Re: [Xen-devel] libxl: error: libxl.c:2150:libxl_set_memory_target new target 0 for dom0 is below the minimum threshold

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-11-23 at 11:23 +0000, Stefano Stabellini wrote:
> On Wed, 23 Nov 2011, Ian Campbell wrote:
> > On Wed, 2011-11-23 at 10:56 +0000, Stefano Stabellini wrote:
> > > On Tue, 22 Nov 2011, Teck Choon Giam wrote:
> > > > >> >> That means default value changed if it is unset for autoballoon?
> > > > >> >> Original one is commented with #autoballoon = 1 in 
> > > > >> >> /etc/xen/xl.conf so
> > > > >> >> it is unset I guess so the default value for autoballoon for c/s 
> > > > >> >> 23110
> > > > >> >> is autoballoon=0 where c/s 23190 is autoballoon=1? ÃÆÃâ?ÃÆÃâÅÃâ 
> > > > >> >> Just some
> > > > >> >> guessing... ...
> > > > >
> > > > > Autoballoon and dom0_mem are incompatible.
> > > > > I don't think there are any relevant differences between 23110 and 
> > > > > 23190
> > > > > on xen-unstable, maybe you used to start dom0 without dom0_mem before?
> > > > 
> > > > Nope... all my servers will always have those dom0_mem set.  It is
> > > > from xen-4.1-testing not xen-unstable for the two changeset.
> > > 
> > > I still cannot see anything in that range. However as I said before, it
> > > is expected that with dom0_mem set autoballoon needs to be disabled.
> > 
> > We still haven't seen the full node IDs of the changesets which I asked
> > for so it is not obvious that we are actually looking at the same range
> > of changesets...
> > 
> > > Maybe it is worth documenting this somewhere, any suggestions where?
> > 
> > xl man page? Perhaps we also need a separate /etc/xl.cfg (or whatever
> > the path is) page?
> > 
> > If there is anywhere on the wiki which recommends dom0_mem= (and there
> > should be) it should simultaneously mention this setting. As should any
> > "getting started" page.
> > 
> > On the other hand I'm not sure why autoballoon and dom0_mem play so
> > badly together. Surely if dom0_mem is used autoballon should just see
> > that there is plenty of free RAM in the system and not do anything?
>  
> Because one of the "memory slack" paramters needed by autoballoon is
> 
> totalmemory - dom0_memory_at_boot

libxl gets dom0_memory_at_boot via the dominfo (in
libxl__fill_dom0_memory_info which I assume happens before any
potentially autoballooning) which should take dom0_mem into account,
doesn't it?

> fortunately libxl is smart enough to trim it to 15% of the total memory
> of the system, just in case a user forgot to disable autoballon but set
> dom0_mem.
> Ah! Now that you make me think about it, I bet the failure comes from a
> system that is missing:
> 
> libxl: Introduce a maximum limit for free_mem_slack (re (dom0) ballooning)
>     
>     This fixes this message:
>      libxl: error: libxl.c:2921:libxl_set_memory_target new target
>                    for dom0 is below the minimum threshold
>     which can occur spuriously if dom0_mem is specified and xl
>     autoballoning is left turned on.

It's in 4.1-testing as 22210:4d95192b2fc8.


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