Re: [Qemu-devel] Abnormal observation during migration: too many "write-not-dirty" pages

[Dr. David Alan Gilbert]

* Chunguang Li (address@hidden) wrote:
> 
> 
> 
> > -----原始邮件-----
> > 发件人: "Dr. David Alan Gilbert" <address@hidden>
> > 发送时间: 2017-11-15 18:11:37 (星期三)
> > 收件人: "Chunguang Li" <address@hidden>
> > 抄送: address@hidden, address@hidden, address@hidden, address@hidden, 
> > address@hidden
> > 主题: Re: [Qemu-devel] Abnormal observation during migration: too many 
> > "write-not-dirty" pages
> > 
> > * Chunguang Li (address@hidden) wrote:
> > > Hi all!
> > > 
> > > I got a very abnormal observation for the VM migration. I found that many 
> > > pages marked as dirty during migration are "not really dirty", which is, 
> > > their content are the same as the old version.
> > > 
> > > 
> > > 
> > > 
> > > I did the migration experiment like this:
> > > 
> > > During the setup phase of migration, first I suspended the VM. Then I 
> > > copied all the pages within the guest physical address space to a memory 
> > > buffer as large as the guest memory size. After that, the dirty tracking 
> > > began and I resumed the VM. Besides, at the end
> > > of each iteration, I also suspended the VM temporarily. During the 
> > > suspension, I compared the content of all the pages marked as dirty in 
> > > this iteration byte-by-byte with their former copies inside the buffer. 
> > > If the content of one page was the same as its former copy, I recorded it 
> > > as a "write-not-dirty" page (the page is written exactly with the same 
> > > content as the old version). Otherwise, I replaced this page in the 
> > > buffer with the new content, for the possible comparison in the future. 
> > > After the reset of the dirty bitmap, I resumed the VM. Thus, I obtain the 
> > > proportion of the write-not-dirty pages within all the pages marked as 
> > > dirty for each pre-copy iteration.
> > > 
> > > I repeated this experiment with 15 workloads, which are 11 CPU2006 
> > > benchmarks, Memcached server, kernel compilation, playing a video, and an 
> > > idle VM. The CPU2006 benchmarks and Memcached are write-intensive 
> > > workloads. So almost all of them did not converge to stop-copy.
> > > 
> > > 
> > > 
> > > 
> > > Startlingly, the proportions of the write-not-dirty pages are quite high. 
> > > Memcached and three CPU2006 benchmarks(zeusmp, mcf and bzip2) have the 
> > > most high proportions. Their proportions of the write-not-dirty pages 
> > > within all the dirty pages are as high as 45%-80%. The proportions of the 
> > > other workloads are about 5%-20%, which are also abnormal. According to 
> > > my intuition, the proportion of write-not-dirty pages should be far less 
> > > than these numbers. I think it should be quite a particular case that one 
> > > page is written with exactly the same content as the former data.
> > > 
> > > Besides, the zero pages are not counted for all the results. Because I 
> > > think codes like memset() may write large area of pages to zero pages, 
> > > which are already zero pages before.
> > > 
> > > 
> > > 
> > > 
> > > I excluded some possible unknown reasons with the machine hardware, 
> > > because I repeated the experiments with two sets of different machines. 
> > > Then I guessed it might be related with the huge page feature. However, 
> > > the result was the same when I turned the huge page feature off in the OS.
> > > 
> > > 
> > > 
> > > 
> > > Now there are only two possible reasons in my opinion. 
> > > 
> > > First, there is some bugs in the KVM kernel dirty tracking mechanism. It 
> > > may mark some pages that do not receive write request as dirty.
> > > 
> > > Second, there is some bugs in the OS running inside the VM. It may issue 
> > > some unnecessary write requests.
> > > 
> > > 
> > > What do you think about this abnormal phenomenon? Any advice or possible 
> > > reasons or even guesses? I appreciate any responses, because it has 
> > > confused me for a long time. Thank you.
> > 
> > Wasn't it you who pointed out last year the other possibility? - The
> > problem of false positives due to sync'ing the whole of memory and then
> > writing the data out, but some of the dirty pages were already written?
> > 
> > Dave
> 
> Yes, you remember that!

Yes, I remember that, and my TODO list told me it was you :-)

> It was me. After that, I did more analysis and experiments. I found that, in 
> fact, both reasons contribute to the "fake dirty" pages (dirty pages that do 
> not need to be resent, because their contents are the same as that in the 
> target node). One is what I pointed out last year, which you have mentioned. 
> The other reason is what I am talking about now, the "write-not-dirty" 
> phenomenon.
> In fact, according to my experiments results, the "wirte-not-dirty" is the 
> main reason resulting to the "fake dirty" pages, while sync'ing the whole of 
> memory contributes less.

How do you differentiate between "fake dirty' and the syncing?

The cases where values change back to what they used to be seem
the most likely to me (e.g. locks/counts that decrement back) - but
that seems a high %.
I wonder if there's any difference between page write protection
based dirtying and PML (that I think can be used on some newer chips).

One way to debug it I guess would be to keep the write protection
and watch the progression of data values within a page - do they
actually change and then change back or do the values never
really change.

Dave


> Chunguang
> > 
> > > 
> > > --
> > > Chunguang Li, Ph.D. Candidate
> > > Wuhan National Laboratory for Optoelectronics (WNLO)
> > > Huazhong University of Science & Technology (HUST)
> > > Wuhan, Hubei Prov., China
> > > 
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
> 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK