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Lost memory, total memory size is not correct

DISCUSSION Jürgen Urban 3 participants 3 messages Jan 19, 2004

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Jürgen Urban

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#0 lore raw

Hello,

I tried to get the amount of total physical memory. I looked at /proc/meminfo and found this line (2.4.18):

MemTotal: 30844 kB

But this is not correct the system have 32768 kB Memory. I looked at kernel sources and I found the variable max_mapnr. Can I use it to detect the correct memory size? It seems that it stores the maximum number of pages usable. So I can convert it with macro K() in linux/fs/proc/proc_misc.c to a value in kB.

But there are 1924 kB not available (32768 kB - 30844 kB). On system boot I get the following message:
Memory: 30780k available (960k kernel code, 392k data, 64k init, 0k highmem) So I calculated:

1924 kB
-960 kB Kernel
- 392 kB Data
- 64 kB Init
--------------------
508 kB

There are 508 kB lost (?) memory. It seems the boot allocator is reserving this memory, but linux doesn't tell for what. I want to know for what the 508 kB are. Is the kernel stack included in the 508 kB or in the 30844 kB. I don't think so, because the value 30844 kB isn't changing after boot. And every process should allocate 8 kB kernel stack.

Best Regards
Jürgen Urban

Signature · 10 lines
Jürgen Urban <[email protected]>
Software Engineer

SYSGO Real-Time Solutions AG
Am Pfaffenstein 14
55270 Klein-Winternheim, Germany

Telefon: +49-6136-9948-0
FAX: +49-6136-9948-10
www.sysgo.com
#1 lore raw
Quoted 5 lines · Jürgen Urban

Hello,

I tried to get the amount of total physical memory. I looked at /proc/meminfo and found this line (2.4.18):

Quoted 7 lines

MemTotal: 30844 kB

But this is not correct the system have 32768 kB Memory. I looked at kernel sources and I found the variable max_mapnr. Can I use it to detect the correct memory size? It seems that it stores the maximum number of pages usable. So I can convert it with macro K() in linux/fs/proc/proc_misc.c to a value in kB.

There are some addresses that cannot be used on PC because there is something mapped (for example System ROM...), you can see these regions doing "cat /proc/iomem".

Here there is my output:

00000000-0009fbff : System RAM
0009fc00-0009ffff : reserved
000a0000-000bffff : Video RAM area
000c0000-000c7fff : Video ROM
000f0000-000fffff : System ROM
00100000-07febfff : System RAM
00100000-00298596 : Kernel code
00298597-00322f27 : Kernel data
07fec000-07feefff : ACPI Tables
07fef000-07ffefff : reserved
07fff000-07ffffff : ACPI Non-volatile Storage de800000-de81ffff : Promise Technology, Inc. 20265 df000000-df0000ff : Cologne Chip Designs GmbH ISDN network controller [HFC-PCI]
df800000-df8000ff : Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+
df800000-df8000ff : 8139too
e0000000-e3dfffff : PCI Bus #01
e0000000-e1ffffff : 3Dfx Interactive, Inc. Voodoo Banshee
e3f00000-e5ffffff : PCI Bus #01
e4000000-e5ffffff : 3Dfx Interactive, Inc. Voodoo Banshee
e6000000-e7ffffff : VIA Technologies, Inc. VT8363/8365 [KT133/KM133] ffff0000-ffffffff : reserved

Bye

Signature · 2 lines
	Paolo Ornati
	Linux v2.4.24
#2 lore raw
Quoted 14 lines · Paolo Ornati
Quoted 5 lines · J?rgen Urban

Hello,

I tried to get the amount of total physical memory. I looked at /proc/meminfo and found this line (2.4.18):

Quoted 7 lines

MemTotal: 30844 kB

But this is not correct the system have 32768 kB Memory. I looked at kernel sources and I found the variable max_mapnr. Can I use it to detect the correct memory size? It seems that it stores the maximum number of pages usable. So I can convert it with macro K() in linux/fs/proc/proc_misc.c to a value in kB.

Quoted 3 lines

There are some addresses that cannot be used on PC because there is something mapped (for example System ROM...), you can see these regions doing "cat /proc/iomem".

That's usually just a few KB, but don't forget that MemTotal is what's left after kernel memory is mapped.