memory 之外的堆空间

Question

My application currently consumes quite a lot of memory because it is running physics simulations. The issue is that consistently, at the 51st simulation, Java will throw an error usually because of a heap space out of memory (my program eventually runs thousands of simulations).

Is there anyway I can not just increase the heap space but modify my program so that the heap space is cleared after every run so that I can run an arbitrary number of simulations?

Edit: Thanks guys. Turns out the simulator software wasn't clearing the information after every run and I had those runs all stored in an ArrayList .

Answer 1

There is no way to dynamically increase the heap programatically since the heap is allocated when the Java Virtual Machine is started.

However, you can use this command

java -Xmx1024M YourClass

to set the memory to 1024

or, you can set a min max

java -Xms256m -Xmx1024m YourClassNameHere

Answer 2

If you are using a lot of memory and facing memory leaks, then you might want to check if you are using a large number of ArrayList s or HashMap s with many elements each.

An ArrayList is implemented as a dynamic array . The source code from Sun/Oracle shows that when a new element is inserted into a full ArrayList , a new array of 1.5 times the size of the original array is created, and the elements copied over. What this means is that you could be wasting up to 50% of the space in each ArrayList you use, unless you call its trimToSize method. Or better still, if you know the number of elements you are going to insert before hand, then call the constructor with the initial capacity as its argument.

I did not examine the source code for HashMap very carefully, but at a first glance it appears that the array length in each HashMap must be a power of two, making it another implementation of a dynamic array. Note that HashSet is essentially a wrapper around HashMap .

Answer 3

There are a variety of tools that you can use to help diagnose this problem. The JDK includes JVisualVM that will allow you to attach to your running process and show what objects might be growing out of control. Netbeans has a wrapper around it that works fairly well. Eclipse has the Eclipse Memory Analyzer which is the one I use most often, just seems to handle large dump files a bit better. There's also a command line option, -XX:+HeapDumpOnOutOfMemoryError that will give you a file that is basically a snapshot of your process memory when your program crashed. You can use any of the above mentioned tools to look at it, it can really help a lot when diagnosing these sort of problems.

Depending on how hard the program is working, it may be a simple case of the JVM not knowing when a good time to garbage collect may be, you might also look into the parallel garbage collection options as well.

Answer 4

I also faced the same problem.I resolved by doing the build by following steps as.

-->Right click on the project select RunAs ->Run configurations

Select your project as BaseDirectory. In place of goals give eclipse:eclipse install

-->In the second tab give -Xmx1024m as VM arguments.

Answer 5

I would like to add that this problem is similar to common Java memory leaks.

When the JVM garbage collector is unable to clear the "waste" memory of your Java / Java EE application over time, OutOfMemoryError: Java heap space will be the outcome.

It is important to perform a proper diagnostic first:

• Enable verbose:gc . This will allow you to understand the memory growing pattern over time.
• Generate and analyze a JVM Heap Dump . This will allow you to understand your application memory footprint and pinpoint the source of the memory leak(s).
• You can also use Java profilers and runtime memory leak analyzer such as Plumbr as well to help you with this task.

Answer 6

Try adding -Xmx for more memory ( java -Xmx1024M YourClass ), and don't forget to stop referencing variables you don't need any more (memory leaks).

Answer 7

Are you keeping references to variables that you no longer need (eg data from the previous simulations)? If so, you have a memory leak. You just need to find where that is happening and make sure that you remove the references to the variables when they are no longer needed (this would automatically happen if they go out of scope).

If you actually need all that data from previous simulations in memory, you need to increase the heap size or change your algorithm.

Answer 8

Java is supposed to clear the heap space for you when all of the objects are no longer referenced. It won't generally release it back to the OS though, it will keep that memory for it's own internal reuse. Maybe check to see if you have some arrays which are not being cleared or something.

Answer 9

No. The heap is cleared by the garbage collector whenever it feels like it. You can ask it to run (with System.gc() ) but it is not guaranteed to run.

First try increasing the memory by setting -Xmx256m