Re: [Xen-devel] Xen balloon driver improvement (version 1)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-10-23 at 17:57 +0200, Roger Pau Monnà wrote:
> El 23/10/14 a les 17.23, Wei Liu ha escrit:
> > On Thu, Oct 23, 2014 at 04:30:24PM +0200, Roger Pau Monnà wrote:
> >> El 22/10/14 a les 18.29, Wei Liu ha escrit:
> >>> Hi all
> >>>
> >>> This is my initial design to improve Xen balloon driver.
> >>>
> >>> PDF version with graphs can be found at
> >>>
> >>> http://xenbits.xen.org/people/liuw/xen-balloon-driver-improvement.pdf
> >>>
> >>> % Xen Balloon Driver Improvement
> >>> % Wei Liu <<wei.liu2@xxxxxxxxxx>>
> >>>
> >>> -------------------------------------------
> >>> Version     Date         Changes
> >>> -------     ----         ------------------
> >>>   1         22/10/2014   Initial version.
> >>> -------------------------------------------
> >>>
> >>> ## Motives
> >>>
> >>> 1. Balloon pages fragments guest physical address space.
> >>> 1. Balloon compaction infrastructure can migrate ballooned pages from
> >>>    start of zone to end of zone, hence creating contiguous guest physical
> >>>    address space.
> >>> 1. Having contiguous guest physical address enables some options to
> >>>    improve performance.
> >>>
> >>> ## Goal of improvement
> >>>
> >>> Balloon driver makes use of as many huge pages as possible,
> >>> defragmenting both guest address space and Xen pages. This should be
> >>> achieved without any particular hypervisor side feature.
> >>>
> >>> ## Design and implementation
> >>>
> >>> When balloon driver is asked to increase / decrease reservation, it
> >>> will always start with huge page. However, due to resource
> >>> availability in both hypervisor and guest, it's not always possible to
> >>> get hold of a huge page. In that case the driver will fall back to use
> >>> normal size page. Balloon driver later will try to coalesce small size
> >>> pages into huge page. As time goes by, both Xen and guest should use
> >>> more and more huge pages.
> >>
> >> All this looks quite complicated IMHO, it's adding a lot of logic to the
> >> balloon driver. Can't you just ask the memory subsystem to allocate a
> >> page (or pages) from a specific physical range, and force it to page
> >> out/move what's there at allocation time?
> >>
> >> For example I know FreeBSD has contigmalloc(9)[1] which I think could be
> >> used to achieve this. You could start asking for pages starting at
> >> maxpfn and go down from there, keeping fragmentation at a minimum.
> >>
> >> [1]
> >> https://www.freebsd.org/cgi/man.cgi?query=contigmalloc&apropos=0&sektion=0&manpath=FreeBSD+10.1-RELEASE&arch=default&format=html
> >>
> > 
> > Good point. Just that Linux doesn't have a counterpart, not that I know
> > of. Memblock infrastructure looks similar but it's supposed to be used
> > when initialising kernel.
> > 
> > Even if Linux has similar API, it's still less desirable because to
> > satisfy a contiguous PA allocation, the system needs to be relative
> > quiet (if NO_WAIT / ATOMIC is set), or the API needs to sleep for
> > indefinite period (wait for memory subsystem to squeeze out pages).
> 
> There's no restriction on the time it might take for a guest to balloon
> out. IMHO I would rather add a new interface to the Linux VM subsystem
> that tries to accomplish this rather than adding a bunch of logic
> specific to the balloon driver.

AIUI this already exists and Wei is simply hooking up the necessary
callbacks into the balloon driver.

Ian.


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