Question

I run the same process on a 32bit machine as on a 64bit machine with the same memory VM settings (-Xms1024m -Xmx1024m) and similar VM version (1.6.0_05 vs 1.6.0_16). However the virtual space used by the 64bit machine (as shown in top under “VIRT”) is almost three times as big as that in 32bit!

I know 64bit VMs will use a little more memory for the larger references, but how can it be three times as big? Am I reading VIRT in top incorrectly?

Full data shown below, showing top and then the result of jmap -heap, first for 64bit, then for 32bit. Note the VIRT for 64bit is 3319m for 32bit is 1220m.

* 64bit *

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
22534 agent     20   0 3319m 163m  14m S  4.7  2.0   0:04.28 java               

$ jmap -heap 22534
Attaching to process ID 22534, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 10.0-b19

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 1073741824 (1024.0MB)
   NewSize          = 2686976 (2.5625MB)
   MaxNewSize       = -65536 (-0.0625MB)
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 88080384 (84.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 268500992 (256.0625MB)
   used     = 247066968 (235.62142181396484MB)
   free     = 21434024 (20.441078186035156MB)
   92.01715277089181% used
From Space:
   capacity = 44695552 (42.625MB)
   used     = 0 (0.0MB)
   free     = 44695552 (42.625MB)
   0.0% used
To Space:
   capacity = 44695552 (42.625MB)
   used     = 0 (0.0MB)
   free     = 44695552 (42.625MB)
   0.0% used
PS Old Generation
   capacity = 715849728 (682.6875MB)
   used     = 0 (0.0MB)
   free     = 715849728 (682.6875MB)
   0.0% used
PS Perm Generation
   capacity = 21757952 (20.75MB)
   used     = 16153928 (15.405586242675781MB)
   free     = 5604024 (5.344413757324219MB)
   74.24378912132907% used

* 32bit *

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND            
30168 agent     20   0 1220m 175m  12m S  0.0  2.2   0:13.43 java               

$ jmap -heap 30168
Attaching to process ID 30168, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 14.2-b01

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 1073741824 (1024.0MB)
   NewSize          = 1048576 (1.0MB)
   MaxNewSize       = 4294901760 (4095.9375MB)
   OldSize          = 4194304 (4.0MB)
   NewRatio         = 8
   SurvivorRatio    = 8
   PermSize         = 16777216 (16.0MB)
   MaxPermSize      = 67108864 (64.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 89522176 (85.375MB)
   used     = 80626352 (76.89128112792969MB)
   free     = 8895824 (8.483718872070312MB)
   90.0629940005033% used
From Space:
   capacity = 14876672 (14.1875MB)
   used     = 14876216 (14.187065124511719MB)
   free     = 456 (4.3487548828125E-4MB)
   99.99693479832048% used
To Space:
   capacity = 14876672 (14.1875MB)
   used     = 0 (0.0MB)
   free     = 14876672 (14.1875MB)
   0.0% used
PS Old Generation
   capacity = 954466304 (910.25MB)
   used     = 10598496 (10.107513427734375MB)
   free     = 943867808 (900.1424865722656MB)
   1.1104107034039412% used
PS Perm Generation
   capacity = 16777216 (16.0MB)
   used     = 11366448 (10.839889526367188MB)
   free     = 5410768 (5.1601104736328125MB)
   67.74930953979492% used

Answer 1

Think of virtual space as memory that program will ever be able to use. It is not the memory it actually uses. On 64-bit OS virtual memory is virtually endless, there is no 2Gb limitation as in 32-bit, so developers are free to choose any virtual size. There is nothing to worry about. The RES is actual physical RAM used.