linux is using different strategy to manage the memory. In windows, the application will use the memory only when they have to. In Linux, when the application is open, it will reserve some space for further use. That is why when you keep using the system, the memory usage will grow till 90%+. Though it looks scary but your system will still be fine.
^_^
Do not worry about the memory usage, it is not a big deal and it will not affect the performance.
For more details, read the kernel source then you will know..
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