[arch-general] Where have my 135 MB of memory gone?

[Evangelos Foutras]

Hi everyone,

This isn't exactly a serious issue, but it's something that's been
bugging me for some time now. Today, after spending a few hours
searching the Internet for information and trying some boot options
(i.e.: setting iommu to stuff like noagp and noaperture,) I've decided
to ask on this mailing list, hoping that someone more knowledgeable
might be able to help me figure this out.

I have 4 GB of memory installed on my Arch x86_64 desktop computer.
The integrated graphics chipset is using 128 MB of those 4096 MB,
along with another 128 MB from the sideport memory that exists on the
motherboard. That should leave me with 3968 MB of usable memory.
However, /proc/meminfo [1] (and `free -m') reports 3833 MB, which is
135 MB less than the expected amount.

In `dmesg' output, I also see the following line:

Memory: 3857352k/4849664k available (3481k kernel code, 787336k
absent, 204976k reserved, 1790k data, 480k init)

which says that ~200 MB are reserved. This doesn't match the 135 MB I
calculated above, but could possibly include it.

So, what's going on here? Where and how is my missing memory being used?

Thanks in advance for any replies/suggestions. :)

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[1] http://codepad.org/SGJbaS05 (/proc/meminfo contents)
[2] http://codepad.org/6vgofmKc (dmesg output)