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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Oct 23, 2014 at 04:30:24PM +0200, Roger Pau Monné 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).

Wei.

> Roger.

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