What is "-XX:+UseCompressedOops" ? – how to reduce of memory by Eclipse ?

Most HotSpot JVM in the last year have had it on by default. This option allows references to be 32-bit in a 64-bit JVM and access close to 32 GB of heap. (more than 32-bit pointers can) (You can have near unlimited off heap memory as well). This can save a significant amount of memory and potentially improve performance.

If you want to use this option I suggest you update to a version which has it on by default as there may have been a good reason, such as bugs, why it wasn't enabled previously. Try Java 6 update 23 or Java 7 update 5.

In short, don't turn it on, use a version which has it on by default.

Of course, the modification of Eclipse settings are in eclipse.ini file. 

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