Java using too much memory on Linux?

Question

I was testing the amount of memory java uses on Linux. When just staring up an application that does absolutely NOTHING it already reports that 11 MB is in use. When doing the same on a Windows machine about 6 MB is in use. These were measured with the top command and the windows task manager. The VM on linux I use is the 1.6_0_11 one, and the hotspot VM is Server 11.2. Starting the application using -client did not influence anything.

Why does java take this much memory? How can I reduce this?

EDIT: I measure memory using the windows task manager and in Linux I open the terminal and type top.

Also, I am only interested in how to reduce this or if I even CAN reduce this. I'll decide for myself whether a couple of megs is a lot or not. It's just that the difference of 5 MB between windows and Linux is strange, and I want to know if I am able to do this on Linux too.

Answer 1

Both Top and TaskManager will report how much memory has been allocated to a process, not how much the process is actually using, so I would say it's not an apples to apples comparison. Regardless, in the age of Gigs of memory what's a couple megs here or there on startup?

Answer 2

You can control the heap size otherwise default values will be used, java -X gives you an explanation of the meaning of these switches

i.g.

set JAVA_OPTS="-Xms6m -Xmx6m"
java ${JAVA_OPTS} MyClass

Answer 3

If you think 11MB is "too much" memory... you'd better avoid using Java entirely. Seriously, the JVM needs to do quite a lot of stuff (bytecode verifier, GC, loading all the essential classes), and in an age where average desktop machines have 4GB of RAM, keeping the base JVM overhead (and memory use in generay) very low is simply not a design priority.

If you need your app to run on an embedded system (pretty much the only case where 11 MB might legitimately be considered "too much"), then there are special JVMs designed for such sytems that use less RAM - but at the cost of lacking many of the features and/or performance of mainstream JVMs.

Answer 4

Linux and Windows are radically different operating systems and use RAM very differently. Windows kind of allocates as you go, and Linux caches more at once, and prepares for the future, so that the next operations are smooth.

This explanation is not quite right, but it's close enough for you.

Answer 5

The question you might really be asking is, "Does windows task manager and Linux top report memory in the same way?" I'm sure there are others that can answer this question better than I, but I suspect that you may not be doing an apples to apples comparison.

Try using the jconsole application on each respective machine to do a more granular inspection. You'll find jconsole on your sdk under the bin directory.

There is also a very extensive discussion of java memory management at http://www.ibm.com/developerworks/linux/library/j-nativememory-linux/

The short answer is that how memory is being allocated is a more complex answer than just looking at a single figure at the top of a user simplifed system utility.