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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 22 May 2013, Jim Fehlig wrote:
> Marek Marczykowski wrote:
> > On 19.04.2013 13:10, Stefano Stabellini wrote:
> >   
> >> 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. 
> >>     
> >
> > Unless it is called before first domain start (which case is covered by my 
> > patch).
> >
> >   
> >> 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.
> >>     
> >
> > Looks good for me, but IIUC it's too late for such change in 4.3 and it
> > doesn't qualify for later backport, right? If so, some approach still is
> > needed in libvirt for xen 4.2 and 4.3 (like my simple patch). This still
> > doesn't make libvirt compatible with dom0_mem, but at least cover one
> > (independent) case. Perhaps additionally autoballoon=auto code should be
> > copied from xl to libvirt to cover dom0_mem case with xen 4.2 and 4.3?
> >   
> 
> After reading this thread again, it sounds like the best option is to
> support the autoballoon option in libvirt, e.g. in
> /etc/libvirt/libxl.conf.  (We currently don't have a config file for the
> libxl driver, but I think we'll need one anyhow for other knobs, similar
> to qemu.conf.)   As you note, even if autoballoon is pushed down to
> libxl, libvirt would need to handle it for older libxl.  And libvirt
> needs to handle the dom0_mem case...
 

Given all the troubles that we had with the autoballoon option in
xl/libxl and how it clashes with dom0_mem, I would strongly advise to
consider implementing something like autoballoon=auto from the start.

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