Outdated: GC Test - Constant Memory Requirement

This section provides a tutorial example on testing garbage collection with a constant memory requirement.

I compiled GCTest.java with J2SDK 1.4.0_02, and executed it with:

java -Xms2m -Xmx8m GCTest 16 1 > tail_16_01.out

I got the following output:

Time   Total   Free   Free   Total   Act.   Dead   Over
sec.   Mem.    Mem.   Per.   Obj.    Obj.   Obj.   Head

   0    1984   1727    87%       0      0      0    257 
   0    1984   1593    80%     128    128      0    263 
   1    1984   1592    80%     128    128      0    264 
   1    1984   1462    73%     256    256      0    266 
   2    1984   1332    67%     384    384      0    268 
   2    1984   1202    60%     512    512      0    270 
....                                                  
   8    3124   1062    33%    1920   1920      0    142 
   8    3124    932    29%    2048   2048      0    144 
   9    3124    803    25%    2176   2176      0    145 
   9    3124    801    25%    2176   2048    128    147 
  10    3124    671    21%    2304   2176    128    149 
  10    3124    669    21%    2304   2048    256    151 
  11    3124    540    17%    2432   2176    256    152 
  11    3124    538    17%    2432   2048    384    154 
  12    3124    408    13%    2560   2176    384    156 
  12    3124    406    12%    2560   2048    512    158 
  13    3124    277     8%    2688   2176    512    159 
  13    3124    275     8%    2688   2048    640    161 
  14    5256   2303    43%    2176   2176      0    777 
  14    5256   2301    43%    2176   2048    128    779 
  15    5256   2171    41%    2304   2176    128    781 
  15    5256   2170    41%    2304   2048    256    782 
  16    5256   2040    38%    2432   2176    256    784 
  16    5256   2038    38%    2432   2048    384    786 
....                                                
  29    5256    330     6%    4096   2176   1920    830 
  29    5256    329     6%    4096   2048   2048    831 
  30    5256    199     3%    4224   2176   2048    833 
  30    5256    197     3%    4224   2048   2176    835 
  31    7848   3358    42%    2176   2176      0   2314
  31    7848   3357    42%    2176   2048    128   2315
  32    7848   3227    41%    2304   2176    128   2317
  32    7848   3225    41%    2304   2048    256   2319
  33    7848   3095    39%    2432   2176    256   2321
  33    7848   3094    39%    2432   2048    384   2322
....                                                  
  54    7848    334     4%    5120   2176   2944   2394
  55    7848    332     4%    5120   2048   3072   2396
  55    7848    203     2%    5248   2176   3072   2397
  56    7848    201     2%    5248   2048   3200   2399
  56    8128   2614    32%    2176   2176      0   3338
  57    8128   2612    32%    2176   2048    128   3340
  57    8128   2482    30%    2304   2176    128   3342
  58    8128   2481    30%    2304   2048    256   3343
  58    8128   2351    28%    2432   2176    256   3345
  59    8128   2349    28%    2432   2048    384   3347
....                                                  
  73    8128    378     4%    4352   2176   2176   3398
  74    8128    377     4%    4352   2048   2304   3399
  74    8128    247     3%    4480   2176   2304   3401
  75    8128    245     3%    4480   2048   2432   3403
  75    8128   3382    41%    2176   2176      0   2570
  76    8128   3381    41%    2176   2048    128   2571
  76    8128   3251    39%    2304   2176    128   2573
  77    8128   3249    39%    2304   2048    256   2575
  77    8128   3119    38%    2432   2176    256   2577
  78    8128   3118    38%    2432   2048    384   2578
....                                                  
  98    8128    358     4%    5120   2176   2944   2650
  99    8128    356     4%    5120   2048   3072   2652
  99    8128    226     2%    5248   2176   3072   2654
 100    8128    225     2%    5248   2048   3200   2655
 100    8128   2614    32%    2176   2176      0   3338
 101    8128   2612    32%    2176   2048    128   3340
 101    8128   2482    30%    2304   2176    128   3342
 102    8128   2481    30%    2304   2048    256   3343
 102    8128   2351    28%    2432   2176    256   3345
 103    8128   2349    28%    2432   2048    384   3347
