The purpose of learning the Java GC mechanism is to analyze the cause and solve it when there is a problem with the JVM. JVM monitoring and tuning mainly focuses on three points: how to configure, how to monitor, and how to optimize.
1. Parameter configuration
In the parameters of the Java virtual machine, there are three representation methods. With the "ps -ef | grep java" command, you can get all the startup parameters and configuration parameters of the current Java process:
- Standard parameters (-), all jvm implementations must implement the functions of these parameters and are backward compatible;
- Non-standard parameters (-X), the default JVM implements the functions of these parameters, but does not guarantee that all JVM implementations are satisfied, and does not guarantee backward compatibility;
- Non-Stable parameters (-XX), such parameters are different for each JVM implementation, may be canceled at any time in the future, and need to be used carefully (however, these parameters are often very useful);
2. Standard parameters
The parameters added after running Java commands, such as java -version, java -jar, etc., enter the command Java -help or java -? to get a list of all Java standard parameters of the current machine.
- -client: Set Java to use client mode, which is generally used on PC machines. It starts quickly, but the performance and memory management efficiency are not high; it is mostly used for desktop applications;
- -server: Using server mode, although the startup speed is slow (about 10% slower than client mode), but the performance and memory management efficiency are very high, suitable for servers, servers used for generation environment, development environment or test environment;
If -server or -client is not specified, the JVM will automatically detect whether the current host is a server when it starts. If it is, it will start in server mode. The 64-bit JVM only has server mode, so the -client parameter cannot be used; by default, different Startup mode, the way to perform GC is different:
| boot mode | Cenozoic GC method | Old generation and persistent generation GC methods |
| client | serial | serial |
| server | parallel | Concurrency |
If no -server or -client mode is specified, the judgment method is as follows:
- -classpath/-cp: The directory path where the JVM loads and searches for files, multiple paths are split with ;. Note that if -classpath is used, the JVM will no longer search the CLASSPATH path defined in the environment variable.
The order of the JVM search path is:
- First search for the jar or zip package that comes with the JVM (Bootstrat, the search path can be obtained with System.getProperty("sun.boot.class.path");
- Search for the jar package under JRE_HOME/lib/ext (Extension, the search path can be obtained with System.getProperty("java.ext.dirs"));
- Search user-defined directories, the order is: current directory (.), CLASSPATH, -cp; (the search path is obtained with System.getProperty("java.class.path"))
- -DpropertyName=value: Define the global property value of the system, such as the configuration file address, etc. If the value has spaces, it can be defined in the form of -Dname="space string", and these can be obtained with System.getProperty("propertyName") The defined property value can also be defined in the code in the form of System.setProperty(""propertyName","value").
- -verbose: This is one of the most commonly used commands to query GC problems. The specific parameters are as follows: ①-verbose: class outputs the relevant information of the jvm loaded class. When the jvm reports that the class cannot be found or the class conflicts, it can be diagnosed. ②-verbose: gc outputs the relevant information of each GC. ③-verbose: jni outputs information about native method calls, which is generally used to diagnose jni call error messages.
