Re: The issues about architecture of the COLO checkpoint

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

* Daniel Cho (danielcho@qnap.com) wrote:
> Hi Hailiang,
> 
> 1.
>     OK, we will try the patch
> “0001-COLO-Optimize-memory-back-up-process.patch”,
> and thanks for your help.
> 
> 2.
>     We understand the reason to compare PVM and SVM's packet. However, the
> empty of SVM's packet queue might happened on setting COLO feature and SVM
> broken.
> 
> On situation 1 ( setting COLO feature ):
>     We could force do checkpoint after setting COLO feature finish, then it
> will protect the state of PVM and SVM . As the Zhang Chen said.
> 
> On situation 2 ( SVM broken ):
>     COLO will do failover for PVM, so it might not cause any wrong on PVM.
> 
> However, those situations are our views, so there might be a big difference
> between reality and our views.
> If we have any wrong views and opinions, please let us know, and correct
> us.

It does need a timeout; the SVM being broken or being in a state where
it never sends the corresponding packet (because of a state difference)
can happen and COLO needs to timeout when the packet hasn't arrived
after a while and trigger the checkpoint.

Dave

> Thanks.
> 
> Best regards,
> Daniel Cho
> 
> Zhang, Chen <chen.zhang@intel.com> 於 2020年2月13日 週四 上午10:17寫道：
> 
> > Add cc Jason Wang, he is a network expert.
> >
> > In case some network things goes wrong.
> >
> >
> >
> > Thanks
> >
> > Zhang Chen
> >
> >
> >
> > *From:* Zhang, Chen
> > *Sent:* Thursday, February 13, 2020 10:10 AM
> > *To:* 'Zhanghailiang' <zhang.zhanghailiang@huawei.com>; Daniel Cho <
> > danielcho@qnap.com>
> > *Cc:* Dr. David Alan Gilbert <dgilbert@redhat.com>; qemu-devel@nongnu.org
> > *Subject:* RE: The issues about architecture of the COLO checkpoint
> >
> >
> >
> > For the issue 2:
> >
> >
> >
> > COLO need use the network packets to confirm PVM and SVM in the same state,
> >
> > Generally speaking, we can’t send PVM packets without compared with SVM
> > packets.
> >
> > But to prevent jamming, I think COLO can do force checkpoint and send the
> > PVM packets in this case.
> >
> >
> >
> > Thanks
> >
> > Zhang Chen
> >
> >
> >
> > *From:* Zhanghailiang <zhang.zhanghailiang@huawei.com>
> > *Sent:* Thursday, February 13, 2020 9:45 AM
> > *To:* Daniel Cho <danielcho@qnap.com>
> > *Cc:* Dr. David Alan Gilbert <dgilbert@redhat.com>; qemu-devel@nongnu.org;
> > Zhang, Chen <chen.zhang@intel.com>
> > *Subject:* RE: The issues about architecture of the COLO checkpoint
> >
> >
> >
> > Hi,
> >
> >
> >
> > 1.       After re-walked through the codes, yes, you are right, actually,
> > after the first migration, we will keep dirty log on in primary side,
> >
> > And only send the dirty pages in PVM to SVM. The ram cache in secondary
> > side is always a backup of PVM, so we don’t have to
> >
> > Re-send the none-dirtied pages.
> >
> > The reason why the first checkpoint takes longer time is we have to backup
> > the whole VM’s ram into ram cache, that is colo_init_ram_cache().
> >
> > It is time consuming, but I have optimized in the second patch
> > “0001-COLO-Optimize-memory-back-up-process.patch” which you can find in my
> > previous reply.
> >
> >
> >
> > Besides, I found that, In my previous reply “We can only copy the pages
> > that dirtied by PVM and SVM in last checkpoint.”,
> >
> > We have done this optimization in current upstream codes.
> >
> >
> >
> > 2．I don’t quite understand this question. For COLO, we always need both
> > network packets of PVM’s and SVM’s to compare before send this packets to
> > client.
> >
> > It depends on this to decide whether or not PVM and SVM are in same state.
> >
> >
> >
> > Thanks,
> >
> > hailiang
> >
> >
> >
> > *From:* Daniel Cho [mailto:danielcho@qnap.com <danielcho@qnap.com>]
> > *Sent:* Wednesday, February 12, 2020 4:37 PM
> > *To:* Zhang, Chen <chen.zhang@intel.com>
> > *Cc:* Zhanghailiang <zhang.zhanghailiang@huawei.com>; Dr. David Alan
> > Gilbert <dgilbert@redhat.com>; qemu-devel@nongnu.org
> > *Subject:* Re: The issues about architecture of the COLO checkpoint
> >
> >
> >
> > Hi Hailiang,
> >
> >
> >
> > Thanks for your replaying and explain in detail.
> >
> > We will try to use the attachments to enhance memory copy.
> >
> >
> >
> > However, we have some questions for your replying.
> >
> >
> >
> > 1.  As you said, "for each checkpoint, we have to send the whole PVM's
> > pages To SVM", why the only first checkpoint will takes more pause time?
> >
> > In our observing, the first checkpoint will take more time for pausing,
> > then other checkpoints will takes a few time for pausing. Does it means
> > only the first checkpoint will send the whole pages to SVM, and the other
> > checkpoints send the dirty pages to SVM for reloading?
> >
> >
> >
> > 2. We notice the COLO-COMPARE component will stuck the packet until
