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

At 15:58 +0100 on 11 Jan (1326297501), Olaf Hering wrote:
> On Tue, Jan 10, George Dunlap wrote:
> > On Mon, 2012-01-09 at 19:21 +0000, Olaf Hering wrote:
> > > So there is that maxmem= setting to let the guest OS configure itself
> > > for a given amount of pseudo-physical memory. Then there is a way to cut
> > > down the guest OS memory usage, both with balloon driver in guest and
> > > later with PoD.
> > > Isnt paging a better (or: just different) way to control the memory
> > > usage of a guest OS (It costs diskspace in dom0)?
> > 
> > On the contrary, hypervisor swapping is definitely *much worse* than
> > using a balloon driver.  The balloon driver was an innovation developed
> > specifically to avoid hypervisor swapping if at all possible[1].  We
> > need hypervisor swapping as a back-stop for situations where the balloon
> > driver is non-existent, or can't function immediately for some reason
> > (e.g., we've been using page-sharing to do memory overcommit and
> > suddenly have a bunch of pages un-shared); but it should always be a
> > last resort, and would ideally be mitigated by the balloon driver as
> > soon as possible.
> 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.

I think it makes sense to have two separate targets at the libxl level
(one for the balloon driver and one for the external pager/PoD), but at the
xl level (i.e. in config files and commands) there should be only one
target for memroy-actually-in-use-by-the-guest and xl should DTRT to
achieve it.  This interface is already baffling enough. :)


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