Re: [Qemu-devel] [POC] colo-proxy in qemu

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Gonglei (arei.gonglei@huawei.com) wrote:
> On 2015/7/30 19:56, Dr. David Alan Gilbert wrote:
> > * Jason Wang (jasowang@redhat.com) wrote:
> >>
> >>
> >> On 07/30/2015 04:03 PM, Dr. David Alan Gilbert wrote:
> >>> * Dong, Eddie (eddie.dong@intel.com) wrote:
> >>>>>> A question here, the packet comparing may be very tricky. For example,
> >>>>>> some protocol use random data to generate unpredictable id or
> >>>>>> something else. One example is ipv6_select_ident() in Linux. So COLO
> >>>>>> needs a mechanism to make sure PVM and SVM can generate same random
> >>>>> data?
> >>>>> Good question, the random data connection is a big problem for COLO. At
> >>>>> present, it will trigger checkpoint processing because of the different random
> >>>>> data.
> >>>>> I don't think any mechanisms can assure two different machines generate the
> >>>>> same random data. If you have any ideas, pls tell us :)
> >>>>>
> >>>>> Frequent checkpoint can handle this scenario, but maybe will cause the
> >>>>> performance poor. :(
> >>>>>
> >>>> The assumption is that, after VM checkpoint, SVM and PVM have identical internal state, so the pattern used to generate random data has high possibility to generate identical data at short time, at least...
> >>> They do diverge pretty quickly though; I have simple examples which
> >>> reliably cause a checkpoint because of simple randomness in applications.
> >>>
> >>> Dave
> >>>
> >>
> >> And it will become even worse if hwrng is used in guest.
> > 
> > Yes; it seems quite application dependent;  (on IPv4) an ssh connection,
> > once established, tends to work well without triggering checkpoints;
> > and static web pages also work well.  Examples of things that do cause
> > more checkpoints are, displaying guest statistics (e.g. running top
> > in that ssh) which is timing dependent, and dynamically generated
> > web pages that include a unique ID (bugzilla's password reset link in
> > it's front page was a fun one), I think also establishing
> > new encrypted connections cause the same randomness.
> > 
> > However, it's worth remembering that COLO is trying to reduce the
> > number of checkpoints compared to a simple checkpointing world
> > which would be aiming to do a checkpoint ~100 times a second,
> > and for compute bound workloads, or ones that don't expose
> > the randomness that much, it can get checkpoints of a few seconds
> > in length which greatly reduces the overhead.
> > 
> 
> Yes. That's the truth.
> We can set two different modes for different scenarios. Maybe Named
> 1) frequent checkpoint mode for multi-connections and randomness scenarios
> and 2) non-frequent checkpoint mode for other scenarios.
> 
> But that's the next plan, we are thinking about that.

I have some code that tries to automatically switch between those;
it measures the checkpoint lengths, and if they're consistently short
it sends a different message byte to the secondary at the start of the
checkpoint, so that it doesn't bother running.   Every so often it
then flips back to a COLO checkpoint to see if the checkpoints
are still really fast.

Dave

> 
> Regards,
> -Gonglei
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK