所以選擇 CMS 收集器。下面的參數也是針對 CMS 收集器的，等之後如果有需要，再補充 G1 收集器的參數。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"CMS 設置","attrs":{}}]},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseConcMarkSweepGC：啓用 CMS 垃圾收集器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:CMSInitiatingOccupancyFraction=80 與 -XX:+UseCMSInitiatingOccupancyOnly：兩個參數需要配合使用，否則第一個參數的 75 只是一個參考值，JVM 會重新計算 GC 的時間。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:MaxTenuringThreshold=15：對象在 Survivor 區熬過多少次 Young GC 後晉升到年老代，默認是 15。Young GC 是最大的應用停頓來源，而新生代裏 GC 後存活對象的多少又直接影響停頓的時間，所以如果清楚 Young GC 的執行頻率和應用裏大部分臨時對象的最長生命週期，可以把它設的更短一點，讓其實不是臨時對象的新生代長期對象趕緊晉升到年老代。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-DisableExplicitGC：允許使用 System.gc() 主動調用 GC。這裏需要說明下，有的 JVM 優化建議是設置-XX:-DisableExplicitGC，關閉手動調用 System.gc()。這是因爲 System.gc() 是觸發 Full GC，頻繁的 Full GC 會嚴重影響性能。但是很多 NIO 框架，比如 Netty，會使用堆外內存，如果沒有 Full GC 的話，堆外內存就無法回收。如果不主動調用 System.gc()，就需要等到 JVM 自己觸發 Full GC，這個時候，就可能引起長時間的停頓（STW），而且機器負載也會升高。所以不能夠完全禁止 System.gc()，又得縮短 Full GC 的時間，那就使用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+ExplicitGCInvokesConcurrent","attrs":{}}],"attrs":{}},{"type":"text","text":"或","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses","attrs":{}}],"attrs":{}},{"type":"text","text":"選項，使用 CMS 收集器來觸發 Full GC。這兩個選項需要配合","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+UseConcMarkSweepGC","attrs":{}}],"attrs":{}},{"type":"text","text":"使用。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+ExplicitGCInvokesConcurrent：使用 System.gc() 時觸發 CMS GC，而不是 Full GC。默認是不開啓的，只有使用-XX:+UseConcMarkSweepGC 選項的時候才能開啓這個選項。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses：使用 System.gc() 時，永久代也被包括進 CMS 範圍內。只有使用-XX:+UseConcMarkSweepGC 選項的時候才能開啓這個選項。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+ParallelRefProcEnabled：默認爲 false，並行的處理 Reference 對象，如 WeakReference，除非在 GC log 裏出現 Reference 處理時間較長的日誌，否則效果不會很明顯。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":8,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+ScavengeBeforeFullGC：在 Full GC 執行前先執行一次 Young GC。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":9,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseGCOverheadLimit： 限制 GC 的運行時間。如果 GC 耗時過長，就拋 OOM。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":10,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseParallelGC：設置並行垃圾收集器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":11,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseParallelOldGC：設置老年代使用並行垃圾收集器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":12,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-UseSerialGC：關閉串行垃圾收集器","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":13,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+CMSParallelInitialMarkEnabled 和 -XX:+CMSParallelRemarkEnabled：降低標記停頓","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":14,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+CMSScavengeBeforeRemark：默認爲關閉，在 CMS remark 前，先執行一次 minor GC 將新生代清掉，這樣從老生代的對象引用到的新生代對象的個數就少了，停止全世界的 CMS remark 階段就短一些。如果看到 GC 日誌裏 remark 階段的時間超長，可以打開此項看看有沒有效果，否則還是不要打開了，白白多了次 YGC。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":15,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:CMSWaitDuration=10000：設置垃圾收集的最大時間間隔，默認是 2000。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":16,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+CMSClassUnloadingEnabled：在 CMS 中清理永久代中的過期的 Class 而不等到 Full GC，JDK7 默認關閉而 JDK8 打開。看自己情況，比如有沒有運行動態語言腳本如 Groovy 產生大量的臨時類。它會增加 CMS remark 的暫停時間，所以如果新類加載並不頻繁，這個參數還是不開的好。","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"GC 日誌","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"GC 過程可以通過 GC 日誌來提供優化依據。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintGCDetails：啓用 gc 日誌打印功能","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"-Xloggc:/path/to/gc.log：指定 gc 日誌位置","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintHeapAtGC：打印 GC 前後的詳細堆棧信息","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintGCDateStamps：打印可讀的日期而不是時間戳","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintGCApplicationStoppedTime：打印所有引起 JVM 停頓時間，如果真的發現了一些不知什麼的停頓，再臨時加上","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+PrintSafepointStatistics -XX: PrintSafepointStatisticsCount=1","attrs":{}}],"attrs":{}},{"type":"text","text":"找原因。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintGCApplicationConcurrentTime：打印 JVM 在兩次停頓之間正常運行時間，與","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+PrintGCApplicationStoppedTime","attrs":{}}],"attrs":{}},{"type":"text","text":"一起使用效果更佳。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PrintTenuringDistribution：查看每次 minor GC 後新的存活週期的閾值","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":8,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseGCLogFileRotation 與 -XX:NumberOfGCLogFiles=10 與 -XX:GCLogFileSize=10M：GC 日誌在重啓之後會清空，但是如果一個應用長時間不重啓，那 GC 日誌會增加，所以添加這 3 個參數，是 GC 日誌滾動寫入文件，但是如果重啓，可能名字會出現混亂。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":9,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:PrintFLSStatistics=1：打印每次 GC 前後內存碎片的統計信息","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"其他參數設置","attrs":{}}]},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"-ea：啓用斷言，這個沒有什麼好說的，可以選擇啓用，或者選擇不啓用，沒有什麼大的差異。完全根據自己的系統進行處理。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseThreadPriorities：啓用線程優先級，主要是因爲我們可以給予週期性任務更低的優先級，以避免干擾客戶端工作。在我當前的環境中，是默認啓用的。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:ThreadPriorityPolicy=42：允許降低線程優先級","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+HeapDumpOnOutOfMemoryError：發生內存溢出時進行 heap-dump","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:HeapDumpPath=/path/to/java_pid.hprof：這個參數與","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"-XX:+HeapDumpOnOutOfMemoryError","attrs":{}}],"attrs":{}},{"type":"text","text":"共同作用，設置 heap-dump 時內容輸出文件。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:ErrorFile=/path/to/hs_err_pid.log：指定致命錯誤日誌位置。一般在 JVM 發生致命錯誤時會輸出類似 hs_err_pid.log 的文件，默認是在工作目錄中（如果沒有權限，會嘗試在/tmp 中創建），不過還是自己指定位置更好一些，便於收集和查找，避免丟失。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:StringTableSize=1000003：指定字符串常量池大小，默認值是 60013。對 Java 稍微有點常識的應該知道，字符串是常量，創建之後就不可修改了，這些常量所在的地方叫做字符串常量池。如果自己系統中有很多字符串的操作，且這些字符串值比較固定，在允許的情況下，可以適當調大一些池子大小。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":8,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+AlwaysPreTouch：在啓動時把所有參數定義的內存全部捋一遍。使用這個參數可能會使啓動變慢，但是在後面內存使用過程中會更快。可以保證內存頁面連續分配，新生代晉升時不會因爲申請內存頁面使 GC 停頓加長。通常只有在內存大於 32G 的時候纔會有感覺。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":9,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-UseBiasedLocking：禁用偏向鎖（在存在大量鎖對象的創建且高度併發的環境下（即非多線程高併發應用）禁用偏向鎖能夠帶來一定的性能優化）","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":10,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:AutoBoxCacheMax=20000：增加數字對象自動裝箱的範圍，JDK 默認-128～127 的 int 和 long，超出範圍就會即時創建對象，所以，增加範圍可以提高性能，但是也是需要測試。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":11,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-OmitStackTraceInFastThrow：不忽略重複異常的棧，這是 JDK 的優化，大量重複的 JDK 異常不再打印其 StackTrace。但是如果系統是長時間不重啓的系統，在同一個地方跑了 N 多次異常，結果就被 JDK 忽略了，那豈不是查看日誌的時候就看不到具體的 StackTrace，那還怎麼調試，所以還是關了的好。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":12,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+PerfDisableSharedMem：啓用標準內存使用。JVM 控制分爲標準或共享內存，區別在於一個是在 JVM 內存中，一個是生成/tmp/hsperfdata_{userid}/{pid}文件，存儲統計數據，通過 mmap 映射到內存中，別的進程可以通過文件訪問內容。通過這個參數，可以禁止 JVM 寫在文件中寫統計數據，代價就是 jps、jstat 這些命令用不了了，只能通過 jmx 獲取數據。但是在問題排查時，jps、jstat 這些小工具是很好用的，比 jmx 這種很重的東西好用很多，所以需要自己取捨。","attrs":{}},{"type":"link","attrs":{"href":"http://www.evanjones.ca/jvm-mmap-pause.html","title":"","type":null},"content":[{"type":"text","text":"這裏","attrs":{}}]},{"type":"text","text":" 有個 GC 停頓的例子。","attrs":{}}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":13,"align":null,"origin":null},"content":[{"type":"text","text":"-Djava.net.preferIPv4Stack=true：這個參數是屬於網絡問題的一個參數，可以根據需要設置。在某些開啓 ipv6 的機器中，通過","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"InetAddress.getLocalHost().getHostName()","attrs":{}}],"attrs":{}},{"type":"text","text":"可以獲取完整的機器名，但是在 ipv4 的機器中，可能通過這個方法獲取的機器名不完整，可以通過這個參數來獲取完整機器名。","attrs":{}}]}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"大神給出的例子","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面貼上大神給出的例子，可以參考使用，不過還是建議在自己的環境中有針對的驗證之後再使用，畢竟大神的環境和自己的環境還是不同。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"性能相關","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-UseBiasedLocking -XX:-UseCounterDecay -XX:AutoBoxCacheMax=20000-XX:+PerfDisableSharedMem（可選） -XX:+AlwaysPreTouch -Djava.security.egd=file:/dev/./urandom","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"內存大小相關 (JDK7)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-Xms4096m -Xmx4096m -Xmn2048m -XX:MaxDirectMemorySize=4096m-XX:PermSize=128m -XX:MaxPermSize=512m -XX:ReservedCodeCacheSize=240M","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果使用 jdk8，就把-XX:PermSize=128m -XX:MaxPermSize=512m 換成-XX:MetaspaceSize=128m -XX:MaxMetaspaceSize=512m，正如前面所說的，這兩套參數是爲了保證安全的，建議還是加上。","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"CMS GC 相關","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=75-XX:+UseCMSInitiatingOccupancyOnly -XX:MaxTenuringThreshold=6-XX:+ExplicitGCInvokesConcurrent -XX:+ParallelRefProcEnabled","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"GC 日誌相關","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-Xloggc:/dev/shm/app-gc.log -XX:+PrintGCApplicationStoppedTime-XX:+PrintGCDateStamps -XX:+PrintGCDetails","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"異常日誌相關","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-XX:-OmitStackTraceInFastThrow -XX:ErrorFile=","attrs":{}},{"type":"katexinline","attrs":{"mathString":"{LOGDIR}/hs_err_%p.log-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath="}},{"type":"text","text":"{LOGDIR}/","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"JMX 相關","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"-Dcom.sun.management.jmxremote.port=${JMX_PORT} -Dcom.sun.management.jmxremote-Djava.rmi.server.hostname=127.0.0.1 -Dcom.sun.management.jmxremote.authenticate=false-Dcom.sun.management.jmxremote.ssl=false","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"參考","attrs":{}}]},{"type":"numberedlist","attrs":{"start":1,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"http://calvin1978.blogcn.com/articles/javatuning.html","title":"","type":null},"content":[{"type":"text","text":"Java 性能優化指南 1.8 版，及唯品會的實戰","attrs":{}}]}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"http://blog.csdn.net/yangzl2008/article/details/43202969","title":"","type":null},"content":[{"type":"text","text":"Java 中的逃逸分析和 TLAB 以及 Java 對象分配","attrs":{}}]}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"http://www.evanjones.ca/jvm-mmap-pause.html","title":"","type":null},"content":[{"type":"text","text":"The Four Month Bug: JVM statistics cause garbage collection pauses","attrs":{}}]}]}]}]},{"type":"horizontalrule","attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"你好，我是看山，10 年老猿，開源貢獻者。遊於碼界，戲享人生。關注公衆號：","attrs":{}},{"type":"link","attrs":{"href":"http://static.howardliu.cn/about/kanshanshuo.png","title":"","type":null},"content":[{"type":"text","text":"看山的小屋","attrs":{}}]},{"type":"text","text":"，領取學習資料。","attrs":{}}]}]}