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

[Olaf Hering <olaf@xxxxxxxxx>]

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.

We just need to right knobs.

So far we have two knobs:
maxmem=  xl mem-max
memory=  xl mem-set  (and guest OS balloon driver via sysfs)

Another knob for paging is needed. A while ago you proposed two new
commands: mem-balloon_target and mem-swap_target. Perhaps these terms
should be used also in the config file to set the initial memory/target
and memory/target-tot_pages values. If the latter is set, start the
pager. And if the latter is called, start the pager if it doesnt run
already.

At some point we will have the code ready so that PoD and paging can
coexist, so that the guests memory usage can grow on guests demand as it
does now. This is just a detail, independent from config options and
commands.


To summarize:

maxmem=  ; xl mem-max
memory=  mem-balloon_target= ; xl mem-balloon_target, xl mem-set
mem-swap_target= ; xl mem-swap_target

The rule could be like this:
mem-swap_target <= mem-balloon_target <= mem-max


> > What if the config format is like this:
> > 
> > Do things as they were done until now (PoD + balloon driver):
> >   memory=3072
> >   maxmem=4096
> >   paging=0 (or not specified at all)
> > 
> > Do things with pager instead of balloon driver and/or PoD:
> >   memory=3072
> >   maxmem=4096
> >   paging=1, or xenpaging=1
> >   xenpaging_extra=[ '-f', '/path/to/pagefile_guestname' ] (optional)
> 
> Except that this makes paging and ballooning mutually exclusive.  What
> we want is to make them work together -- to have paging as a back-up
> when ballooning fails (or isn't fast enough).

ballooning in the guest will still work. For example via sysfs, the
guest driver can release pages any time it wants to. But with the above
knobs the balloon driver can still be tweaked from the host.


Olaf

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