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  How to get the size of a primitive/object?  (Read 644 times)
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Offline Ecumene
« Posted 2014-07-12 17:14:03 »

Is there any way I could tell the size of an object/primitive inside of the ram? I may be completely incorrect on how this works. Also, is there any way to tell how much ram is being used by your program?
Offline Rayvolution

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« Reply #1 - Posted 2014-07-12 17:21:26 »

For specific object usage, find a good profiler. There are many out there. YourKit Java Profiler is awesome, but it's expensive (they do offer a trial though) Smiley

There's also many ways to figure out how much ram you're using (in general) without a profiler, you can see how much the JVM is using (but that doesn't really mean the game is actually using though) by hitting CTRL+Alt+Delete and checking the taskbar. But, if you want something more accurate without using a profiler, what I did was add a debug menu to my game, inside I have a few checkers that show memory usage in megabytes. Mine prints out to a drawString, I edited it to output a println though. But you can stick it in a drawString and display it in your game easily enough if you want.

EDIT: Code corrections
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public class YourClass{
   private Runtime runtime = Runtime.getRuntime();

private void grabMemUsage(){
   System.out.println("Free Memory: "+(runtime.freeMemory() / (1024*1024)));
   System.out.println("Total Memory: "+(runtime.totalMemory() / (1024*1024)));
   System.out.println("Maximum Memory: "+(runtime.maxMemory() / (1024*1024)));
}

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Offline princec

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« Reply #2 - Posted 2014-07-12 20:47:13 »

Is there any way I could tell the size of an object/primitive inside of the ram? I may be completely incorrect on how this works. Also, is there any way to tell how much ram is being used by your program?
Heh, accidentally whacked "Appreciate" instead of "Quote". You're welcome Wink

Primitives are all of a specific size according to the Java Language Spec (eg. 4 bytes for an int or float, 8 for longs and doubles, etc). Hotspot has documented somewhere what the overhead is for objects and arrays (which for example in the 32-bit JVM, is 8 bytes overhead for an ordinary Object and 12 bytes for an array). It's possibly the same for 64-bit with compressed OOPs, and possibly twice as big without.

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Offline GoToLoop

Junior Devvie


Medals: 2
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« Reply #3 - Posted 2014-07-12 21:06:12 »

http://docs.Oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
http://Java-Performance.info/overview-of-memory-saving-techniques-java/
Offline GoToLoop

Junior Devvie


Medals: 2
Exp: 1 year



« Reply #4 - Posted 2014-07-12 21:37:20 »

According to the link I've provided, an object instance w/o any instance variables consumes 12 bytes.  
But due to 8 byte alignment, it jumps to 16 bytes. Also, generally we're gonna need to store its reference.
It means more 4 extra bytes. In short 16 bytes for each object at minimum + 4 bytes for each of its reference!   persecutioncomplex

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int bytes = 30;  // 12 overhead + 18 instance variables.
int align = (bytes-1)/8*8 + 8;
System.out.println(align); // 32 bytes consumed.
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