Re: Xen balloon driver improvement (version 1)

[Wei Liu <wei.liu2@citrix.com>]

On Thu, Oct 23, 2014 at 11:09:19AM +0100, David Vrabel wrote:
[...]
> > ### Worker thread to coalesce small size pages
> > 
> > Worker thread wakes up periodically to check if there's enough pages
> > in normal size page queue to coalesce into a huge page. If so, it will
> > try to exchange that huge page into a number of normal size pages with
> > XENMEM\_exchange hypercall.
> 
> I don't think you need a new worker thread for this,  the existing page
> migration is already trying to keep the ballooned zone contiguous so
> after migrating pages you need only try and move contiguous ballooned 4k
> pages to the 2M list.
> 

After some more thought on this, a new worker thread is not needed. It's
possible that current balloon thread does both ballooning work and
coalescing work, as they are mutually exclusive workload so one thread
should be enough.

As for moving contiguous ballooned pages from 4K list to 2M list,
unfortunately I see a problem with this proposal: The 4K pages list is
not sorted. Sorting it requires hooking into core balloon driver -- that
is, to grab multiple locks to avoid racing with page migration thread,
which is prone to error.

Wei.