Home / server / Questions / 1157110
Accepted
Rama
Asked: 2024-03-30 01:23:23 +0800 CST

Unidentified RAM

  • 772

Greetings! A server which is equipped with 512GB RAM(16 slots * 32GB) which OS is Ubuntu 22.04. During boot up, server reserving 16259356K(15GB) RAM showed in dmesg that resulted only 498GB is available for the OS. I was unable to identify where 15GB is allocated. Could you please help me.

1.) Dmidecode for RAM.
root@apac2-prd241:~# dmidecode -t memory |grep Size
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB
    Size: 32 GB


2.) dmesg log.
root@apac2-prd241:~# grep Memory /var/log/dmesg
[    3.919928] kernel: Memory: 520513460K/536773076K available (16393K kernel code, 4395K rwdata, 10908K rodata, 3360K init, 18708K bss, 16259356K reserved, 0K cma-reserved)
[    8.227673] kernel: x86/mm: Memory block size: 2048MB


3.) System RAM.
root@apac2-prd241:~# free -g
               total        used        free      shared  buff/cache   available
Mem:             497          39         439          17          18         438
Swap:              0           0           0
memory

1 Answers

  1. Best Answer

    Each 4K memory page commands a 64B struct page overhead, meaning 8 GB of unavailable memory for 512 GB of RAM. Adding some reserved memory due to crashkernel, memory mapped devices (ie: graphics adapter) and similar ends with ~15 GB unavailable on your server.

    EDIT: you have 6 GB allocated for crashkernel (which does not show as visible OS allocated memory via free), so 8+6=14 GB reserved just by the OS for its own. dmesg | grep -i reserved shows hardware reserved ranges, which you can compare with the output of cat /proc/iomap (note: the latter shows all ranges, even if they do not really use any physical memory).

    • 1