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why 1G memory is missing?

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  why 1G memory is missing? <loushanguan2015@sina.com> - 2021-08-03 00:20 +0200
    Re: why 1G memory is missing? IL Ka <kazakevichilya@gmail.com> - 2021-08-03 00:40 +0200
      Re: why 1G memory is missing? Marco Möller <talby@debianlists.mobilxpress.net> - 2021-08-03 18:00 +0200
        Re: why 1G memory is missing? IL Ka <kazakevichilya@gmail.com> - 2021-08-03 18:30 +0200
          Re: why 1G memory is missing? Marco Möller <talby@debianlists.mobilxpress.net> - 2021-08-06 20:40 +0200

#238069 — why 1G memory is missing?

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i have 2 memory slotsmemtest86+ shows each has 2G, but total is 3Gafter booting linux, top shows total is 3Gwhy 1 G is missing? Thanks!

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#238070

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> i have 2 memory slots
> memtest86+ shows each has 2G, but total is 3G
> after booting linux, top shows total is 3G
>
why 1 G is missing? Thanks!
>


You probably have 32bit OS
https://en.wikipedia.org/wiki/3_GB_barrier

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#238091

On 03.08.21 00:42, IL Ka wrote:
> 
>     i have 2 memory slots
>     memtest86+ shows each has 2G, but total is 3G
>     after booting linux, top shows total is 3G
> 
>     why 1 G is missing? Thanks!
> 
> 
> 
> You probably have 32bit OS
> https://en.wikipedia.org/wiki/3_GB_barrier 
> <https://en.wikipedia.org/wiki/3_GB_barrier>
> 

Not as easy. I have this same problem on two hardware systems.
Using a 64bit Debian OS, installed to an USB-Stick and then booted on 
different hardware fully uses the 8 GB in one equipment physically 
providing 8 GB RAM, but in two other systems only 3 GB out of physically 
present 4 GB RAM are used by DEBIAN. MS Windows is using all the 4 GB, 
and doing this without any changes in the BIOS.

uname shows that a 64 bit OS is in use:
Linux XXXXXX 5.10.0-8-amd64 #1 SMP Debian 5.10.46-3 (2021-07-28) x86_64 
GNU/Linux

inxi shows that only 3 GB are available as the TOTAL, although it finds 
the 4 GB to be physically installed:
$ sudo inxi -m -x
Memory:    RAM: total: 2.88 GiB used: 2.11 GiB (73.2%)
Array-1: capacity: 4 GiB slots: 2 EC: None max module size: 2 GiB note: est.
Device-1: M1 size: 2 GiB speed: 667 MT/s type: DDR2
Device-2: M2 size: 2 GiB speed: 667 MT/s type: DDR2

So, like for the original post, also on my system, only 3 GB out of the 
present 4 GB can be used. This problem seems to be a Debian (Linux in 
general?) problem, because MS Win makes full use of the 4 GB RAM as 
physically present.

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#238094

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>
> inxi shows that only 3 GB are available as the TOTAL, although it finds
> the 4 GB to be physically installed:
> $ sudo inxi -m -x
> Memory:    RAM: total: 2.88 GiB used: 2.11 GiB (73.2%)
> Array-1: capacity: 4 GiB slots: 2 EC: None max module size: 2 GiB note:
> est.
> Device-1: M1 size: 2 GiB speed: 667 MT/s type: DDR2
> Device-2: M2 size: 2 GiB speed: 667 MT/s type: DDR2
>
>
It could be a different problem (Windows may calculate RAM differently in
different places, where did you check it?).
I'd start with a memory map (see Sven Hartge's comment in this thread),
also see https://en.wikipedia.org/wiki/E820
A similar tool for Windows is "Rammap".

With 32bit system you would never be able to use 4GB (because part of
address space is used for MMIO by PCI-(express) devices).
With 64bit address space is much larger, but some ram is used by kernel.
Some tools exclude such ram.

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#238278

On 03.08.21 18:26, IL Ka wrote:
>     inxi shows that only 3 GB are available as the TOTAL, although it finds
>     the 4 GB to be physically installed:
>     $ sudo inxi -m -x
>     Memory:    RAM: total: 2.88 GiB used: 2.11 GiB (73.2%)
>     Array-1: capacity: 4 GiB slots: 2 EC: None max module size: 2 GiB
>     note: est.
>     Device-1: M1 size: 2 GiB speed: 667 MT/s type: DDR2
>     Device-2: M2 size: 2 GiB speed: 667 MT/s type: DDR2
> 
> 
> It could be a different problem (Windows may calculate RAM differently 
> in different places, where did you check it?).
> I'd start with a memory map (see Sven Hartge's comment in this thread), 
> also see https://en.wikipedia.org/wiki/E820 
> <https://en.wikipedia.org/wiki/E820>
> A similar tool for Windows is "Rammap".
> 
> With 32bit system you would never be able to use 4GB (because part of 
> address space is used for MMIO by PCI-(express) devices).
> With 64bit address space is much larger, but some ram is used by kernel. 
> Some tools exclude such ram.
> 

Although the original question got a valid answer already, in this case 
the chipset did not support 4 GB and not even a 64bit OS, I here add to 
some sub-part of the thread (see above in this email what I am referring 
to) some information for completeness - I got time to search in old 
threads on the German spoken Debian mailing list where this topic was 
also up some time ago.

Sven Hartge pointed out and explained that the missing 1 GB on a 64 bit 
system could be related to what is called the "PCI hole" and also the 
following link was provided:

https://en.wikipedia.org/wiki/PCI_hole

I wasn't able to understand all technical explanations by Sven Hartge, 
neither everything what's published in the wikipedia article, this all 
is obviously a topic for real hardware experts, but it at least 
convinced me that it should not be a configuration failure in my Debian 
installation - however, why MS Windows on the identical hardware shows 
the full 4 GB to be available never became clear to me.

Saying all this, I will not be able to further discuss this topic here 
in this thread as I am for far too large extend not understanding the 
necessary basics, but at least wanted to leave this information about 
the "PCI hole".

Always stay in good spirits, Marco!

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