....                                                  
 117    8128    378     4%    4352   2176   2176   3398
 118    8128    376     4%    4352   2048   2304   3400
 118    8128    247     3%    4480   2176   2304   3401
 119    8128    245     3%    4480   2048   2432   3403
 119    8128   3382    41%    2176   2176      0   2570
 120    8128   3380    41%    2176   2048    128   2572
 120    8128   3251    39%    2304   2176    128   2573
 121    8128   3249    39%    2304   2048    256   2575
 121    8128   3119    38%    2432   2176    256   2577
 122    8128   3117    38%    2432   2048    384   2579
....                                                  
 142    8128    358     4%    5120   2176   2944   2650
 143    8128    356     4%    5120   2048   3072   2652
 143    8128    226     2%    5248   2176   3072   2654
 144    8128    224     2%    5248   2048   3200   2656
 144    8128   2614    32%    2176   2176      0   3338
 145    8128   2612    32%    2176   2048    128   3340
 145    8128   2482    30%    2304   2176    128   3342
 146    8128   2481    30%    2304   2048    256   3343
 146    8128   2351    28%    2432   2176    256   3345
 147    8128   2349    28%    2432   2048    384   3347
....                                                  
 161    8128    378     4%    4352   2176   2176   3398
 162    8128    376     4%    4352   2048   2304   3400
 162    8128    247     3%    4480   2176   2304   3401
 163    8128    245     3%    4480   2048   2432   3403
 163    8128   3382    41%    2176   2176      0   2570
 164    8128   3380    41%    2176   2048    128   2572
 164    8128   3251    39%    2304   2176    128   2573
 165    8128   3249    39%    2304   2048    256   2575
 165    8128   3119    38%    2432   2176    256   2577
 166    8128   3117    38%    2432   2048    384   2579
....                                                  
 186    8128    358     4%    5120   2176   2944   2650
 187    8128    356     4%    5120   2048   3072   2652
 187    8128    226     2%    5248   2176   3072   2654
 188    8128    224     2%    5248   2048   3200   2656
 188    8128   2614    32%    2176   2176      0   3338
 189    8128   2612    32%    2176   2048    128   3340
 189    8128   2482    30%    2304   2176    128   3342
 190    8128   2481    30%    2304   2048    256   3343
 190    8128   2351    28%    2432   2176    256   3345
 191    8128   2349    28%    2432   2048    384   3347
....                                                  
 205    8128    378     4%    4352   2176   2176   3398
 206    8128    376     4%    4352   2048   2304   3400
 206    8128    247     3%    4480   2176   2304   3401
 207    8128    245     3%    4480   2048   2432   3403
 207    8128   3382    41%    2176   2176      0   2570
 208    8128   3380    41%    2176   2048    128   2572
 208    8128   3251    39%    2304   2176    128   2573
 209    8128   3249    39%    2304   2048    256   2575
 209    8128   3119    38%    2432   2176    256   2577
 210    8128   3117    38%    2432   2048    384   2579
....                                                  
 230    8128    486     5%    4992   2048   2944   2650
 231    8128    356     4%    5120   2176   2944   2652
 231    8128    226     2%    5248   2176   3072   2654
 232    8128    224     2%    5248   2048   3200   2656
 232    8128   2614    32%    2176   2176      0   3338
 233    8128   2612    32%    2176   2048    128   3340
 233    8128   2482    30%    2304   2176    128   3342
 234    8128   2481    30%    2304   2048    256   3343
 234    8128   2351    28%    2432   2176    256   3345
 235    8128   2349    28%    2432   2048    384   3347
....                                                  
 250    8128    376     4%    4352   2048   2304   3400
 250    8128    247     3%    4480   2176   2304   3401
 251    8128    245     3%    4480   2048   2432   3403
 251    8128   3382    41%    4608   2048   2560    138 
 252    8128   3380    41%    2176   2176      0   2572
 252    8128   3379    41%    2176   2048    128   2573
 253    8128   3249    39%    2304   2176    128   2575
 253    8128   3247    39%    2304   2048    256   2577
 254    8128   3117    38%    2432   2176    256   2579
 254    8128   3116    38%    2432   2048    384   2580
....

