Browsing the linux kernel output dmesg on 6 different systems (4 server, and for the sake of comparison also 2 laptops) I am shocked by the large amount of BIOS/UEFI reserved RAM
Three servers systems have this output
3 x server with 128GiB RAM show this
[ 0.000000] Memory: 5491432k/136314880k available (7668k kernel code, 2636180k absent, 2440136k reserved, 6052k data, 1876k init)
=> ~2.5GiB RAM "BIOS-reserverd"
1 x server with 512GiB RAM show this:
[ 5.892498] Memory: 527942676K/536409480K available (10252K kernel code, 1241K rwdata, 3320K rodata, 1592K init, 2272K bss, 8466804K reserved, 0K cma-reserved)
=> ~8.5GiB "lost to BIOS"
1 x latop with this info
[ 0.086548] Memory: 16115288K/16583384K available (14339K kernel code, 1524K rwdata, 7928K rodata, 1660K init, 2928K bss, 467840K reserved, 0K cma-reserved)
=> ~450MiB reserved (256 is VGA, but still 200MB out of 16GiB is )
1 x laptop (thinkpad x200s no uefi) yields:
[ 0.432824] Memory: 8022988K/8282140K vailable (14339K kernel code, 1524K rwdata, 7912K rodata, 1660K init, 2928K bss, 258892K reserved, 0K cma-reserved)
=> 258 MiB (however this being almost entirely the graphic 256MiB)
I can understand, that some memory is retained by the firmware (UEFI/BIOS), however the loss of 8.5GiB or ECC RAM seems not easy to understand.
I wonder how I should think about the reseverd -and hence lost- RAM.
To me it seems that the EFI systems have a tendency to take roughly 1.5% of total physical RAM.
Can those values be corroborated?
Can there be an explanation why the UEFI uses so much RAM? (8.5GiB is more than most laptop systems I ran a full flagged desktop multi media system on, have used).
