640/400/40MB Test on HotSpot Client 1.7

JVM Tutorials - Herong's Tutorial Examples

∟640/400/40MB Test on HotSpot Client 1.7

This section provides a tutorial example on running GCTest2.java on HotSpot Client JVM 1.7.0_45 with 640MB memory limit and a 400MB base plus 40MB fluctuating memory usage with object size of 1.25MB.

Here is my first test with GCTest2.java with HotSpot Client 1.7.0_45 and maximum memory of 640MB on my Windows 7 system:

>\Progra~1\java\jdk1.7.0_45\bin\javac GCTest2.java

>\Progra~1\java\jdk1.7.0_45\bin\java -Xmx640m GCTest2
Test parameters:
   Object size: 1280KB
   Initial objects and data size: 320, 409600KB
   Added objects and data size: 32, 40960KB
Time   Total    Free    Used  Free  Total    Act.    Dead    Over
sec.    Mem.    Mem.    Mem.    %.   Obj.    Obj.    Obj.    Head
   1  633536  182776  450760  28%  450560  409600   40960     200
   3  633536  141817  491719  22%  450560  409600   40960   41159
   4  633536  100857  532679  15%  491520  409600   81920   41159
   5  633536   59896  573640   9%  532480  409600  122880   41160
   6  633536   18936  614600   2%  573440  409600  163840   41160
   7  633536    7417  626119   1%  450560  409600   40960  175559
   8  633536  141829  491707  22%  450560  409600   40960   41147
   9  633536  100868  532668  15%  491520  409600   81920   41148
  10  633536   59908  573628   9%  532480  409600  122880   41148
  11  633536   18939  614597   2%  573440  409600  163840   41157
  12  633536   18944  614592   2%  450560  409600   40960  164032
  14  633536  141833  491703  22%  450560  409600   40960   41143
  15  633536  100869  532667  15%  491520  409600   81920   41147
  16  633536   59908  573628   9%  532480  409600  122880   41148
  17  633536   18944  614592   2%  573440  409600  163840   41152
  18  633536   18947  614589   2%  450560  409600   40960  164029
  19  633536  141834  491702  22%  449280  409600   39680   42422
  20  633536  100872  532664  15%  490240  409600   80640   42424
  21  633536   59907  573629   9%  531200  409600  121600   42429
  22  633536   18945  614591   2%  572160  409600  162560   42431
  23  633536  182796  450740  28%  449280  409600   39680    1460
  25  633536  141833  491703  22%  490240  409600   80640    1463
  26  633536  100871  532665  15%  531200  409600  121600    1465
  27  633536   59906  573630   9%  572160  409600  162560    1470
  28  633536   18944  614592   2%  613120  409600  203520    1472
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at GCTest2$MyObject.<init>(GCTest2.java:70)
        at GCTest2.myTest(GCTest2.java:37)
        at GCTest2.main(GCTest2.java:22)

I am very surprised to see OutOfMemoryError exception. Let's review the output to understand what happened in the JVM. Note that the output was reformatted with white spaces.

At second 1, the "while" loop already finished 1 step, leaving 32 dead objects, 40960KB, to be freed by the GC.
At second 6, these is only %2 of free memory with 163840KB to be freed. Noticed that the "overhead" memory usage went up from 200KB to 41160KB. May be this is due to other JVM system threads started to run and took about 41MB.
At second 7, the GC process performed successfully and removed all dead objects freeing almost 163840KB. But the "overhead" usage went up again to 175559KB, taking more memory than what GC released. The resulting free memory was only about 1%.
The JVM continued to perform GC on dead objects and some "overhead" memory to maintain free memory between 22% and 2%.
At second 23, GC was very lucky to free up almost all dead objects and all "overhead" memory and restored free memory to 28%.
Then at second 28, a "disaster" happened. The free memory is about 2% and dead objects occupied 203520KB. The JVM decided not to perform GC and let the program to ran out of memory. Or may be the JVM did not have enough memory to run the GC, since the "overhead" memory is only 1472KB.

I repeated this test many times. "OutOfMemoryError" exception occured about 70% of the time.

Conclusion, HotSpot Client JVM 1.7.0_45 requires more than 640MB to run a program that has a 400MB base memory usage plus 40MB fluctuating memory usage with object size of 1.25MB. Basically, the JVM requires more than 200MB to manage dead objects and other system tasks.

Last update: 2014.

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Downloading and Installing JDK 1.8.0 on Windows

Downloading and Installing JDK 1.7.0 on Windows

java.lang.Runtime Class - The JVM Instance

java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

Class Class - Class Reflections

Sun's JVM - Java HotSpot VM

JRockit JVM 28.2.7 by Oracle Corporation

JVM Runtime Data Areas