How to clean up buffer or cache momory?

[黄小伟]

 Hi
     I encounter a problem recentely. My system supports USB strorage and other network applications.
     But after I put a file to usb storage, it will consume some many momory.
(buffer + cache).
     And these buffered memory can not be reclaimed(use free command).
So I want to know how to reclaim the buffer memory in a short time.
  
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