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Re: [Qemu-discuss] Qemu reports slightly less memory on PXE boot


From: Jakob Bohm
Subject: Re: [Qemu-discuss] Qemu reports slightly less memory on PXE boot
Date: Mon, 4 Jan 2016 16:55:58 +0100
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0

On 04/01/2016 15:21, Prem Anand Haridoss wrote:

On booting an iso (from the cdrom) with 8G memory, after bootup, I see 8179816 kB as the total memory available.

However on PXE booting the same vCPE livecd iso over a nic, with the same 8G memory, we see only 7857512 kB as the total memory available

 

Is this a bug in qemu? I am using version 2.5.0 (Debian 1:2.5+dfsg-1)

 

Regards

Prem

 

Logs:

=====

On booting iso using cdrom

--------------------------

address@hidden:~/work/tplaf/ qemu-system-x86_64 -name Test -nographic -enable-kvm -cpu Haswell -smp 2 -m 8G -cdrom $iso -boot d

 

ISOLINUX 4.05 20121210  Copyright (C) 1994-2011 H. Peter Anvin et al

Press control and F then 1 for help, or ENTER to

boot: live-serial init=/bin/bash

Loading, please wait...

bash: no job control in this shell

root@(none):/#

root@(none):/# cat /proc/meminfo | head -n3

MemTotal:        8179816 kB

MemFree:         8138960 kB

MemAvailable:    8131960 kB

 

On pxe booting the iso over a nic

---------------------------------

address@hidden:~/ qemu-system-x86_64 -name Test -nographic -enable-kvm -cpu Haswell -smp 2 -m 8G -netdev user,id=net0,tftp=$HOME/TPLAF//TB2/rtr2/tftp,bootfile=/gpxelinux.0 -device rtl8139,netdev=net0 -boot

 

PXELINUX 6.03 PXE 6.03-pre20 Copyright (C) 1994-2014 H. Peter Anvin et al

boot: Build109.11

Loading memdisk... ok

Loading <deleted>...ok

 

ISOLINUX 4.05 20121210  Copyright (C) 1994-2011 H. Peter Anvin et al

Press control and F then 1 for help, or ENTER to

boot: live-serial init=/bin/bash

Loading, please wait...

bash: no job control in this shell

root@(none):/#

root@(none):/# cat /proc/meminfo | head -n3

MemTotal:        7857512 kB

MemFree:         7816764 kB

MemAvailable:    7809988 kB

Presumably, the PXE BIOS needs some RAM for the network
stack and CD-ROM emulation.


Perhaps it simply loads the entire ISO image into RAM
before booting it, so the boot process can play with
the network adapter without loosing access to its "CD".


Check if the difference is close to the size of the ISO
file.


Enjoy

Jakob
-- 
Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
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