Xen users - Jul 2008 - Ballooning and Free memory in xm info

Hi,


Is it expected behavior that when ballooning is enabled the amount of 
free memory visisble is 28k (thus: 0) in free_memory?


Stefan

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On Saturday 19 July 2008, Stefan de Konink wrote:
> Hi,
>
>
> Is it expected behavior that when ballooning is enabled the amount of
> free memory visisble is 28k (thus: 0) in free_memory?
Do you mean autoballooning of dom0?  Before you''ve started and
destroyed any
guests then I''d guess this is normal - dom0 will have been sized to use
all
available memory.  From Xen''s PoV the memory is not free because dom0 
currently owns it; dom0 will free it to Xen before starting a domU when 
autoballooning is active.

Or did you mean something else?

Cheers,
Mark


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Mark Williamson schreef:
> On Saturday 19 July 2008, Stefan de Konink wrote:
>> Is it expected behavior that when ballooning is enabled the amount of
>> free memory visisble is 28k (thus: 0) in free_memory?
> 
> Do you mean autoballooning of dom0?  Before you''ve started and
destroyed any
> guests then I''d guess this is normal - dom0 will have been sized
to use all
> available memory.  From Xen''s PoV the memory is not free because
dom0
> currently owns it; dom0 will free it to Xen before starting a domU when 
> autoballooning is active.
> 
> Or did you mean something else?
Even with one domain active it is still 0. I''m currently creating an 
cluster economy. One part of the bidding process is new domain is of 
course free memory. I''m using libvirt, and this is
''smartly'' reporting
only 28k free memory with ballooning on. That is probably equal to xm 
info does in megabytes.

I have ''solved'' the issue by specifying dom0_mem=... at the
Xen cmdline,
but I guess there must be a way to get the value:

total_free = xen_total - (dom0_total - dom0_free)

(from the ballooning driver)



Stefan

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On Saturday 19 July 2008, Stefan de Konink wrote:
> Mark Williamson schreef:
> > On Saturday 19 July 2008, Stefan de Konink wrote:
> >> Is it expected behavior that when ballooning is enabled the amount
of
> >> free memory visisble is 28k (thus: 0) in free_memory?
> >
> > Do you mean autoballooning of dom0?  Before you''ve started
and destroyed
> > any guests then I''d guess this is normal - dom0 will have
been sized to
> > use all available memory.  From Xen''s PoV the memory is not
free because
> > dom0 currently owns it; dom0 will free it to Xen before starting a
domU
> > when autoballooning is active.
> >
> > Or did you mean something else?
>
> Even with one domain active it is still 0.
I think that''s still expected behaviour; dom0 has given up the memory
and it''s
all been allocated to the domU.  If you were to destroy the domU, then 
there''d be free memory (apparently it isn''t reclaimed by
dom0).

There used to be more free memory floating about in the system to act as a 
pool for the network driver page flipping but this mechanism is not used 
anymore unless you have a really old guest.  This may mean there''s less
free
memory hanging around with domUs running than you remember seeing before!
> I''m currently creating an 
> cluster economy
Ah, a nice plan.
> One part of the bidding process is new domain is of 
> course free memory. I''m using libvirt, and this is
''smartly'' reporting
> only 28k free memory with ballooning on. That is probably equal to xm
> info does in megabytes.
>
> I have ''solved'' the issue by specifying dom0_mem=... at
the Xen cmdline,
> but I guess there must be a way to get the value:
>
> total_free = xen_total - (dom0_total - dom0_free)
>
> (from the ballooning driver)
I was going to suggest you use dom0_mem :-)  I think that''s the
pragmatic way
to go about this...

I guess you could try to do some kind of calculation based on the amount of 
free memory *in* dom0 that it might be able to give back to Xen through 
ballooning.  You should get more predictable performance by just having dom0 
at a set memory footprint instead of snatching memory off it occasionally, so 
unless you *really* need to have it autoballooning I think your approach 
dom0_mem is appropriate.

Make sure to disable autoballooning so you don''t confuse things by 
accidentally shrinking dom0 further :-)

Cheers,
Mark


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Mark Williamson schreef:
>> I''m currently creating an 
>> cluster economy
> 
> Ah, a nice plan.
It is based around my current REST http-interface to libvirt, so an 
avahi request is put on the network (offer), and the available cluster 
nodes can give their tender, lowest will get it. For now I keep it 
simple; that means no exact migration costs, just the cost of 
overcommitting with respect to free memory.

Our setup require this to work before ip address allocation... but I 
suppose it would work for anyone with just a switched network as a charm.
> Make sure to disable autoballooning so you don''t confuse things by
> accidentally shrinking dom0 further :-)
Will I get away with dom0_mem or should I pass dom0-min-mem=0 in the 
xendconfig? Or is there a cmdline param that disables ballooning?


Stefan

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On Saturday 19 July 2008, Stefan de Konink wrote:
> > Make sure to disable autoballooning so you don''t confuse
things by
> > accidentally shrinking dom0 further :-)
>
> Will I get away with dom0_mem or should I pass dom0-min-mem=0 in the
> xendconfig? Or is there a cmdline param that disables ballooning?
The xend config setting is the way to do this.  If dom0_mem on the command 
line is less than dom0-min-mem in the config file then autoballooning
won''t
happen anyway; it seems sensible to make it explicit, though.

Cheers,
Mark


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