Re: [Xen-devel] [PATCH 4 of 4] xenpaging: initial libxl support

[Tim Deegan <tim@xxxxxxx>]

At 17:38 +0100 on 11 Jan (1326303483), Olaf Hering wrote:
> On Wed, Jan 11, Tim Deegan wrote:
> 
> > > Isnt that up to the host admin to decide where to take the memory from?
> > > So if its acceptable to swap parts of a VM (independent from what the
> > > guest OS thinks it has), so be it.
> > 
> > Why?  The _only_ reason I can imagine for wanting to use paging is when
> > the balloon driver can't or won't do its job.  There's no advantage to
> > paging except that you can always force it to happen.
> 
> Isnt that the whole point of paging, to make it happen at will without
> the guest (or the application at process level) noticing it?

Yes, but that's a _bad_ thing. :)  If the guest can co-operate, you'll
get way better eviction choices, better performance, and better
accounting (since the I/O is done by the guest to guest-owned disk).

That's why I think both mechanisms should be visible up to the libxl
layer, but xl itself should just implement the one sensible policy:
try ballooning first, then page if that fails.  

Tim.



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