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

[Yang Hongyang <yanghy@cn.fujitsu.com>]

On 07/30/2015 09:59 PM, Dr. David Alan Gilbert wrote:
> * zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
>> On 2015/7/30 20:30, Dr. David Alan Gilbert wrote:
>>> * 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.
>>>
>>
>> Do you mean if there are consistent checkpoint requests, not do checkpoint but just send a special message to SVM?
>> Resume to common COLO mode until the checkpoint lengths is so not short ?
>
>    We still have to do checkpoints, but we send a special message to the SVM so that
> the SVM just takes the checkpoint but does not run.
>
>    I'll send the code after I've updated it to your current version; but it's
> quite rough/experimental.
>
> It works something like
>
>   -----------run PVM     run SVM
>       COLO     <long gap>
>       mode       miscompare
>                  checkpoint
>   -----------run PVM     run SVM
>       COLO     <short gap>
>       mode       miscompare
>                  checkpoint
>   -----------run PVM     run SVM
>       COLO     <short gap>
>       mode       miscompare         < After a few short runs
>                  checkpoint
>   -----------run PVM     SVM idle   \
>     Passive    <fixed delay>        |  - repeat 'n' times
>       mode       checkpoint         /
>   -----------run PVM     run SVM
>       COLO     <short gap>          < Still a short gap
>       mode       miscompare
>   -----------run PVM     SVM idle   \
>     Passive    <fixed delay>        |  - repeat 'n' times
>       mode       checkpoint         /
>   -----------run PVM     run SVM
>       COLO     <long gap>          < long gap now, stay in COLO
>       mode       miscompare
>                  checkpoint
>   -----------run PVM     run SVM
>       COLO     <long gap>
>       mode       miscompare
>                  checkpoint
>
> So it saves the CPU time on the SVM, and the comparison traffic, and is
> automatic at switching into the passive mode.
>
> It used to be more useful, but your minimum COLO run time that you
> added a few versions ago helps a lot in the cases where there are miscompares,
> and the delay after the miscompare before you take the checkpoint also helps
> in the case where the data is very random.

This is great! This is exactly what we were thinking about, when random
scenario will fallback to MC/Remus like FT. Thank you very much!
I have a question, do you also modify colo-proxy kernel module? because
in the fixed checkpoint mode, I think we need to buffer the network
packets, and release them at checkpoint.

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

-- 
Thanks,
Yang.