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Re: [Qemu-devel] 答复: What's the advantages of POSTCOPY over CPU-THROTTL


From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] 答复: What's the advantages of POSTCOPY over CPU-THROTTLE?
Date: Thu, 7 Jan 2016 09:14:13 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

* Zhangbo (Oscar) (address@hidden) wrote:
> Thank you David and Jason!
> 
> BTW, I noticed that Vmware did the same work alike us, but the situation is a 
> little different:
>     they proposed postcopy(in the name of QuickResume) in vSphere4.1, but 
> they substituted it with SDPS(similar to CPU-THROTTLE) from vSphere5, do you 
> know the reason behind this?
>     Reference: 
> https://qianr.wordpress.com/2013/10/14/vmware-vm-live-migration-vmotion/
> It's told that they've already introduced a shared storage to avoid losing 
> the guest when the network connection is lost. So what's their concern of 
> disposing QuickResume? 

Hmm that's a good summary; I'd not seen any detail of how vmware's systems 
worked before.
I'm not exactly sure; but I think that's saying that they store the
outstanding pages that haven't been transferred in a file on disk
so that they could be used to recover the VM later if the network failed.
It's not clear to me from that description if they do that only when the
network fails, or as part of a normal postcopy flow.

> Are there any other prices we need to pay to have postcopy?

The precopy phase in postcopy mode is slower than normal precopy migration,
but I think that's the only other penalty.

Dave

> 
> -----邮件原件-----
> 发件人: Jason J. Herne [mailto:address@hidden 
> 发送时间: 2016年1月7日 3:43
> 收件人: Dr. David Alan Gilbert; Zhangbo (Oscar)
> 抄送: zhouyimin Zhou(Yimin); Zhanghailiang; Yanqiangjun; Huangpeng (Peter); 
> address@hidden; Herongguang (Stephen); Linqiangmin; Huangzhichao; Wangyufei 
> (James)
> 主题: Re: [Qemu-devel] What's the advantages of POSTCOPY over CPU-THROTTLE?
> 
> On 01/06/2016 04:57 AM, Dr. David Alan Gilbert wrote:
> > * Zhangbo (Oscar) (address@hidden) wrote:
> >> Hi all:
> >>   Postcopy is suitable for migrating guests which have large page change 
> >> rates. It
> >>      1 makes the guest run at the destination ASAP.
> >>      2 makes the downtime of the guest small enough.
> >>      If we don't take the 1st advantage into account, then, its benefit 
> >> seems similar with CPU-THROTTLE: both of them make the guest's downtime 
> >> small during migration.
> >>
> >>      CPU-THROTTLE would make the guest's dirtypage rate *smaller than the 
> >> network bandwidth*, in order to make the to_send_page_number in each 
> >> iteration convergent and achieve the small-enough downtime during the last 
> >> iteration.
> >>      If we adopt POST-COPY here, the guest's dirtypage rate would *become 
> >> equal to the bandwidth*, because we have to fetch its memory from the 
> >> source side, via the network.
> >>      Both of them would introduce performance degradations of the guest, 
> >> which may in turn cause downtime larger.
> >>
> >>      So, here comes the question: If we just compare POSTCOPY with 
> >> CPU-THROTTLE for their advantages in decreasing downtime, POSTCOPY seems 
> >> has no pos over CPU-THROTTLE, is that right?
> >>
> >>      Meanwhile, Are there any other benifits of POSTCOPY besides the 2 
> >> mentioned above?
> >
> > It's a good question and they do both try and help solve the same problem.
> > One problem with cpu-throttle is whether you can throttle the CPU 
> > enough to get the dirty-rate below the rate of the network, and the 
> > answer to that is very workload dependent.  On a large, many-core VM, 
> > even a little bit of CPU can dirty a lot of memory.  Postcopy is 
> > guaranteed to finish migration, irrespective of the workload.
> >
> > Postcopy is pretty fine-grained, in that only threads that are 
> > accessing pages that are still on the source are blocked, since it 
> > allows the use of async page faults, that means it's even finer 
> > grained than the vCPU level, so many threads come back up to full 
> > performance pretty quickly even if there are a few pages left.
> >
> 
> Good answer Dave. FWIW, I completely agree. Using cpu throttling can help the 
> situation depending on workload. Postcopy will *always* work. 
> One possible side effect of Postcopy is loss of the guest if the network 
> connection dies during the postcopy phase of migration. This should be a very 
> rare occurrence however. So both methods have their uses.
> 
> --
> -- Jason J. Herne (address@hidden)
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK



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