I have tried to make minecraft run with more ram, but when I edit the arguments it seems that it can't create enough ram even for 2G. I have almost 4G of ram usable though. arguments: -Xmx2G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
Looking at the JVM arguments you are using I don't think there would be a problem.
As I stated in my comment, the
Xmx argument is a maximum setting and will only ever be reached if Minecraft has enough chunks loaded, so to put it simply having a higher
Xmx value will allow to have a higher render distance set in game (this is an extreme over simplification).
That being said there is an additional option you can specify that will allow you to tell the JVM to allocate that memory immediately, rather than as needed; the option looks like
-Xms2G. However this will only provide a very minimal performance increase if any at all.
I would also try removing the
Xmn argument, this setting is used for things in memory that are short lived and it will likely be cutting into the 2G overall max (don't quote me on that I could be wrong)
I am not too knowledgeable on the other JVM arguments you have specified but I did find this article that mentions to avoid using both
CMSIncrementalMode and that
UseAdaptiveSizePolicy are contradictory settings
A quick final note, if you are trying to get Minecraft to use more ram to improve performance, it is much more likely that your processor or video card are performance bottlenecks here than your ram. I especially think this because you are already providing it with more memory than it is using.