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javaarraysmemoryram
Christian Daley
Christian Daley

Reputation: 789

Using too much Ram in Java

I'm writing a program in java which has to make use of a large hash-table, the bigger the hash-table can be, the better (It's a chess program :P). Basically as part of my hash table I have an array of "long[]", an array of "short[]", and two arrays of "byte[]". All of them should be the same size. When I set my table size to ten-million however, it crashes and says "java heap out of memory". This makes no sense to me. Here's how I see it:

1 Long + 1 Short + 2 Bytes = 12 bytes
x 10,000,000 = 120,000,000 bytes
/ 1024 = 117187.5 kB
/ 1024 = 114.4 Mb

Now, 114 Mb of RAM doesn't seem like too much to me. In total my CPU has 4Gb of RAM on my mac, and I have an app called FreeMemory which shows how much RAM I have free and it's around 2Gb while running this program. Also, I set the java preferences like -Xmx1024m, so java should be able to use up to a gig of memory. So why won't it let me allocate just 114Mb?

Upvotes: 0

Views: 1726

Answers (4)

Peter Lawrey
Peter Lawrey

Reputation: 533870

You predicted that it should use 114 MB and if I run this (on a windows box with 4 GB)

public static void main(String... args) {
    long used1 = memoryUsed();
    int Hash_TABLE_SIZE = 10000000;
    long[] pos = new long[Hash_TABLE_SIZE];
    short[] vals = new short[Hash_TABLE_SIZE];
    byte[] depths = new byte[Hash_TABLE_SIZE];
    byte[] flags = new byte[Hash_TABLE_SIZE];
    long used2 = memoryUsed() - used1;
    System.out.printf("%,d MB used%n", used2 / 1024 / 1024);
}

private static long memoryUsed() {
    return Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
}

prints

114 MB used

I suspect you are doing something else which is the cause of your problem.

I am using Oracle HotSpot Java 7 update 10

Upvotes: 1

Marko Topolnik
Marko Topolnik

Reputation: 200296

From what I have read about BlueJ, and serious technical information is almost impossible to find, BlueJ VM is quite likely not to support primitive types at all; your arrays are actually of boxed primitives. BlueJ uses a subset of all Java features, with emphasis on object orientation.

If that is the case, plus taking into consideration that performance and efficiency are quite low on BlueJ VM's list of priorities, you may actually be using quite a bit more memory than you think: a whole order of magnitude is quite imaginable.

Upvotes: 0

ggrandes
ggrandes

Reputation: 2143

Has not taken into account that each object is a reference and also use memory, and more "hidden things"... we must also take into account also the alignment... byte is not always a byte ;-)

To see how much memory is really in use, you can use a profiler:

If you are using standard HashMap (or similar from JDK), each "long" (boxing/unboxing) really are more than 8bytes), you can use this as a base... (use less memory)

Upvotes: 0

Layla
Layla

Reputation: 5476

I believe one way it would be to clean the heap memory after each execution, one link is here:

Java heap space out of memory

Upvotes: -2

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