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

On Thu, 11 Apr 2013, Marek Marczykowski wrote:
> 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).

This is the (in)famous incompatibility between autoballoon and dom0_mem.

> 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.

Calling "xl set-mem 0" manually should be compatible with
autoballoon. However it wouldn't work with dom0_mem.

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

Could we introduce something similar to autoballoon=auto to libvirt?

Maybe we should push down the autoballoon option to libxl: we should
probably rename autoballoon to libxl_memory_management, enable it
automatically during initialization (and call
libxl__fill_dom0_memory_info) unless dom0_mem has been passed as an
option, in which case we would disable it. If we do it at the libxl
level we would cover xl as well as libvirt and also fix the "xl set-mem
0" case.

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