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Trying to get more than 4GB RAM on 32 bit CPUs with PAE, but can't.

~# grep -o pae /proc/cpuinfo 
pae
pae
pae
pae
~# uname -a
Linux debian 4.9.0-6-686-pae #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) i686 GNU/Linux
~# grep HIGHMEM /boot/config-4.9.0-6-686-pae 
# CONFIG_NOHIGHMEM is not set
# CONFIG_HIGHMEM4G is not set
CONFIG_HIGHMEM64G=y
CONFIG_HIGHMEM=y
# CONFIG_DEBUG_HIGHMEM is not set
~# dmidecode -t memory | grep Enabled
    Enabled Error Correcting Capabilities:
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)
    Enabled Size: 1024 MB (Single-bank Connection)

Still got

~# free -h
              total        used        free      shared  buff/cache   available
Mem:           3.8G         29M        3.6G         10M        244M        3.5G
Swap:            9G          0B          9G

Maybe someone can help, where do I look?

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  • What motherboard do you have? Some motherboards are limited to 4GB. See Hardware support on this Wikipedia page.
    – Timothy Martin
    Mar 28, 2018 at 21:26
  • Does the BIOS show more than 4GiB? Are there related options in the BIOS setup ("remapping")?
    – Hauke Laging
    Mar 28, 2018 at 21:42
  • @TimothyMartin Motherboard is 8IPXDREL-GG, specs say up to 12GB
    – mikst
    Mar 28, 2018 at 22:25
  • @HaukeLaging BIOS shows (and checks at POST) 6GB. I could not find any related options in BIOS.
    – mikst
    Mar 28, 2018 at 22:27
  • What does dmidecode -t memory | grep -i "size\|maximum" report?
    – Timothy Martin
    Mar 29, 2018 at 0:24

1 Answer 1

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To diagnose memory problems, the best place to start is the kernel boot messages; the kernel prints out everything it finds concerning the system’s memory. Start with the output of dmesg, looking for “memory”.

As you discovered, in your case this shows

[ 0.000000] WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 2176MB of RAM.
[ 0.000000] WARNING: CPU: 0 PID: 0 at /build/linux-rtbsTs/linux-4.9.82/arch/x86/kernel/cpu/mtrr/cleanup.c:978 mtrr_trim_uncached_memory+0x2f5/0x326

which suggests that the BIOS isn’t setting things up properly for a 6GiB system.

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