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

["Chunguang Li" <lichunguang@hust.edu.cn>]

> -----Original Messages-----
> From: "Juan Quintela" <quintela@redhat.com>
> Sent Time: 2017-11-15 17:45:44 (Wednesday)
> To: "Chunguang Li" <lichunguang@hust.edu.cn>
> Cc: qemu-devel@nongnu.org, dgilbert@redhat.com, amit.shah@redhat.com, pbonzini@redhat.com, stefanha@redhat.com
> Subject: Re: Abnormal observation during migration: too many "write-not-dirty" pages
> 
> "Chunguang Li" <lichunguang@hust.edu.cn> wrote:
> > Hi all! 
> 
> Hi
> 
> Sorry for the delay, I was on vacation an still getting up to speed.

Hi, Juan, thanks for your reply.

> 
> > 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 think your test is quite good, and I am also ashamed that 80% of
> "false" dirty pages is really a lot.
> 
> > 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. 
> 
> 
> vhost and friends could make a small difference here, but in general,
> this approach should be ok.
> 
> > 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. 
> 
> That is the impressive part, 15 workloads.  Thanks for taking the effor.
> 
> BTW, do you have your qemu changes handy, just to be able to test
> locally, and "review" how do you measure things.

Sorry, I do not have my changes handy. But don't worry, I will send them to you tomorrow morning. It's night here.

> 
> 
> > 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%.
> 
> Or the workload does really stupid things like:
> 
> a = 0;
> a = 1;
> a = 0;
> 
> This makes no sense at all.
> 
> Just in case, could you try to test this with xbzrle?  It should go well
> with this use case (but you need to get a big enough buffer to cache
> enough memory).

In fact, I have tested these workloads (the 45%-80% ones) with xbzrle. And when the buffer is big enough, they really go well. While they do not converge to stop-copy before, now they finish migration quickly.

> 
> 
> > 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. 
> 
> I agree with that.
> 
> > 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. 
> 
> Huge page could have caused that.  Remember that we have transparent
> huge pages.  I have to look at that code.

In fact, the results are the same no matter I turn on or turn off the transparent huge pages in the OS.

Later, Chunguang.

> 
> > 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. 
> 
> That is a posibilty.
> 
> > 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.
> 
> I would like to reproduce this.
> 
> Thanks for bringing this to our attention.
> 
> Later, Juan.


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