[help] kernel boot parameter "mem=xx" disparity

[help] kernel boot parameter "mem=xx" disparity

[Jun Hu]

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Hi Guys:

My machine has 8G memory, when I use kernel boot parameter mem=5G , it only display ¡°4084 M¡± using ¡°free ¨Cm ¡°. 
where the ¡°5120-4084 = 1036M ¡° memory run?

# /bin/uname -a
Linux dom0-93 3.0.13-0.27-default #1 SMP Wed Feb 15 13:33:49 UTC 2012 (d73692b) x86_64 x86_64 x86_64 GNU/Linux

dom0-93:~ # free -m
total used free shared buffers cached
Mem: 4084 259 3824 0 6 84
-/+ buffers/cache: 169 3915
Swap: 1916 0 1916
dom0-93:~ # cat /proc/cmdline 
root=/dev/disk/by-id/cciss-3600508b100104839535656314933001b-part1 resume=/dev/disk/by-id/cciss-3600508b100104839535656314933001b-part2 splash=silent crashkernel=256M-:128M vga=0x317 mem=5120M


when I didn't append "mem=" parmeter, "free -m " shows that total memory is  7892M ,
it only eat up  300M (8192-7892), though I don't know where the 300M memory go.

why? 

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Re: [help] kernel boot parameter "mem=xx" disparity

[KOSAKI Motohiro]

On Tue, Oct 16, 2012 at 8:55 PM, Jun Hu <duanshuidao@hotmail.com> wrote:
> Hi Guys:
>
> My machine has 8G memory, when I use kernel boot parameter mem=5G , it only
> display “4084 M” using “free –m “.
> where the “5120-4084 = 1036M “ memory run?

mem is misleading parameter. It is not specify amount memoy. It is specify
maximum recognized address. Thus when your machine have some memory
hole, you see such result. Don't worry, recent regular machine often have
 ~1G hole.

Detailed memory map is logged in /var/log/messages as a part of boot messages.

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Re: [help] kernel boot parameter "mem=xx" disparity

[Jun Hu]

Yes, I  understand now,  through google "memory hole" ; thanks a lot.

recent regular machine often have  ~1G hole. It  is  true.

I found  from boot msg:

55:<6>[    0.000000] Memory: 4171008k/5242880k available (4404k kernel code, 
788584k absent, 283288k reserved, 7685k data, 1340k init)

it tells me where 1G memory went .



-----a??a??e?(R)a>>?----- 
From: KOSAKI Motohiro
Sent: Wednesday, October 17, 2012 9:43 AM
To: Jun Hu
Cc: linux-mm
Subject: Re: [help] kernel boot parameter "mem=xx" disparity

On Tue, Oct 16, 2012 at 8:55 PM, Jun Hu <duanshuidao@hotmail.com> wrote:
> Hi Guys:
>
> My machine has 8G memory, when I use kernel boot parameter mem=5G , it 
> only
> display a??4084 Ma?? using a??free a??m a??.
> where the a??5120-4084 = 1036M a?? memory run?

mem is misleading parameter. It is not specify amount memoy. It is specify
maximum recognized address. Thus when your machine have some memory
hole, you see such result. Don't worry, recent regular machine often have
~1G hole.

Detailed memory map is logged in /var/log/messages as a part of boot 
messages.

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