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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Chunguang Li (lichunguang@hust.edu.cn) 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.
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> 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.
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> 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.
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> 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.
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> 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.
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> 
> 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

> 
> --
> 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 / dgilbert@redhat.com / Manchester, UK