Re: [Qemu-devel] Migration dirty bitmap: should only mark pages as dirty after they have been sent

[Amit Shah]

On (Mon) 26 Sep 2016 [22:55:01], Chunguang Li wrote:
> 
> 
> 
> > -----原始邮件-----
> > 发件人: "Dr. David Alan Gilbert" <address@hidden>
> > 发送时间: 2016年9月26日 星期一
> > 收件人: "Chunguang Li" <address@hidden>
> > 抄送: address@hidden, address@hidden, address@hidden, address@hidden, 
> > address@hidden
> > 主题: Re: [Qemu-devel] Migration dirty bitmap: should only mark pages as 
> > dirty after they have been sent
> > 
> > * Chunguang Li (address@hidden) wrote:
> > > Hi all!
> > > I have some confusion about the dirty bitmap during migration. I have 
> > > digged into the code. I figure out that every now and then during 
> > > migration, the dirty bitmap will be grabbed from the kernel space through 
> > > ioctl(KVM_GET_DIRTY_LOG), and then be used to update qemu's dirty bitmap. 
> > > However I think this mechanism leads to resendness of some NON-dirty 
> > > pages.
> > > 
> > > Take the first iteration of precopy for instance, during which all the 
> > > pages will be sent. Before that during the migration setup, the 
> > > ioctl(KVM_GET_DIRTY_LOG) is called once, so the kernel begins to produce 
> > > the dirty bitmap from this moment. When the pages "that haven't been 
> > > sent" are written, the kernel space marks them as dirty. However I don't 
> > > think this is correct, because these pages will be sent during this and 
> > > the next iterations with the same content (if they are not written again 
> > > after they are sent). It only makes sense to mark the pages which have 
> > > already been sent during one iteration as dirty when they are written.
> > > 
> > > 
> > > Am I right about this consideration? If I am right, is there some advice 
> > > to improve this?
> > 
> > I think you're right that this can happen; to clarify I think the
> > case you're talking about is:
> > 
> >   Iteration 1
> >     sync bitmap
> >     start sending pages
> >     page 'n' is modified - but hasn't been sent yet
> >     page 'n' gets sent
> >   Iteration 2
> >     sync bitmap
> >        'page n is shown as modified'
> >     send page 'n' again
> >
> 
> Yes，this is right the case I am talking about.
>  
> > So you're right that is wasteful; I guess it's more wasteful
> > on big VMs with slow networks where the length of each iteration
> > is large.
> 
> I think this is "very" wasteful. Assume the workload writes the pages dirty 
> randomly within the guest address space, and the transfer speed is constant. 
> Intuitively, I think nearly half of the dirty pages produced in Iteration 1 
> is not really dirty. This means the time of Iteration 2 is double of that to 
> send only really dirty pages.

It makes sense, can you get some perf numbers to show what kinds of
workloads get impacted the most?  That would also help us to figure
out what kinds of speed improvements we can expect.


                Amit