linux-kernel - Re: missing 700 MB of RAM

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Date:	Mon, 22 Nov 2010 23:37:45 +0100
From:	Jiri Slaby <jirislaby@...il.com>
To:	Andreas Hartmann <andihartmann@...19freenet.de>
CC:	linux-kernel@...r.kernel.org
Subject: Re: missing 700 MB of RAM

You should reply-to-all.

On 11/22/2010 10:58 PM, Andreas Hartmann wrote:
> Jiri Slaby wrote:
>> On 11/21/2010 08:02 PM, Andreas Hartmann wrote:
>>> I've got 8 GB RAM installed, but linux reports 7,3 GB. Where is the
>>> missing RAM (700 MB)?
>> ...
>>> The motherboard is a GA-MA78GM-S2H (64bit, AMD) with a on board Radeon
>>> HD 3200 VGA card. The card uses 256 MB of RAM - so 444 MB of RAM are
>>> missing. Where are they gone?
>>
>> Hi, ask your BIOS:
>>
>>> BIOS-provided physical RAM map:
>>> BIOS-e820: 0000000000000000 - 000000000009f800 (usable)
>>> BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
>>> BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
>>> BIOS-e820: 0000000000100000 - 00000000afde0000 (usable)
>>> BIOS-e820: 00000000afde0000 - 00000000afde3000 (ACPI NVS)
>>> BIOS-e820: 00000000afde3000 - 00000000afdf0000 (ACPI data)
>>> BIOS-e820: 00000000afdf0000 - 00000000afe00000 (reserved)
>>> BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
>>> BIOS-e820: 00000000fec00000 - 0000000100000000 (reserved)
>>> BIOS-e820: 0000000100000000 - 0000000230000000 (usable)
>>
>> It reports 7677M of usable mem. Or maybe your loader is cheating on you?
>> I doubt so...
> 
> Ok. That's right. Anyway, I'm wondering about the difference to free:
> 
>             total       used       free     shared    buffers     cached
> Mem:      7706580    5271096    2435484          0      79232    3049120
> 
> which is 7526M. This means, there are missing 151M of usable RAM?!

Yes, and if you look into dmesg, you will see:
Your BIOS doesn't leave a aperture memory hole
Please enable the IOMMU option in the BIOS setup
This costs you 32 MB of RAM
Mapping aperture over 32768 KB of RAM @ 20000000

Then there is some kernel code+data (11M), mem_map (108M) etc.
7677-108-32-11=7526

> I would have expected, that free reprots the same amount as the kernel
> reports to be usable.
> 
> On the other hand, I would have expected, that the amount, reported by
> free + gfx-memory should nearly be 8 GB. Here it is: 7782. Thus there
> are missing 410 MB between the installed hardware and free + gfx-memory.

Since the BIOS is not reporting the memory, kernel can do only a little
about that.

> Removing the difference between free and physical ram map (410 - 151),
> ~259 MB are remaing. This is nearly the amount for the gfx-card, which
> has already been removed. Could it be, that the gfx-memory is reserved
> twice - once by the bios and another time by the kernel?!

I don't understand where you got the numbers...

regards,
-- 
js
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