Spring Boot memory consumption increases beyond -Xmx option

Question

I noticed Spring Boot application does not obey the amount of memory set via Xmx option. For example: java -Xss64m -Xmx64m -jar test.jar

I also printed on console the amount of memory really used by application at startup, and shows: Max memory: 61M

long maxBytes = Runtime.getRuntime().maxMemory();
System.out.println("Max memory: " + maxBytes / 1024 / 1024 + "M");

When I open Windows processes before accessing any web page, it shows +-105M, so how can Java say 61M?

After accessing any web page it goes from +-125M to +-135M. Why is there such increase? It should give "java.lang.OutOfMemoryError: PermGen space" but do not increase this way.

It makes me get worried, if many applications could run out of memory at the server. By the way, I am using Java 1.8_45

Answer 1

After monitoring the Spring Boot application, I found out some possible reasons such as:

1. Number of http threads (Undertow starts around 50 threads per default, but you can increase / decrease via property the amount of threads needed)
2. Access to native routines (.dll, .so) via JNI
3. Static variables
4. Use of cache (memcache, ehcache, etc)
5. If a VM is 32 bit or 64 bit, 64 bit uses more memory to run the same application, so if you don't need a heap bigger than 1.5GB, so keep your application runnnig over 32 bit to save memory.