Re: eliminating 166G limit (was Re: [Xen-devel] Problem with nr_nodes on large memory NUMA machine)

[beth kon <eak@xxxxxxxxxx>]

Has there been any more thought on this subject? The discussion seems to 
have stalled, and we're hoping to find a way past this 166G limit...
Jan Beulich wrote:

> > > > Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 27.11.07 10:21 >>>
> > > >        
> > On 27/11/07 09:00, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:
> >
> >    
> >
> > > > I don't get how your netback approach works. The pages we transfer do not
> > > > originate from netback, so it has little control over them. And, even if it
> > > > did, when we allocate pages for network receive we do not know which
> > > > domain's packet will end up in each buffer.
> > > >        
> > > Oh, right, I mixed up old_mfn and new_mfn in netbk_gop_frag(). Nevertheless
> > > netback could take care of this by doing the copying there, as at that point i
> > > already knows the destination domain.
> > >      
> > You may not know constraints on that domain's max_mfn though. We could add
> > an interface to Xen to interrogate that, but generally it's not something we
> > probably want to expose outside of Xen and the guest itself.
> >    
> What constraints other than the guest's address size influence its max_mfn?
> Of course, if there's anything beyond the address size, then having a way to
> obtain the constraint explicitly would be desirable. But otherwise (and as
> fallback) using 37 bits (128G) seems quite reasonable.
>
>  
>
> > > > Personally I think doing it in Xen is perfectly good enough for supporting
> > > > this very out-of-date network receive mechanism.
> > > >        
> > > I'm not just concerned about netback here. The interface exists, and other
> > > users might show up and/or exist already. Whether it would be acceptable
> > > for them to do allocation and copying is unknown. You'd therefore either
> > > need a way to prevent future users of the transfer mechanism, or set proper
> > > requirements on its use. I think that placing extra requirements on the user
> > > of the interface is better than introducing extra (possibly hard to reproduce/
> > > recognize/debug) possibilities of failure.
> > >      
> > The interface is obsolete.
> >    
> Then it should be clearly indicated as such, e.g. by a mechanism similar to
> deprecated_irq_flag() in Linux 2.6.22. And as a result, its use in netback 
> should
> then probably be conditional upon an extra config option, which could at once
> be used to provide a note to Xen that the feature isn't being used so that the
> function could return -ENOSYS and the clipping could be avoided/reverted.
>
> Jan
>
>
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>  

--
Elizabeth Kon (Beth)
IBM Linux Technology Center
Open Hypervisor Team
email: eak@xxxxxxxxxx


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