Re: [Xen-devel] [libvirt] [PATCH RESENT 04/12] libxl: populate xenstore memory entries at startup

[Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On 11.04.2013 09:52, Ian Campbell wrote:
> On Thu, 2013-04-11 at 05:09 +0100, Jim Fehlig wrote:
>>> +    /* This will fill xenstore info about free and dom0 memory - if 
>>> missing,
>>> +     * should be called before starting first domain */
>>> +    if (libxl_get_free_memory(libxl_driver->ctx, &free_mem)) {
>>> +        VIR_ERROR(_("cannot get free memory info"));
>>> +        goto error;
>>> +    }
>>>   
>>
>> Should failure of libxl_get_free_memory() really be fatal and prevent
>> the driver from loading?
> 
> I'm not sure it is intended to be called like this...
> 
> I think it is intended to be called as part of starting every domain, to
> check if there is enough free memory for that domain, rather than
> calling it once at start of day.
> 
> In that context if it fails or returns less than the required amount of
> memory then that would be fatal for starting that domain.
> 
> In xl we use this as part of the auto balloon of dom0, see
> xl_cmdimplg.c:freemem. Does libvirt do autoballooning or does it require
> dom0_mem? Perhaps this is handled at a higher level?

The problem is how libxl set initial value for freemem-slack. If, for any
reason, dom0 hasn't (almost) all memory assigned before creating first domain,
15% of host memory will no longer be used at all. This "any reason" can be
dom0_mem, which is covered by "auto" value for autoballoon in xen-unstable
(actually only for xl, not libxl in general). But this can also happen if
somebody calls xl set-mem 0 <some value>. The later case doesn't mean the user
want to disable autoballoon completely.

And to answer you question - libvirt rely on libxl autoballoon.

-- 
Best Regards / Pozdrawiam,
Marek Marczykowski
Invisible Things Lab

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