Re: [Xen-users] System-RAM: 3325MB question

[Frank Schubert <f.schubert@xxxxxxxxxxxx>]

Petersson, Mats wrote:
>  
> 
>> -----Original Message-----
>> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
>> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
>> Frank Schubert
>> Sent: 28 April 2006 09:47
>> To: xen-users Mailinglist
>> Subject: [Xen-users] System-RAM: 3325MB question
>>
>> Hi,
>>
>> first I want to send many thanks to the Xen developers and to 
>> this list.
>>
>> Here's my question:
>> Today i upgraded a server to 4GB RAM but Xen only shows 3325MB RAM?:
>>
>>  Xen version 3.0.2-2 (xenod@xxxxxxxxxxxx) (gcc version 3.3.3 
>> 20040412 (Red Hat Linux 3.3.3-7)) Thu Apr 13 17:34:06 BST 
>> 2006  Latest ChangeSet: Thu Apr 13 15:18:37 2006 +0100 
>> 9617:5802713c159b
>>
>>
>> Do i already need PAE extension for this?
>> (I thought PAE is only needed if i want more than 4GB.)
> 
> Short answer: Yes.
> 
> The reason you only see 3.3GB is that the PCI space has to live
> somewhere - and for practical purposes, the BIOS deciedes that SOMEWHERE
> is below 4GB - so you get your PCI devices taking up around 700MB, and
> that is what eats up the memory. 
> 
> The RAM will then be "redirected" to above 4GB - so you need PAE or
> 64-bit addressing (which is PAE with a fourth level of page-tables
> added) to see more than 4GB of address space, and since some of your ram
> is above 4GB, you need PAE on. 
> 
> --
> Mats

Thank you for your explanation.

Just because I'm curious:

xm dmesg says:
[...]
(XEN) Using scheduler: Simple EDF Scheduler (sedf)
(XEN) PAE disabled.
(XEN) found SMP MP-table at 000ff780
[...]

Can I simply enable PAE with a kernel option in grub?
(The output looks like i could do that.)

>From other postings I know that I should recompile...

Regards,
Frank

-- 
Frank Schubert
Systemadministration
EsPresto AG
Breite Str. 30-31
10178 Berlin/Germany
Tel: +49.(0)30.90 226.750
Fax: +49.(0)30.90 226.760
f.schubert@xxxxxxxxxxxx

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