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Re: [Xen-users] Ram Calculation

To: ShaunR <mailinglists@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Ram Calculation
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Mon, 21 Dec 2009 10:39:43 +0200
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On Sun, Dec 20, 2009 at 01:01:33PM -0800, ShaunR wrote:
> Years ago when i first started using xen i specified a dom0_mem=xxx in 
> grub.  I no longer do this, what i do now is set the dom0_min-mem 
> setting in xend-config.sxp to 1024.  I also have a few scripts that run 
> at boot and one of them does a `xm mem-set 0 1024`.  after that a xm 
> list 0 shows dom0 allocated 1024 Ram.  The problem comes on teh system 
> when i run free or free -m, free shows dom0 using 1336096 with only 
> 1160512 used.  This also prevents the other command i run `xm mem-max 0 
> 1024` from running correctly.  Anybody know if this is a bug or what may 
> be going on here.  Is there anyway to force dom0 to 1024 with out having 
> to specify it in grub?

You still should use dom0_mem= parameter in grub.conf. 

If you boot dom0 with 32 GB of RAM visible to it, and then balloon it
down to only 1 GB, the dom0 linux kernel will have problems.. for example some
network parameters are calculated based on the boot time amount of
memory, and some data structures for memory pages are reserved also based on 
the boot
time amount of memory.. so you'll waste a lot of RAM in dom0.

> Next problem is the mem usage from xm info.  xm info shows the host 
> having a total of 32766 Ram, and a free memory of 12531.  when i do a xm 
> list and total the ram for all running domains (including dom0) it 
> doesn't come out to 12531.  How can i get accurate reporting of how much 
> ram the system has, how much each guest is really using, and how much 
> ram is really free?

Xen hypervisor uses some memory itself, and there's some overhead for
every page of guest memory. Also some memory is reserved for DMA
operations etc.

The formula to calculate Xen memory overhead: 8kB per 1MB of guest plus
1MB per guest virtual cpu.

Also see "xm dmesg" for more information about memory

Hopefully that helps.

-- Pasi

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