JVM parameter configuration details

1. Common parameter configuration

• -XX:+PrintGC       Every time a trigger GC When printing related logs
• -XX:+UseSerialGC       Serial recovery
• -XX:+PrintGCDetails  More detailed GC journal
• -Xms               Initial value of pile
• -Xmx               Maximum heap availability
• -Xmn               Maximum usable value of the new generation reactor
• -XX:SurvivorRatio  For setting up the Cenozoic eden Space and from/to The proportion of space .
• -XX:NewRatio       Allocate the proportion of new generation and old generation 1:2
• Including -XX:SurvivorRatio=eden/from=den/to

summary : In practice , We can directly equal the initial heap size to the maximum heap size , This advantage is to reduce the number of garbage collection when the program is running , To improve efficiency .

2. Heap memory size configuration

Examples of use :  -Xmx20m -Xms5m 

explain ： The present Java The maximum available memory of the application is 20M, The initial memory is 5M

// byte[] b = new byte[4 * 1024 * 1024]; // System.out.println(" Allocated 4M Space to array "); System.out.print(" Maximum memory "); System.out.println(Runtime.getRuntime().maxMemory() / 1024.0 / 1024 + "M"); System.out.print(" Available memory "); System.out.println(Runtime.getRuntime().freeMemory() / 1024.0 / 1024 + "M"); System.out.print(" Memory used "); System.out.println(Runtime.getRuntime().totalMemory() / 1024.0 / 1024 + "M");

 

3. Set the Cenozoic scale parameters

Examples of use :-Xms20m -Xmx20m -Xmn1m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC

explain ： Heap memory initialization value 20m, Maximum heap memory 20m, New generation maximum available 1m,eden Space and from/to The proportion of space is 2/1

Examples of use : -Xms20m -Xmx20m -XX:SurvivorRatio=2 -XX:+PrintGCDetails -XX:+UseSerialGC

4. Set the proportion parameters of the new generation and the old generation

-XX:NewRatio=2

explain ： Heap memory initialization value 20m, Maximum heap memory 20m, New generation maximum available 1m,eden Space and from/to The proportion of space is 2/1

The proportion of the new generation and the old generation is 1/2

5. The difference between memory overflow and memory leak

Java Memory leak is the failure to clean up memory garbage in time , As a result, the system can no longer provide you with memory resources （ Out of memory ）;

and Java Memory overflow is when you ask for more memory than the system can give you , The system can't meet the demand , So there's overflow .

out of memory , That's easy to understand , The storage space is not large enough . It's like pouring too much water , It's spilled from the top of the cup .
Memory leak , The principle is , Used memory space is not released in time , Take up memory for a long time , Eventually, there's not enough memory , And there's a memory overflow .

6. actual combat OutOfMemoryError abnormal

 

Java Heap overflow

The reason for the error : java.lang.OutOfMemoryError: Java heap space Heap memory overflow

terms of settlement : Set heap memory size // -Xms1m -Xmx10m -XX:+PrintGCDetails -XX:+HeapDumpOnOutOfMemoryError

// -Xms1m -Xmx10m -XX:+PrintGCDetails -XX:+HeapDumpOnOutOfMemoryError List<Object> listObject = new ArrayList<>(); for (inti = 0; i < 10; i++) { System.out.println("i:" + i); Byte[] bytes = new Byte[1 * 1024 * 1024]; listObject.add(bytes); } System.out.println(" Add success ...");

 

Virtual machine stack overflow

The reason for the error : java.lang.StackOverflowError  Stack memory overflow

Stack overflow   Generated from recursive calls , Loop traversal will not , But recursive calls are generated in loop methods ,  Stack overflow can also occur .

terms of settlement : Set thread maximum call depth

-Xss5m Set the maximum call depth

publicclass JvmDemo04 {            privatestaticintcount;            publicstaticvoid count(){                    try {                               count++;                               count();                    } catch (Throwable e) {                              System.out.println(" Maximum depth :"+count);                              e.printStackTrace();                    }            }            publicstaticvoid main(String[] args) {                     count();           } }

 

7 The difference between memory overflow and memory leak

Java Memory leak is the failure to clean up memory garbage in time , As a result, the system can no longer provide you with memory resources （ Out of memory ）;

and Java Memory overflow is when you ask for more memory than the system can give you , The system can't meet the demand , So there's overflow .

out of memory , That's easy to understand , The storage space is not large enough . It's like pouring too much water , It's spilled from the top of the cup .
Memory leak , The principle is , Used memory space is not released in time , Take up memory for a long time , Eventually, there's not enough memory , And there's a memory overflow .