'free' and '/proc/meminfo/ report less total memory than 'dmidecode'

We have a server in a colocation centre (a real physical server, not a VM), running 64-bit Debian (uname -r: 3.16.0-4-amd64).

/proc/meminforeports about 4 GiB total memory:

$ head -n 1 /proc/meminfo
MemTotal:        4051692 kB

free reports the same (I only looked at the total column; I'm not talking about used, free, shared, buffers, cached):

$ free -k
             total       used       free     shared    buffers     cached
Mem:       4051692    3867356     184336     220908      63948    1203596
-/+ buffers/cache:    2599812    1451880
Swap:     15728208     652540   15075668

And so does dmesg | grep Memory:

$ dmesg | grep Memory
[    0.000000] Memory: 4034240K/4185236K available (5287K kernel code, 949K rwdata, 1836K rodata, 1208K init, 840K bss, 150996K reserved)

But dmidecode reports 4 * 2 GiB = 8 GiB of RAM, if I understand it correctly:

$ sudo dmidecode --type memory

# dmidecode 2.12
SMBIOS 2.6 present.

Handle 0x0008, DMI type 5, 24 bytes
Memory Controller Information
    Error Detecting Method: 64-bit ECC
    Error Correcting Capabilities:
        Single-bit Error Correcting
    Supported Interleave: One-way Interleave
    Current Interleave: One-way Interleave
    Maximum Memory Module Size: 4096 MB
    Maximum Total Memory Size: 16384 MB
    Supported Speeds:
        Other
    Supported Memory Types:
        DIMM
        SDRAM
    Memory Module Voltage: 3.3 V
    Associated Memory Slots: 4
        0x0009
        0x000A
        0x000B
        0x000C
    Enabled Error Correcting Capabilities:
        Single-bit Error Correcting

Handle 0x0009, DMI type 6, 12 bytes
Memory Module Information
    Socket Designation: DIMM1A
    Bank Connections: 0 1
    Current Speed: Unknown
    Type: DIMM SDRAM
    Installed Size: 2048 MB (Single-bank Connection)
    Enabled Size: 2048 MB (Single-bank Connection)
    Error Status: OK

Handle 0x000A, DMI type 6, 12 bytes
Memory Module Information
    Socket Designation: DIMM1B
    Bank Connections: 2 3
    Current Speed: Unknown
    Type: DIMM SDRAM
    Installed Size: 2048 MB (Single-bank Connection)
    Enabled Size: 2048 MB (Single-bank Connection)
    Error Status: OK

Handle 0x000B, DMI type 6, 12 bytes
Memory Module Information
    Socket Designation: DIMM2A
    Bank Connections: 4 5
    Current Speed: Unknown
    Type: DIMM SDRAM
    Installed Size: 2048 MB (Single-bank Connection)
    Enabled Size: 2048 MB (Single-bank Connection)
    Error Status: OK

Handle 0x000C, DMI type 6, 12 bytes
Memory Module Information
    Socket Designation: DIMM2B
    Bank Connections: 6 7
    Current Speed: Unknown
    Type: DIMM SDRAM
    Installed Size: 2048 MB (Single-bank Connection)
    Enabled Size: 2048 MB (Single-bank Connection)
    Error Status: OK

Handle 0x002A, DMI type 16, 15 bytes
Physical Memory Array
    Location: System Board Or Motherboard
    Use: System Memory
    Error Correction Type: Single-bit ECC
    Maximum Capacity: 16 GB
    Error Information Handle: Not Provided
    Number Of Devices: 4

Handle 0x002C, DMI type 17, 28 bytes
Memory Device
    Array Handle: 0x002A
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 2048 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM1A
    Bank Locator: BANK0
    Type: DDR3
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Micron        
    Serial Number: 501C6FDC
    Asset Tag: AssetTagNum0
    Part Number: 9JSF25672AZ-1G4D1 
    Rank: Unknown

Handle 0x002E, DMI type 17, 28 bytes
Memory Device
    Array Handle: 0x002A
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 2048 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM1B
    Bank Locator: BANK1
    Type: DDR3
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Micron        
    Serial Number: 2A1C6FDC
    Asset Tag: AssetTagNum1
    Part Number: 9JSF25672AZ-1G4D1 
    Rank: Unknown

Handle 0x0030, DMI type 17, 28 bytes
Memory Device
    Array Handle: 0x002A
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 2048 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM2A
    Bank Locator: BANK2
    Type: DDR3
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Micron        
    Serial Number: 511C6FDC
    Asset Tag: AssetTagNum2
    Part Number: 9JSF25672AZ-1G4D1 
    Rank: Unknown

Handle 0x0032, DMI type 17, 28 bytes
Memory Device
    Array Handle: 0x002A
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 2048 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM2B
    Bank Locator: BANK3
    Type: DDR3
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Micron        
    Serial Number: 4B1C6FDC
    Asset Tag: AssetTagNum3
    Part Number: 9JSF25672AZ-1G4D1 
    Rank: Unknown

What am I missing? It's a server in a colocation center, so unfortunately I can't easily see what is installed physically.

Edit: man dmidecode says "More often than not, information contained in the DMI tables is inaccurate, incomplete or simply wrong.". Maybe dmidecode simply reports wrong data?

Edit: This is not a duplicate of Why is Linux reporting “free” memory strangely?. That question is about free memory, and confusion arising from buffers and cache taking away from that. I am not concerned about free memory, only about total memory. Don't let my usage of the free command fool you: I didn't use it to look at the amount of free memory, only the amount of total memory. If someone still thinks this question is a duplicate, please explain me why because I don't understand.

Edit: dmidecode -t1 as requested by Lenniey

$ sudo dmidecode -t1
# dmidecode 2.12
SMBIOS 2.6 present.

Handle 0x0001, DMI type 1, 27 bytes
System Information
        Manufacturer: Supermicro
        Product Name: X8SIL
        Version: 0123456789
        Serial Number: 0123456789
        UUID: 49434D53-0200-9037-2500-379025009946
        Wake-up Type: Power Switch
        SKU Number: To Be Filled By O.E.M.
        Family: To Be Filled By O.E.M.

Solution 1:

The manual for that Supermicro X8SIL motherboard is available at: http://www.supermicro.com/manuals/motherboard/3420/MNL-1130.pdf

On page 32 (aka 2-10) it indicates that if Unbuffered (UDIMM) single-rank memory is used, the maximum amount of memory supported will be only 4 GB when using 1 GB DIMMs and 8 GB with 2 GB DIMMs.

With dual-rank UDIMMs, the maximum capacity would be 16 GB.

The ultimate maximum capacity of 32 GB can only be reached by using Registered (RDIMM) quad-rank memory modules, and the memory bus speed will take a hit when using them.

And a bit of googling on "Micron 9JSF25672AZ-1G4D1" brought me here: https://www.compuram.biz/memory_module/mt9jsf25672az-1g4d1/micron.htm

It seems to confirm that Micron Technology (MT) 9JSF25672AZ-1G4D1 is indeed an unbuffered single-rank memory module of size 2 GB.

On page 34 (or 2-13) of the manual it indicates that when only 4 GB of RAM is used, a significant amount of it will be allocated to system devices and won't be usable. This might account for at least some of the missing memory with 8 GB installed too. Unfortunately the manual doesn't describe in detail the system device allocations in the 8 GB case.