does UseParallelOldGC guarantee a better full gc performance

[the.6th.month at gmail.com]

hi all:
I'm currently using jdk 6u26. I just enabled UseParallelOldGC, expecting
that would enhance the full gc efficiency and decrease the mark-sweep time
by using multiple-core. The JAVA_OPTS is as below:
-XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintTenuringDistribution
-Xloggc:gc.log-server -Xms4000m -Xmx4000m -Xss256k -Xmn256m
-XX:PermSize=256m -XX:+UseParallelOldGC  -server
-Dorg.apache.jasper.compiler.Parser.STRICT_QUOTE_ESCAPING=false
as shown in jinfo output, the settings have taken effect, and the
ParallelGCThreads is 4 since the jvm is running on a four-core server.
But what's strange is that the mark-sweep time remains almost unchanged (at
around 6-8 seconds), do I miss something here? Does anyone have the same
experience or any idea about the reason behind?
Thanks very much for help

All the best,
Leon
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