Why does Java use so much more memory on my Linux server?

Question

I have two Java programs. On my computer, one of them uses 9MB of RAM and the other uses 77MB. But when I upload them to a server, the same programs use 382MB and 186MB! Is there a way to stop this from happening?

Answer 1

1. How do you measure the memory usage in each case? Different operating systems have a different concept of what constitutes "memory usage".

2. 64-bit systems require more memory than 32-bit systems due to the increased pointer (reference in Java speak) size. Are you using a 32-bit OS and JVM on your desktop computer?

3. Are you using different JVM options? A moderately active Java application usually ends up using all the memory that is permitted by the -Xmx option, to minimize the CPU time spent on garbage collection. In addition, the default maximum heap space is determined in relation to the available physical memory - if the server has more memory, the Java applications are bound to use more memory as well.

4. Server JVMs (see the -server JVM option) have different settings and favor performance over memory usage. The -server option is the default on 64-bit systems.

5. Are you absolutely certain that the application load is the same in both cases?

Answer 2

It is quite common for applications to allocate virtual memory in large chunks to improve performance and efficiency. Nobody bothers to optimize such things because they have no effect. If you don't actually have a problem, there's nothing to fix.

Virtual memory is not a scarce resource. Attempting to reduce the consumption of vm is wasted effort.

Answer 3

How did you measure that numbers?

Comparing the numbers of Windows Task Manager and ps(1) on Linux is futile because they are computed differently. For example shared libraries and shared memory are attributed differently on both platforms. Also the memory management is completely different.

If, on the other hand, you refer to numbers gathered from within your apps via Runtime (or similar), then you have to look how the JVM was started with what parameters. Most important are the parameters -Xmx and -Xms but you might lookup several others in the doc of either java or javaw .

Related to point 1:

How to measure actual memory usage of an application or process?

Answer 4

除非您进行了明确设置（例如-Xmx128M之类的命令行参数），否则JVM的默认最大堆大小取决于可用的RAM数量 。