> > receive packets from PVM and SVM, as this rule, when we add the
> > COLO-COMPARE to PVM, its network will stuck until SVM start. So it is an
> > other issue to make PVM stuck while setting COLO feature. With this issue,
> > could we let colo-compare to pass the PVM's packet when the SVM's packet
> > queue is empty? Then, the PVM's network won't stock, and "if PVM runs
> > firstly, it still need to wait for The network packets from SVM to
> > compare before send it to client side" won't happened either.
> >
> >
> >
> > Best regard,
> >
> > Daniel Cho
> >
> >
> >
> > Zhang, Chen <chen.zhang@intel.com> 於 2020年2月12日 週三 下午1:45寫道：
> >
> >
> >
> > > -----Original Message-----
> > > From: Zhanghailiang <zhang.zhanghailiang@huawei.com>
> > > Sent: Wednesday, February 12, 2020 11:18 AM
> > > To: Dr. David Alan Gilbert <dgilbert@redhat.com>; Daniel Cho
> > > <danielcho@qnap.com>; Zhang, Chen <chen.zhang@intel.com>
> > > Cc: qemu-devel@nongnu.org
> > > Subject: RE: The issues about architecture of the COLO checkpoint
> > >
> > > Hi,
> > >
> > > Thank you Dave,
> > >
> > > I'll reply here directly.
> > >
> > > -----Original Message-----
> > > From: Dr. David Alan Gilbert [mailto:dgilbert@redhat.com]
> > > Sent: Wednesday, February 12, 2020 1:48 AM
> > > To: Daniel Cho <danielcho@qnap.com>; chen.zhang@intel.com;
> > > Zhanghailiang <zhang.zhanghailiang@huawei.com>
> > > Cc: qemu-devel@nongnu.org
> > > Subject: Re: The issues about architecture of the COLO checkpoint
> > >
> > >
> > > cc'ing in COLO people:
> > >
> > >
> > > * Daniel Cho (danielcho@qnap.com) wrote:
> > > > Hi everyone,
> > > >      We have some issues about setting COLO feature. Hope somebody
> > > > could give us some advice.
> > > >
> > > > Issue 1:
> > > >      We dynamic to set COLO feature for PVM(2 core, 16G memory),  but
> > > > the Primary VM will pause a long time(based on memory size) for
> > > > waiting SVM start. Does it have any idea to reduce the pause time?
> > > >
> > >
> > > Yes, we do have some ideas to optimize this downtime.
> > >
> > > The main problem for current version is, for each checkpoint, we have to
> > > send the whole PVM's pages
> > > To SVM, and then copy the whole VM's state into SVM from ram cache, in
> > > this process, we need both of them be paused.
> > > Just as you said, the downtime is based on memory size.
> > >
> > > So firstly, we need to reduce the sending data while do checkpoint,
> > actually,
> > > we can migrate parts of PVM's dirty pages in background
> > > While both of VMs are running. And then we load these pages into ram
> > > cache (backup memory) in SVM temporarily. While do checkpoint,
> > > We just send the last dirty pages of PVM to slave side and then copy the
> > ram
> > > cache into SVM. Further on, we don't have
> > > To send the whole PVM's dirty pages, we can only send the pages that
> > > dirtied by PVM or SVM during two checkpoints. (Because
> > > If one page is not dirtied by both PVM and SVM, the data of this pages
> > will
> > > keep same in SVM, PVM, backup memory). This method can reduce
> > > the time that consumed in sending data.
> > >
> > > For the second problem, we can reduce the memory copy by two methods,
> > > first one, we don't have to copy the whole pages in ram cache,
> > > We can only copy the pages that dirtied by PVM and SVM in last
> > checkpoint.
> > > Second, we can use userfault missing function to reduce the
> > > Time consumed in memory copy. (For the second time, in theory, we can
> > > reduce time consumed in memory into ms level).
> > >
> > > You can find the first optimization in attachment, it is based on an old
> > qemu
> > > version (qemu-2.6), it should not be difficult to rebase it
> > > Into master or your version. And please feel free to send the new
> > version if
> > > you want into community ;)
> > >
> > >
> >
> > Thanks Hailiang!
> > By the way, Do you have time to push the patches to upstream?
> > I think this is a better and faster option.
> >
> > Thanks
> > Zhang Chen
> >
> > > >
> > > > Issue 2:
> > > >      In
> > > > https://github.com/qemu/qemu/blob/master/migration/colo.c#L503,
> > > > could we move start_vm() before Line 488? Because at first checkpoint
> > > > PVM will wait for SVM's reply, it cause PVM stop for a while.
> > > >
> > >
> > > No, that makes no sense, because if PVM runs firstly, it still need to
> > wait for
> > > The network packets from SVM to compare before send it to client side.
> > >
> > >
> > > Thanks,
> > > Hailiang
> > >
> > > >      We set the COLO feature on running VM, so we hope the running VM
> > > > could continuous service for users.
> > > > Do you have any suggestions for those issues?
> > > >
> > > > Best regards,
> > > > Daniel Cho
> > > --
> > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> >
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK