Re: [RFC] postcopy livemigration proposal

[Dor Laor <dlaor@redhat.com>]

On 08/08/2011 06:59 PM, Anthony Liguori wrote:
> On 08/08/2011 10:36 AM, Avi Kivity wrote:
>> On 08/08/2011 06:29 PM, Anthony Liguori wrote:
>>>
>>>>>> - Efficient, reduce needed traffic no need to re-send pages.
>>>>>
>>>>> It's not quite that simple. Post-copy needs to introduce a protocol
>>>>> capable of requesting pages.
>>>>
>>>> Just another subsection.. (kidding), still it shouldn't be too
>>>> complicated, just an offset+pagesize and return page_content/error
>>>
>>> What I meant by this is that there is potentially a lot of round trip
>>> overhead. Pre-copy migration works well with reasonable high latency
>>> network connections because the downtime is capped only by the maximum
>>> latency sending from one point to another.
>>>
>>> But with something like this, the total downtime is
>>> 2*max_latency*nb_pagefaults. That's potentially pretty high.
>>
>> Let's be generous and assume that the latency is dominated by page copy
>> time. So the total downtime is equal to the first live migration pass,
>> ~20 sec for 2GB on 1GbE. It's distributed over potentially even more
>> time, though. If the guest does a lot of I/O, it may not be noticeable
>> (esp. if we don't copy over pages read from disk). If the guest is
>> cpu/memory bound, it'll probably suck badly.
>>
>>>
>>> So it may be desirable to try to reduce nb_pagefaults by prefaulting
>>> in pages, etc. Suffice to say, this ends up getting complicated and
>>> may end up burning network traffic too.

It is complicated but can help (like pre faulting working set size 
pages). Beyond that async page fault will help a bit.
Lastly, if a guest has several apps, those that are memory intensive 
might suffer but light weight apps will function nicely.
It provides extra flexibility over the current protocol (that still has 
value for some of the loads).

>>
>> Yeah, and prefaulting in the background adds latency to synchronous
>> requests.
>>
>> This really needs excellent networking resources to work well.
>
> Yup, it's very similar to other technologies using RDMA (single system
> image, lock step execution, etc.).
>
> Regards,
>
> Anthony Liguori
>
>>
>
>