Re: [Xen-devel] Free memory reported but not used in new domain creation?

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Mon, Aug 12, 2013 at 03:55:17PM +0800, G.R. wrote:
> On Mon, Aug 12, 2013 at 2:15 PM, Pasi Kärkkäinen <pasik@xxxxxx> wrote:
> > On Mon, Aug 12, 2013 at 01:46:30PM +0800, G.R. wrote:
> >> Hi guys,
> >> I need your help understanding this behavior:
> >>
> >> My host has 8GB RAM installed. Xen reports about 1.8G free memory with
> >> three domains running.
> >>
> >> xl info|grep memory
> >> total_memory           : 7887
> >> free_memory            : 1833
> >>
> >> xl list
> >> Name                                        ID   Mem VCPUs      State   
> >> Time(s)
> >> Domain-0                                     0   768     8     r-----     
> >> 650.2
> >> nas                                          1  3072     2     -b----     
> >> 704.0
> >> windom                                       3  2047     4     r-----     
> >> 520.7
> >>
> >> Now, when I tried to launch a forth domain with 1G memory assigned,
> >> here is the result:
> >>
> >> xl list
> >> Name                                        ID   Mem VCPUs      State   
> >> Time(s)
> >> Domain-0                                     0   380     8     r-----     
> >> 654.1
> >> nas                                          1  3072     2     -b----     
> >> 706.7
> >> windom                                       3  2047     4     r-----     
> >> 529.8
> >> freebsd                                      5  1024     1     -b----      
> >>  0.7
> >>
> >> xl info|grep memory
> >> total_memory           : 7887
> >> free_memory            : 1184
> >>
> >> As you can see, hypervisor is ballooning memory from dom0 instead of
> >> allocating from free_memory. If I increase the guest memory of the new
> >> domain to 1.5G or 2G. It simply fails to launch, saying something like
> >> dom0 memory is below minimal threshold.
> >>
> >> This gives me the impression that the free_memory is fake. Is it an
> >> expected behavior?
> >> What are the remaining 1GB memory for?
> >>
> >> PS: I'm on the xen 4.3.0 release.
> >>
> >
> > Hello,
> >
> > This is a long-standing bug in xl.
> > The workaround is to disable xl autoballooning..
> >
> > http://wiki.xen.org/wiki/Xen_Best_Practices
> >
> 
> Thanks for the info. It'll be great if you can point me to the bug /
> thread for reference.
> 

I don't think there's a "bug entry" about it, 
but it has been discussed multiple times here on xen-devel,
and multiple people have reported the issue..

So far the response has mainly been "disable autoballoon". 
But I think it'd be better if it worked out-of-the-box!


> I thought I was following that page, but maybe I made some mistake and
> the config does not take effect:
> 
> xen_commandline        : placeholder
> dom0_mem=1G,max:1536M,min:768M,dom0_max_vcpus=6,vpmu=1
> 
> If there are any syntax error, shouldn't it raise error?
> 

So you need to set autoballoon=0 in /etc/xen/xl.conf

-- Pasi


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