I'm used to kernel's using most of the memory for caching. Here's a comparison of two machines running Debian stable - one using 3.16 kernel and one using 4.5 kernel
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$ uname -vr
3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08)
$ top
top - 21:21:34 up 108 days, 11:11, 17 users, load average: 0.23, 0.21, 0.17
Tasks: 290 total, 1 running, 288 sleeping, 0 stopped, 1 zombie
%Cpu(s): 1.0 us, 0.4 sy, 0.0 ni, 98.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 16306084 total, 15959684 used, 346400 free, 400096 buffers
KiB Swap: 15622140 total, 1020 used, 15621120 free. 12279592 cached Mem
$ free -h
total used free shared buffers cached
Mem: 15G 15G 337M 643M 390M 11G
-/+ buffers/cache: 3.1G 12G
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$ uname -vr
4.5.0-0.bpo.2-amd64 #1 SMP Debian 4.5.4-1~bpo8+1 (2016-05-13)
$ top
top - 11:23:15 up 6:17, 5 users, load average: 0.34, 0.18, 0.15
Tasks: 229 total, 2 running, 226 sleeping, 0 stopped, 1 zombie
%Cpu(s): 2.3 us, 0.7 sy, 0.0 ni, 96.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 32856480 total, 2690676 used, 30165804 free, 75472 buffers
KiB Swap: 3997692 total, 0 used, 3997692 free. 1141724 cached Mem
$ free -h
total used free shared buffers cached
Mem: 31G 2.6G 28G 50M 73M 1.1G
-/+ buffers/cache: 1.4G 29G
Swap: 3.8G 0B 3.8G