Re: [Xen-users] Dom0 Memory?

["Ian Tobin" <itobin@xxxxxxxxxxxxx>]

Ok makes sense now, I didn't realise it wasn't compatible anymore as I'm sure 
many others didn't but as a default config once compiled with dom_mem being 
recommended as best practice it is easy to fall into the trap I guess.

Thanks

Ian

 
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On 11 May 2012, at 17:48, "Stefano Stabellini" 
<stefano.stabellini@xxxxxxxxxxxxx> wrote:

> On Fri, 11 May 2012, Stefano Stabellini wrote:
>>> On Fri, May 11, 2012 at 3:27 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 
>>> wrote:
>>>> On Fri, 2012-05-11 at 09:11 +0100, Ian Tobin wrote:
>>> 
>>>>> Xl info
>>>>> 
>>>>> total_memory à à à à à : 15869
>>>>> free_memory à à à à à à : 13124
>>> 
>>> 
>>>>> Now I create a new config file for VM2 and I set the RAM
>>>>> 
>>>>> memory = 12288
>>> 
>>>>> libxl: error: libxl.c:2150:libxl_set_memory_target new target 0 for dom0 
>>>>> is
>>> below the minimum threshold
>>>>> 
>>>>> failed to free memory for the domain
>>>>> 
>>>>> Why it is trying to take memory from dom0??
>>>> 
>>>> You need to set "autoballoon=0" in /etc/xen/xl.conf if you don't want xl
>>>> to automatically balloon dom0 down when starting a domain -- for example
>>>> if you use the dom0_mem option.
>>>> 
>>>> Ian.
>>> 
>>> Isn't that a bug? Shouldn't xl try to use free_memory first, and only
>>> try to baloon-down memory from dom0 if it still needs more?
>> 
>> Even if this is an unsupported configuration, it could actually be a
>> bug.
>> Could you please post the output of xenstore-ls?
> 
> I didn't notice that you are actually trying to create a 12G VM.
> 
> The problem is that autoballoon=1 in xl.conf is fundamentally
> incompatible with dom0_mem on the Xen command line.
> 
> The reason is that autoballoon calculates the amount of memory that
> should be left free on the system but measuring how much memory is free
> after dom0 boots, assuming that dom0 gets all the memory.
> So if you pass dom0_mem=512M, then at boot time the free memory is
> 16G-512M => 15.5G.
> Not so long ago xl autoballoon would have tried to keep 15.5G of memory
> free in the system at all times, therefore preventing you from starting
> any guests at all.
> Fortunately we have a safety net for wrong configurations like this one:
> the amount of memory that should be left free in the system is capped to
> 15% of the total memory, in this case 0.15*16G => 2.4G.
> 
> So the reason why you cannot start a 12G guest is that 12G > 13124M - 2.4G.
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