This section provides a tutorial example on how to view shared memory among JVM processes using vadump.exe.
As suggested by Steve Bohne from Sun.COM, if you want to see the shared memory usage
of JVM processes, you should install vadump.exe from Microsoft.
vadump.exe is a command line tool that shows very detailed information of a running process on a Windows system.
There are 3 options you need to use with vadump.exe to get the private memory and shared memory usages:
"-p PID" - Specifies a running process by the process ID, which can be obtained by the tasklist.exe command
on a Windows XP system.
"-o" - Tells vadump.exe to report a snapshot of the workingset of the specified process.
"-s" - Tells vadump.exe to print summary information only.
With vadump.exe and tasklist.exe ready, let's see how much memory will be shared by multiple JVM processes
with CDS turned off:
(Open first command window)
C:\>\progra~1\java\jdk1.6.0_02\bin\java -cp . -Xshare:off
LongSleep
Free memory: 4997088
Total memory: 5177344
(Open second command window)
C:\>\progra~1\java\jdk1.6.0_02\bin\java -cp . -Xshare:off
LongSleep
Free memory: 4997088
Total memory: 5177344
(Open third command window)
C:\>tasklist
Image Name PID Session Name Session# Mem Usage
============= ====== ============ ======== =========
...
java.exe 2312 0 6,580 K
java.exe 3920 0 6,548 K
(tasklist.exe reports the total memory usage
in the same way as Windows Task Manager)
C:\>\progra~1\resour~1\vadump -o -s -p 2312
Category Total Private Shareable Shared
Pages KBytes KBytes KBytes KBytes
...
Total Modules 794 3176 300 0 2876
Total Dynamic Data 816 3264 3120 4 140
Total System 45 180 180 0 0
Grand Total Working Set 1655 6620 3600 4 3016
C:\>\progra~1\resour~1\vadump -o -s -p 3920
Category Total Private Shareable Shared
Pages KBytes KBytes KBytes KBytes
...
Total Modules 794 3176 300 0 2876
Total Dynamic Data 804 3216 3088 4 124
Total System 49 196 196 0 0
Grand Total Working Set 1647 6588 3584 4 3000
Now we know that:
Even without CDS feature turned on, there is about 3 MB of memory shared by all JVM processes.
This shared memory is resulted from DLL files used by JVM like: jvm.dll, java.dll, ntdll.dll, kernel32.dll, etc.
LongSleep.java application only need about 3.5 MB of private memory to run, which
is about half of the total memory reported by Windows Task Manager.
Now let's repeat the same test with CDS turned on:
(Open first command window)
C:\>\progra~1\java\jdk1.6.0_02\bin\java -cp . -Xshare:on
LongSleep
Free memory: 4997088
Total memory: 5177344
(Open second command window)
C:\>\progra~1\java\jdk1.6.0_02\bin\java -cp . -Xshare:on
LongSleep
Free memory: 4997088
Total memory: 5177344
(Open third command window)
C:\>tasklist
Image Name PID Session Name Session# Mem Usage
============= ====== ============ ======== =========
...
java.exe 1772 0 7,032 K
java.exe 420 0 7,032 K
(tasklist.exe reports the total memory usage
in the same way as Windows Task Manager)
C:\>\progra~1\resour~1\vadump -o -s -p 1772
Category Total Private Shareable Shared
Pages KBytes KBytes KBytes KBytes
...
Total Modules 784 3136 300 0 2836
Total Dynamic Data 925 3700 2124 4 1572
Total System 59 236 236 0 0
Grand Total Working Set 1768 7072 2660 4 4408
C:\>\progra~1\resour~1\vadump -o -s -p 420
Category Total Private Shareable Shared
Pages KBytes KBytes KBytes KBytes
...
Total Modules 784 3136 300 0 2836
Total Dynamic Data 925 3700 2124 4 1572
Total System 59 236 236 0 0
Grand Total Working Set 1768 7072 2660 4 4408
Comparing with CDS turned off, the results show that:
The shared memory has been increased from 3000 KB to 4408 KB.
The increased shared memory amount 1408 KB was mainly coming from
the Dynamic Data category, which is probably how the CDS shared archive file
was used by JVM processes.
Of course, turning on the CDS feature costed bout 7072 - 6588 = 484 KB
extra memory usage.
The net saving on private memory usage is about 1408 - 484 = 924 KB,
or 3584 - 2660 = 924 KB.
