On Windows, you can use the following command to find out the defaults on the system where your applications runs.
java -XX:+PrintFlagsFinal -version | findstr HeapSize
Look for the options MaxHeapSize (for -Xmx) and InitialHeapSize for -Xms.
On a Unix/Linux system, you can do
java -XX:+PrintFlagsFinal -version | grep HeapSize
I believe the resulting output is in bytes.
Answer from stones333 on Stack OverflowOn Windows, you can use the following command to find out the defaults on the system where your applications runs.
java -XX:+PrintFlagsFinal -version | findstr HeapSize
Look for the options MaxHeapSize (for -Xmx) and InitialHeapSize for -Xms.
On a Unix/Linux system, you can do
java -XX:+PrintFlagsFinal -version | grep HeapSize
I believe the resulting output is in bytes.
For Java SE 5: According to Garbage Collector Ergonomics [Oracle]:
initial heap size:
Larger of 1/64th of the machine's physical memory on the machine or some reasonable minimum. Before J2SE 5.0, the default initial heap size was a reasonable minimum, which varies by platform. You can override this default using the -Xms command-line option.
maximum heap size:
Smaller of 1/4th of the physical memory or 1GB. Before J2SE 5.0, the default maximum heap size was 64MB. You can override this default using the -Xmx command-line option.
UPDATE:
As pointed out by Tom Anderson in his comment, the above is for server-class machines. From Ergonomics in the 5.0 JavaTM Virtual Machine:
In the J2SE platform version 5.0 a class of machine referred to as a server-class machine has been defined as a machine with
- 2 or more physical processors
- 2 or more Gbytes of physical memory
with the exception of 32 bit platforms running a version of the Windows operating system. On all other platforms the default values are the same as the default values for version 1.4.2.
In the J2SE platform version 1.4.2 by default the following selections were made
- initial heap size of 4 Mbyte
- maximum heap size of 64 Mbyte
Increase heap size in Java - Stack Overflow
Java 21 occupying more memory in ram than the heap size
Java21 impressed memory usage!
Essential JVM Heap Settings: What Every Java Developer Should Know
You can increase to 2GB on a 32 bit system. If you're on a 64 bit system you can go higher. No need to worry if you've chosen incorrectly, if you ask for 5g on a 32 bit system java will complain about an invalid value and quit.
As others have posted, use the cmd-line flags - e.g.
java -Xmx6g myprogram
You can get a full list (or a nearly full list, anyway) by typing java -X.
It is possible to increase heap size allocated by the JVM by using these command line options:
-Xms<size> set initial Java heap size
-Xmx<size> set maximum Java heap size
-Xss<size> set java thread stack size
In the following example, minimum heap size is set to 16mb, and the maximum to 64mb:
java -Xms16m -Xmx64m ClassName
Hi all... I have created a service in Java 21 using the latest springboot version 3.x.x series. When I deploy the service in live. I had allocated 2gb Ram and 1 Core Cpu for the pod. I was using internal cache that is EHCache, this tells why I have used 2gb Ram. After serving the requests for some time, the memory percentage of the pod had reached 95%, this was not expected as it was serving low numberiof requests. So I took a heap and analysed it. Below are the observations.
-
Used heap size is 113mb
-
Large memory object is EHCache 60mb (expected)
-
Unreferenced objects 400mb
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GC algorithm used ( SerialGC) By taking heap dump I could not find much information. But what I observed is much memory objects were unreferenced objects. But GC should have cleared these. I saw online insstackoverflow, articles were telling most of them had faced same problem but did not post solutions to it. Many suggested to use different GC algorithm, so I ran the pod with G1GC algorithm. There was no significant observation seen. I am out of options now. Can somebody help me if they faced same issue and kindly post your solution. Thanks in Advance
Recently, our team updated our Spring Boot service to Java 21 for a project. Since we had already updated to Java 17 with Spring Boot 3.x version, the update to Java 21 was completed very easily, except for some issues with test cases.
However, a very significant change was observed in the memory usage of the service deployed on EKS. The heap memory usage decreased by nearly 50%, and native memory usage reduced by about 30%. We conservatively maintained the existing G1GC for garbage collection, yet the usage still decreased. After monitoring, we plan to halve the requested memory capacity within Docker.
Apart from this, CPU usage remained within the margin of error (we weren't using CPU close to the limit anyway). However, the minor GC count increased significantly.
We believe these effects are due to the preventive G1GC garbage collection patch introduced in Java 20.
We're curious if others have experienced similar changes when updating to Java 21 compared to previous versions.
You could use java -Xmx4096m in order to set your heap to 4 GB.
you could then add export _JAVA_OPTIONS=-Xmx4096m to your shell by their (.bashrc, .zshrc) file.
Hmmmm....Java performance is not necessarily tied to the size of the heap size. By increasing the max size you do prevent one source of out of memory errors (there are other things that generate this java error).
The only efficiency is gained by reducing the number of calls to increase stack memory size if you set the preallocated heap to the maximum size.
Hmm...I guess it might also slow down the garbage collector a little bit since it would not have to react as quickly as a smaller heap size.
Both of the above heap size adjustments are available on the java command line.