Class Data Sharing (CDS) and memory analys with pmap

Question

Hi everybody,
I read that CDS can decrease the memory footprint by sharing some librairies.
I did some tests on PowerPC board and embeddedJVM 1.5 with 128Mo ROM
I wrote a HelloWorld with a sleep(10) and see the results :
# java -Xshare:off HelloWorld
mapped:   172568 KB writable/private: 165708 KB shared: 32 KB
# java -Xshare:on HelloWorld
mapped:   201112 KB writable/private: 188604 KB shared: 5680 KBThe shared memory increase, normal...
The memory mapped and writable/private increase, I thought that memory will decrease...
so, where is the benefits ?
And the mapped memory is > to board memory ! why ?
thanks for your help
Obelix

Answer 1

somebody can do this tests please :
Write a HelloWord.java with a sleep() of 10s and execute this scripts :
#!/bin/sh
java -version >> testsCDSoff.txt 2>&1
java -Xshare:off HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSoff.txt
java -Xshare:off HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSoff.txt
java -Xshare:off HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSoff.txtand
#!/bin/sh
java -version >> testsCDSon.txt 2>&1
java -Xshare:on HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSon.txt
java -Xshare:on HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSon.txt
java -Xshare:on HelloWorld &
PID=$!
sleep 2
pmap -d $PID | grep mapped >> testsCDSon.txtand then dump testsCDSoff.txt and testsCDSon.txt here :D
Look at mines :
testsCDSoff.txt
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)
mapped: 237220K    writeable/private: 186972K    shared: 44740K
mapped: 237220K    writeable/private: 186972K    shared: 44740K
mapped: 237220K    writeable/private: 186972K    shared: 44740K
testsCDSon.txt
java version "1.5.0_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)
mapped: 265808K    writeable/private: 210252K    shared: 50048K
mapped: 265808K    writeable/private: 210252K    shared: 50048K
mapped: 265808K    writeable/private: 210252K    shared: 50048Kwhere is the benefits ? I did an error somewhere or I don't understand CDS ?