Re: [Qemu-devel] [PATCH kernel 0/2] speed up live migration by skipping free pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 25, 2016 at 05:38:30PM +0530, Amit Shah wrote:
> On (Mon) 25 Apr 2016 [14:04:06], Michael S. Tsirkin wrote:
> > On Mon, Apr 25, 2016 at 11:36:41AM +0530, Amit Shah wrote:
> > > On (Tue) 19 Apr 2016 [22:34:32], Liang Li wrote:
> > > > Current QEMU live migration implementation mark all guest's RAM pages
> > > > as dirtied in the ram bulk stage, all these pages will be processed
> > > > and it consumes quite a lot of CPU cycles and network bandwidth.
> > > > 
> > > > From guest's point of view, it doesn't care about the content in free
> > > > page. We can make use of this fact and skip processing the free
> > > > pages, this can save a lot CPU cycles and reduce the network traffic
> > > > significantly while speed up the live migration process obviously.
> > > > 
> > > > This patch set is the kernel side implementation.
> > > > 
> > > > The virtio-balloon driver is extended to send the free page bitmap
> > > > from guest to QEMU.
> > > > 
> > > > After getting the free page bitmap, QEMU can use it to filter out
> > > > guest's free pages. This make the live migration process much more
> > > > efficient.
> > > > 
> > > > In order to skip more free pages, we add an interface to let the user
> > > > decide whether dropping the cache in guest during live migration.
> > > 
> > > So if virtio-balloon is the way to go (i.e. speed is acceptable), I
> > > just have one point then.  My main concern with using (or not using)
> > > virtio-balloon was that a guest admin is going to disable the
> > > virtio-balloon driver entirely because the admin won't want the guest
> > > to give away pages to the host, esp. when the guest is to be a
> > > high-performant one.
> > 
> > The result will be the reverse of high-performance.
> > 
> > If you don't want to inflate a balloon, don't.
> > 
> > If you do but guest doesn't respond to inflate requests,
> > it's quite reasonable for host to kill it -
> > there is no way to distinguish between that and
> > guest being malicious.
> 
> With the new command I'm suggesting, the guest will let the host know
> that it has enabled this option, and it won't free up any RAM for the
> host.
> 
> Also, just because a guest doesn't release some memory (which the
> guest owns anyway) doesn't make it malicious, and killing such guests
> is never going to end well for that hosting provider.
> 
> > I don't know of management tools doing that but
> > it's rather reasonable. What does happen is
> > some random guest memory is pushed it out to swap,
> > which is likely much worse than dropping unused memory
> > by moving it into the balloon.
> 
> Even if the host (admin) gave a guarantee that there won't be any
> ballooning activity involved that will slow down the guest, a guest
> admin can be paranoid enough to disable ballooning.  If, however, this
> is made known to the host, it's likely a win-win situation because the
> host knows the guest needs its RAM, and the guest can still use the
> driver to send stats which the host can use during migration for
> speedups.
> 
> 
> 		Amit

We'd need to understand the usecase better to design a good interface
for this. AFAIK the normal usecase for ballooning is for
memory overcommit: asking guest to free up memory might work
better than swap which makes host initiate a bunch of IO.
How is not inflating in this case a good idea?
I'm afraid I don't understand why was inflating balloon
requested if we do not want the guest to inflate the balloon.  What does
"paranoid" mean in this context?  This seems to imply some kind of
security concern. Is guest likely to need all of its memory or a
specific portion of it? Is it likely to be a static configuration or a
dynamic one? If dynamic, does guest also want to avoid deflating the
balloon or only inflating it?


-- 
MST