Re: [Qemu-devel] [RFC PATCH RDMA support v4: 03/10] more verbose documentation of the RDMA transport

[Michael R. Hines]

On 03/20/2013 04:37 PM, Michael S. Tsirkin wrote:
> On Wed, Mar 20, 2013 at 04:24:14PM -0400, Michael R. Hines wrote:
> > On 03/20/2013 11:55 AM, Michael S. Tsirkin wrote:
> > > Then, later, in a separate patch, I can implement /dev/pagemap support.
> > >
> > > When that's done, RDMA dynamic registration will actually take effect and
> > > benefit from actually verifying that the page is mapped or not.
> > >
> > > - Michael
> > > Mapped into guest? You mean e.g. for ballooning?
> > Three scenarios are candidates for mapped checking:
> >
> > 1. anytime the virtual machine has not yet accessed a page (usually
> > during the 1st-time boot)
> So migrating booting machines is faster now?  Why is this worth
> optimizing for?
Yes, it helps both the TCP migration and RDMA migration simultaneously.
> > 2. Anytime madvise(DONTNEED) happens (for ballooning)
> This is likely worth optimizing.
> I think a better the way to handling this one is by tracking
> ballooned state. Just mark these pages as unused in qemu.

Paolo said somebody attempted that, but stopped work on it for some reason?

> > 3.  Anytime cgroups kicks out a zero page that was accessed and
> > faulted but not dirty that is a clean candidate for unmapping.
> >         (I did a test that seems to confirm that cgroups is pretty
> > "smart" about that)
> > Basically, anytime the pagemap says "this page is *not* swap and
> > *not* mapped
> > - then the page is not important during the 1st iteration.
> > On the subsequent iterations, we come along as normal checking the
> > dirty bitmap as usual.
> >
> > - Michael
> If it will never be dirty you will never migrate it?
> Seems wrong - it could have guest data on disk - AFAIK clean does not
> mean no data, it means disk is in sync with memory.

Sorry, yes - that was a mis-statement: clean pages are always mapped (or 
swapped) and would have to
be transmitted at least once.

- Michael