[Xen-devel] RE: tmem - really default to on?

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> > of tmem on a per-domain basis. Then I can't see why it should be
> > only our Dom0 to be affected. And finally I can't see how the same
> > couldn't happen when only DomU-s use tmem.
> 
> I'm suggesting disabling CONFIG_TMEM for default dom0 compile

Oops, I see in re-reading your earlier posts that you
are enabling it by default for domU as well.  In that
case I agree, sadly, that your best choice might be to
disable tmem completely in your Xen hypervisor, at least
until the Xen fragmentation issues are resolved.

> -----Original Message-----
> From: Dan Magenheimer
> Sent: Monday, February 08, 2010 10:19 AM
> To: Jan Beulich
> Cc: Keir Fraser; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: tmem - really default to on?
> 
> > >This has likely been avoided by luck when lots of memory is
> > >flushed from tmem and returned to the Xen heap and consolidated.
> > >
> > >Are you suggesting that the domain structure could/should have
> > >two sizes, dynamically chosen by machine size?  Or something
> > >else?
> >
> > No, it just should be split into parts each of which fits in a page
> > independent of architecture. But that's nothing I would consider
> > realistic for 4.0.
> 
> OK.  Agreed this is too big a change for 4.0 but I'm thinking
> about post-4.0.
> 
> The order=2 shadow page allocation should also probably be
> considered a "bug" for post-4.0 as, I think, even ballooning
> will eventually fragment memory and theoretically 75% of
> physical memory might be unused and domain creation (or PV
> migration) will fail.
> 
> Since (I think) this affects other Xen 4.0 dynamic memory
> utilization solutions, I'll post a separate basenote to
> discuss that.
> 
> > >In any case, I'd still suggest turning tmem off in your dom0
> > >is the best short-term solution.
> >
> > I'm still not following you here: For one, I can't recall a way to
> turn
> > of tmem on a per-domain basis. Then I can't see why it should be
> > only our Dom0 to be affected. And finally I can't see how the same
> > couldn't happen when only DomU-s use tmem.
> 
> I'm suggesting disabling CONFIG_TMEM for default dom0 compile
> (for all dom0 for now).  Then only environments that consciously
> run a domU with a tmem-enabled kernel could be affected.
> The failure can only occur if at least one domU/dom0 enables
> tmem, and even then should only occur in certain workloads,
> though I suppose eventually sufficient fragmentation may
> occur in any workload.
> 
> Dan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel