Java Heap Space Error - How To Increase Maximum Amount Able To Be Reserved? [Cannot Exceed 1505M]

Question

I am running 64-bit windows 7 with 4GB of RAM. I have 32-bit java I am trying to run a graph search algorithm in eclipse. I commented absolutely everything out except for a simple println("Hello World")
After a lot of tinkering, I found that I cannot reserve more than 1505M-1507M (it varies between that-- I've no idea why). That is to say, I set the following as my JVM arguments:

-Xms1505M

I read online that I should be able to reserve a maximum of 2G. A quick ctrl-alt-del check showed that I have 2400M available and 1200 cached.
Here is where things get strange: As a stupid experiment, I opened 50 tabs of on google chrome such that I had 400 available memory, 450 cached. I ran my eclipse program with the flag above and it still ran. I reserved 1500M of non-existent RAM.
Someone please help! This program is for a grade and I've been stuck on this for hours.

Answer 1

A 32-bit windows program runs in an emulated 32-bit environment which is designed to work just like Windows XP for compatibility. This means it also has the same limitations as 32-bit windows and you cannot have a heap larger than 1.2 - 1.4 GB depending on what you have run before.

The simplest solution is; don't use 32-bit Java. The 64-bit Java will run better/faster unless you are forced to use 32-bit DLL. In that case I suggest you have one JVM running in 32-bit and you communicate (RMI/messaging/shared memory) with it from a 64-bit program which does all the real work.

I read online that I should be able to reserve a maximum of 2G.

That was never possible with windows 32-bit in Java. The problem was that the heap had to be continous and use what memory was left after all the shareed libraries were loaded.

A quick ctrl-alt-del check showed that I have 2400M available and 1200 cached.

Time to get some more memory I think. I wouldn't buy a laptop with less than 8 GB if you want to use memory seriously and I wouldn't buy a PC with less than 32 GB.

Here is where things get strange: As a stupid experiment, I opened 50 tabs of on google chrome such that I had 400 available memory, 450 cached. I ran my eclipse program with the flag above and it still ran. I reserved 1500M of non-existent RAM.

The OS has access to more memory, it just won't let you use it in a 32-bit emulation. You can have 32 GB of main memory and a 32-bit JVM still will not be able to allocate more.

Answer 2

-Xms is to set the minimum heap, in your case you need to change the max heap using -Xmx

There are other posts here in SO that discuss -Xms vs -Xmx, here is one of them

Answer 3

An operating system with virtual memory can perform strange tricks, and the memory-usage statistics may not always tell you what you think they are. Some of the memory may be swapped out to disk, which sounds like what you're describing here, but some of the memory that's listed for each program is actually shared (e.g., copies of system libraries that are used by each program, but only one copy is loaded in memory).

The more fundamental question is why your graph algorithm is taking up such an inordinate amount of memory; unless you're trying to work on the global Internet routing table, you're probably implementing the algorithm incorrectly.