Lost memory on Linux - not cached, not buffers

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http://stackoverflow.com – My Ubuntu 12 server is mysteriously losing/wasting memory. It has 64GB of ram. About 46GB are shown as used even when I shutdown all my applications. This memory is not reported as used for buffers or caching. The result of top (while my apps are running; the apps use about 9G): top - 21:22:48 up 46 days, 10:12, 1 user, load average: 0.01, 0.09, 0.12 Tasks: 635 total, 1 running, 633 sleeping, 1 stopped, 0 zombie Cpu(s): 0.2%us, 0.2%sy, 0.0%ni, 99.6%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 65960100k total, 55038076k used, 10922024k free, 271700k buffers Swap: 0k total (HowTos)