Ubuntu Server 16.04 64bit had only 8Gb of RAM when 16GB are installed

Question

I have one DIMM installed on a Lenovo x3550 M5 server.

The OS is Ubuntu 16.04.2 64 bit uname -ra

Linux hostname 4.4.0-62-generic #83-Ubuntu SMP Wed Jan 18 14:10:15 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

free -hm gives:

              total        used        free      shared  buff/cache   available
Mem:           7.2G        238M        6.5G         20M        474M        6.7G
Swap:          7.4G          0B        7.4G

And sudo lshw -c memory gives:

*-memory
   description: System Memory
   physical id: 5
   slot: System board or motherboard
   size: 16GiB
 *-bank:0
      description: DIMM Synchronous 2133 MHz (0.5 ns)
      product: M393A2G40EB1-CPB
      vendor: Samsung
      physical id: 0
      serial: 88045A3F
      slot: Dimm 1
      size: 16GiB
      width: 64 bits
      clock: 2133MHz (0.5ns)

other banks are empty.

In the dmesg I do not see anything out of the ordinary:

 dmesg | grep memory
 [    0.000000] Scanning 1 areas for low memory corruption
 [    0.000000] Base memory trampoline at [ffff880000096000] 96000 size 24576
 [    0.000000] Early memory node ranges
 [    0.000000] PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
 [    0.000000] PM: Registered nosave memory: [mem 0x0009c000-0x0009ffff]
 [    0.000000] PM: Registered nosave memory: [mem 0x000a0000-0x000dffff]
 [    0.000000] PM: Registered nosave memory: [mem 0x000e0000-0x000fffff]
 [    0.000000] PM: Registered nosave memory: [mem 0x432a8000-0x4b2affff]
 [    0.000000] PM: Registered nosave memory: [mem 0x4bfdd000-0x4bfebfff]
 [    0.000000] PM: Registered nosave memory: [mem 0x646b2000-0x793fefff]
 [    0.000000] PM: Registered nosave memory: [mem 0x793ff000-0x7b3fefff]
 [    0.000000] PM: Registered nosave memory: [mem 0x7b3ff000-0x7b787fff]
 [    0.000000] PM: Registered nosave memory: [mem 0x7b800000-0x8fffffff]
 [    0.000000] PM: Registered nosave memory: [mem 0x90000000-0xfed1bfff]
 [    0.000000] PM: Registered nosave memory: [mem 0xfed1c000-0xfed1ffff]
 [    0.000000] PM: Registered nosave memory: [mem 0xfed20000-0xfeffffff]
 [    0.000000] PM: Registered nosave memory: [mem 0xff000000-0xffffffff]
 [    0.043027] Initializing cgroup subsys memory
 [    0.044460] Freeing SMP alternatives memory: 32K (ffffffff820b8000 -  ffffffff820c0000)
 [    1.122178] Freeing initrd memory: 36936K (ffff8800337cc000 - ffff880035bde000)
 [    1.122674] Scanning for low memory corruption every 60 seconds
 [    2.616860] Freeing unused kernel memory: 1488K (ffffffff81f44000 - ffffffff820b8000)
 [    2.629739] Freeing unused kernel memory: 1792K (ffff880001840000 - ffff880001a00000)
 [    2.638571] Freeing unused kernel memory: 136K (ffff880001dde000 - ffff880001e00000)