Java 8 中的默认 Xmxsize（最大堆大小）

Question

In the oracle documentation I found:

-Xmxsize Specifies the maximum size (in bytes) of the memory allocation pool in bytes ... The default value is chosen at runtime based on system configuration.

What does system configuration mean?

Answer 1

It varies on implementation and version, but usually it depends on the VM used (eg client or server, see -client and -server parameters) and on your system memory.

Often for client the default value is 1/4th of your physical memory or 1GB (whichever is smaller).

Also Java configuration options (command line parameters) can be "outsourced" to environment variables including the -Xmx , which can change the default (meaning specify a new default). Specifically the JAVA_TOOL_OPTIONS environment variable is checked by all Java tools and used if exists (more details here and here ).

You can run the following command to see default values:

java -XX:+PrintFlagsFinal -version

It gives you a loooong list, -Xmx is in MaxHeapSize , -Xms is in InitialHeapSize . Filter your output (eg |grep on linux) or save it in a file so you can search in it.

Answer 2

Like you have mentioned, The default -Xmxsize (Maximum HeapSize) depends on your system configuration.

Java8 client takes Larger of 1/64th of your physical memory for your Xmssize (Minimum HeapSize) and Smaller of 1/4th of your physical memory for your -Xmxsize (Maximum HeapSize).

Which means if you have a physical memory of 8GB RAM, you will have Xmssize as Larger of 8*(1/64) and Smaller of -Xmxsize as 8*(1/4).

You can Check your default HeapSize with

In Windows :

java -XX:+PrintFlagsFinal -version | findstr /i "HeapSize PermSize ThreadStackSize"

In Linux :

java -XX:+PrintFlagsFinal -version | grep -iE 'HeapSize|PermSize|ThreadStackSize'

These default values can also be overrided to your desired amount.

Answer 3

Surprisingly this question doesn't have a definitive documented answer. Perhaps another data point would provide value to others looking for an answer. On my systems running CentOS (6.8,7.3) and Java 8 (build 1.8.0_60-b27, 64-Bit Server):

default memory is 1/4 of physical memory, not limited by 1GB.

Also, -XX:+PrintFlagsFinal prints to STDERR so command to determine current default memory presented by others above should be tweaked to the following:

java -XX:+PrintFlagsFinal 2>&1 | grep MaxHeapSize

The following is returned on system with 64GB of physical RAM:

uintx MaxHeapSize                                  := 16873684992      {product}

Answer 4

As of 8, May, 2019:

JVM heap size depends on system configuration, meaning:

a) client jvm vs server jvm

b) 32bit vs 64bit .

Links:

1) updation from J2SE5.0: https://docs.oracle.com/javase/6/docs/technotes/guides/vm/gc-ergonomics.html
2) brief answer: https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/ergonomics.html
3) detailed answer: https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/parallel.html#default_heap_size
4) client vs server: https://www.javacodegeeks.com/2011/07/jvm-options-client-vs-server.html

Summary: (Its tough to understand from the above links. So summarizing them here)

1) Default maximum heap size for Client jvm is 256mb (there is an exception, read from links above).

2) Default maximum heap size for Server jvm of 32bit is 1gb and of 64 bit is 32gb (again there are exceptions here too. Kindly read that from the links).

So default maximum jvm heap size is: 256mb or 1gb or 32gb depending on VM, above.

Answer 5

On my Ubuntu VM, with 1048 MB total RAM, java -XX:+PrintFlagsFinal -version | grep HeapSize java -XX:+PrintFlagsFinal -version | grep HeapSize printed : uintx MaxHeapSize := 266338304 , which is approx 266MB and is 1/4th of my total RAM.