As the output shows, the JVM (HotSpot) started with 2MB of total memory, with about 87% of free memory. As the program starts to allocate objects during the initialization phase, free memory starts to decrease. I don't know why.

When the free memory decreased to 14%, about 294KB, still enough for 2 more objects, the JVM decided to request more total memory from the operating system.

At second 8, the program finished the initialization phase, and entered into the loop testing phase. It started to allocate one object, then release the oldest object from the list. The released objects became dead objects and waiting to be collected as garbage.

As you can see from the output, the dead objects were not immediately collected by the garbage collector. The first collection of dead objects happened at second 14, when free memory decreased to 8%, about 275KB. The JVM collected the dead objects and request more total memory at about the same time. I don't know why.

As the program continued to repeatedly allocate and release objects, dead objects were accumulated again, and the free memory continued to decrease. When free memory decreased to 3%, 197KB, at second 30, the JVM decided to collect dead objects and request more total memory again.

The next time the JVM decided to collect dead objects and request more total memory happened at second 56. Total memory reached 8128KB, about the same as the maximum memory size specified by the command line option, -Xmx8m.

When the free memory decreased again to 3% at second 75, the JVM could not get any more total memory. It only collected the dead objects, and raised the free memory back to 41%.

While reviewing the rest of the output, I noticed that at second 100, the JVM collected the dead objects with 3200KB of memory. However, the free memory reported by the JVM only increased by 2389KB (2614KB-225KB). So where is the other 800KB collected from the dead objects? Can any one explain this?

19 seconds later, the JVM collected only 2432KB of dead objects, but the free memory increased by 3137KB. This time, about 700KB more memory comes out from nowhere. May be certain amount of memory was reserved at the previous garbage collection time.

After looking at the over head memory size, I found that each time the JVM collected garbage, it also reserved the same amount of memory as the garbage collected. This explains well why at second 100, 800KB of collected memory was not giving back as free memory. At that time, 8128 (total memory) - 2176 (active objects) - 3338 (reserved memory) = 2614 (free memory), where the reserved memory (3338KB) is about the same as the collected memory (3200KB).

Couple of quick observations:

Last update: 2003.

Table of Contents

 About This Book

 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

 Memory Management and Garbage Collectors

 Garbage Collection Tests

 JVM Stack, Frame and Stack Overflow

 Thread Testing Program and Result

 CPU Impact of Multi-Thread Applications

 I/O Impact of Multi-Thread Applications

 CDS (Class Data Sharing)

 Micro Benchmark Runner and JVM Options

 Micro Benchmark Tests on "int" Operations

 Micro Benchmark Tests on "long" Operations

 Micro Benchmark Tests in JIT Compilation Mode

 Micro Benchmark Tests on "float" and "double" Operations

Outdated Tutorials

 Outdated: Download and Install Java SE 1.6 Update 2

 Outdated: Installing JRockit JVM 8.0

 Outdated: Testing with LongWhile.java

 Outdated: Testing with LongSleep.java

 Outdated: GCTest.java - Garbage Collection Test Program

Outdated: GC Test - Constant Memory Requirement

 Outdated: GC Test - Periodical Memory Requirement

 Outdated: GC Test - Releasing Old vs. New Objects

 Outdated: GC Test - JDK 1.4.0 vs. JDK 1.3.1

 Outdated: GC Test - Client vs. Server

 Outdated: StringBuffer Testing Program

 Outdated: Installing JRockit JVM 7.0

 Outdated: Running JRockit JVM with Management Console

 References

 PDF Printing Version