xl vs. xm: memory consumption

xl vs. xm: memory consumption

[Christoph Egger]

Hi!

Is it possible that xl needs more memory than xm?
Does xl log somewhere how much memory is free and how much it does want?

xl create -c <guest.conf>
Parsing config file <guest.conf>
failed to free memory for the domain

xm create -c <guest.conf>
[guest boot messages]

Christoph


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Re: xl vs. xm: memory consumption

[Stefano Stabellini]

On Fri, 5 Nov 2010, Christoph Egger wrote:
> 
> Hi!
> 
> Is it possible that xl needs more memory than xm?
> Does xl log somewhere how much memory is free and how much it does want?
> 
> xl create -c <guest.conf>
> Parsing config file <guest.conf>
> failed to free memory for the domain
> 
> xm create -c <guest.conf>
> [guest boot messages]
> 

That's because of freemem-slack: a basic amount of memory left free
on the system otherwise memory allocation for new VMs fails.
It is calculated using the amount of free memory at boot time, assuming
that dom0 was booted using all the memory in the system.
If you boot dom0 passing dom0_mem to xen, then you have to disable
autoballooning (autoballoon=0 in /etc/xen/xl.conf).
Given the other bug you reported on libxl__fill_dom0_memory_info, I
suspect that the freemem-slack value might be wrong on your system.
On the other hand if your configuration is correct, we might have to
lower the maximum value of freemem-slack that currently is 15% of the
total memory in the system.

Re: xl vs. xm: memory consumption

[Ian Jackson]

Stefano Stabellini writes ("Re: [Xen-devel] xl vs. xm: memory consumption"):
> It is calculated using the amount of free memory at boot time, assuming
> that dom0 was booted using all the memory in the system.

Perhaps it would be worth spotting if freemem_slack is very large and
issuing a warning ?  That would help spot this particular
configuration error which I suspect may become common.

Ian.

Re: xl vs. xm: memory consumption

[Stefano Stabellini]

On Wed, 10 Nov 2010, Ian Jackson wrote:
> Stefano Stabellini writes ("Re: [Xen-devel] xl vs. xm: memory consumption"):
> > It is calculated using the amount of free memory at boot time, assuming
> > that dom0 was booted using all the memory in the system.
> 
> Perhaps it would be worth spotting if freemem_slack is very large and
> issuing a warning ?  That would help spot this particular
> configuration error which I suspect may become common.
 
I agree. Currently we limit freemem_slack to 15% of the system memory,
maybe we should lower that limit?