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

[Chunguang Li]

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.


--
Chunguang Li, Ph.D. Candidate
Wuhan National Laboratory for Optoelectronics (WNLO)
Huazhong University of Science & Technology (HUST)
Wuhan, Hubei Prov., China