2. Non-standard parameters
Non-standard parameters are parameters extended on the basis of standard parameters. Enter the "java -X" command to obtain a list of all non-standard parameters supported by the current JVM. In different types of JVM, the parameters used are different. Please refer to the following figure to have a visual understanding of the size of the memory area:
- -Xmn：新生代内存大小的最大值，包括E区和两个S区的总和。-Xmn只能使用在JDK1.4或之后的版本中，之前的1.3和1.4版本中，可使用-XX:NewSize设置年轻代大小，用-XX:MaxNewSize设置年轻代最大值；如果同时设置了-Xmn和-XX:NewSize,-XX:MaxNewSize，则谁设置在后面，谁就生效；如果同时设置了-XX:newSize -XX:MaxNewSize与-XX:NewRatio则实际生效的值是：min(MaxNewSize,Max(newSize,Heap/(NewRatio+1)))，在开发、测试环境，可以-XX:NewSize和-XX:MaxNewSize来设置新生代大小，但在线上生产环境，使用-Xmn一个即可，或者-XX:NewSize和-XX:MaxNewSize设置为同一个值，这样能够防止在每次GC之后都要调整堆的大小(即：抖动，抖动会严重影响新能)。
- -Xms：初始堆的大小，也是堆大小的最小值，默认值是总共的物理内存/64(且小于1G)，默认情况下，当堆中可用内存小于40%(这个值可以用-XX：MinHeapFreeRatio调整)时，堆内存会开始增加，一直增加到-Xmx的大小；
- -Xmx:堆的最大值，默认值是总共的物理内存/64(且小于1G)，如果Xms和Xmx都不设置，则两者大小会相同，默认情况下，当堆中可用内存大于70%时，堆内存会开始减少，一直减少到-Xms的大小；这个堆的大小=年轻代大小+年老代大小，堆的大小不包括持久代的大小，如果增大了年轻代，年老代会相应减少，官方默认的设置年老代大小/年轻代大小=2/1左右（使用-XX:NewRatio可以设置）。建议在开发测试环境可以用Xms和Xmx分别设置大小值最大值，但是在生产环境上，Xms和Xmx设置的值必须一样，原因与年轻代一样--防止抖动。
- -Xss：这个参数用于设置每个线程的栈内存，默认1M，一般来说是不需要修改的。除非代码不多，可以设置的小点，另外一个相似的参数是-XX:ThreadStackSize，这两个参数在1.6以前，都是谁设置在后面，谁就生效；1.6版本以后，-Xss设置在后面，则以-Xss为准，-XXThreadStackSize设置在后面，则主线程以-Xss为准，其他线程以-XX:ThreadStackSize为准。
- -Xrs：减少JVM对操作系统信号的使用，当此参数设置之后，JVM将不接收控制台的控制handler，以防止与在后台以服务形式运行的JVM冲突。
- -Xprof：跟踪正运行的程序，并将跟踪数据在标准输出输出；适合于开发环境调试；
- -Xnoclassgc：关闭针对class的gc功能；因为其阻止内存回收，所以可能会导致OutOfMemoryError错误，慎用。
- -Xincgc：开启增量gc（默认为关闭）；这有助于减少长时间GC时应用程序出现的停顿；但由于可能和应用程序并发执行，所以会降低CPU对应的处理能力。
- -Xloggc:file：与-verbose:gc功能类似，只是将每次GC事件的相关记录到一个文件中，文件的位置最好在本地，以避免网络的潜在问题。若与verbose命令同时出现在命令行中，则以-Xloggc为准。
三、非Stable参数（非静态参数）
以-XX表示的非Stable参数，虽然在官方文档中是不正确的，不健壮的，各个公司的实现也各不相同，但往往非常实用，所以这部分参数对于GC非常重要。JVM（Hotspot)中主要的参数可以大致分为3类：
- 性能参数（Performance Options）：用于JVM的性能调优和内存分配控制，如初始化内存大小的设置；
- 行为参数（Behavioral Options):用于改变JVM的基础行为，如GC的方法和算法的选择；
- 调试参数（Debugging Options）：用于监控、打印、输出等JVM参数，用于显示JVM更详细的信息；
对于非Stable参数，使用方法有4种：
- -XX:+<option>启用选项
- -XX:-<option>不启用选项
- -XX:<option>=<number>给选项设置一个数字类型值，可跟单位，例如32k,1024m,2g
- -XX:<option>=<String>给选项设置一个字符串值，例如-XX:HeapDumpPath=./dump.core
首先介绍性能参数，性能参数往往用来定义内存分配的大小和比例，相比于行为参数和调试参数。一个比较明显的区别是性能参数后面往往跟的有数值，常用如下：
常用的行为参数，主要用来选择使用什么样的垃圾收集器组合，以控制运行中的GC策略。
常用的调试参数，用于监控和打印GC的信息：
这些参数将为我们进行GC的监控与调试提供很大帮助，是我们进行GC相关操作的重要工具。
转自:http://www.cnblogs.com/zhguang/p/Java-JVM-GC.html