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Re: The issues about architecture of the COLO checkpoint


From: Dr. David Alan Gilbert
Subject: Re: The issues about architecture of the COLO checkpoint
Date: Thu, 20 Feb 2020 19:43:52 +0000
User-agent: Mutt/1.13.3 (2020-01-12)

* Daniel Cho (address@hidden) wrote:
> Hi Hailiang,
> 
> I have already patched the file to my branch, but there is a problem while
> doing migration.
> Here is the error message from SVM
> "qemu-system-x86_64: /root/download/qemu-4.1.0/memory.c:1079:
> memory_region_transaction_commit: Assertion `qemu_mutex_iothread_locked()'
> failed."

It's probably worth getting the full backtrace.

Dave

> Do you have this problem?
> 
> Best regards,
> Daniel Cho
> 
> Daniel Cho <address@hidden> 於 2020年2月20日 週四 上午11:49寫道:
> 
> > Hi Zhang,
> >
> > Thanks, I will configure on code for testing first.
> > However, if you have free time, could you please send the patch file to
> > us, Thanks.
> >
> > Best Regard,
> > Daniel Cho
> >
> >
> > Zhang, Chen <address@hidden> 於 2020年2月20日 週四 上午11:07寫道:
> >
> >>
> >> On 2/18/2020 5:22 PM, Daniel Cho wrote:
> >>
> >> Hi Hailiang,
> >> Thanks for your help. If we have any problems we will contact you for
> >> your favor.
> >>
> >>
> >> Hi Zhang,
> >>
> >> " If colo-compare got a primary packet without related secondary packet
> >> in a certain time , it will automatically trigger checkpoint.  "
> >> As you said, the colo-compare will trigger checkpoint, but does it need
> >> to limit checkpoint times?
> >> There is a problem about doing many checkpoints while we use fio to
> >> random write files. Then it will cause low throughput on PVM.
> >> Is this situation is normal on COLO?
> >>
> >>
> >> Hi Daniel,
> >>
> >> The checkpoint time is designed to be user adjustable based on user
> >> environment(workload/network status/business conditions...).
> >>
> >> In net/colo-compare.c
> >>
> >> /* TODO: Should be configurable */
> >> #define REGULAR_PACKET_CHECK_MS 3000
> >>
> >> If you need, I can send a patch for this issue. Make users can change the
> >> value by QMP and qemu monitor commands.
> >>
> >> Thanks
> >>
> >> Zhang Chen
> >>
> >>
> >>
> >> Best regards,
> >> Daniel Cho
> >>
> >> Zhang, Chen <address@hidden> 於 2020年2月17日 週一 下午1:36寫道:
> >>
> >>>
> >>> On 2/15/2020 11:35 AM, Daniel Cho wrote:
> >>>
> >>> Hi Dave,
> >>>
> >>> Yes, I agree with you, it does need a timeout.
> >>>
> >>>
> >>> Hi Daniel and Dave,
> >>>
> >>> Current colo-compare already have the timeout mechanism.
> >>>
> >>> Named packet_check_timer,  It will scan primary packet queue to make
> >>> sure all the primary packet not stay too long time.
> >>>
> >>> If colo-compare got a primary packet without related secondary packet in
> >>> a certain time , it will automatic trigger checkpoint.
> >>>
> >>> https://github.com/qemu/qemu/blob/master/net/colo-compare.c#L847
> >>>
> >>>
> >>> Thanks
> >>>
> >>> Zhang Chen
> >>>
> >>>
> >>>
> >>> Hi Hailiang,
> >>>
> >>> We base on qemu-4.1.0 for using COLO feature, in your patch, we found a
> >>> lot of difference  between your version and ours.
> >>> Could you give us a latest release version which is close your
> >>> developing code?
> >>>
> >>> Thanks.
> >>>
> >>> Regards
> >>> Daniel Cho
> >>>
> >>> Dr. David Alan Gilbert <address@hidden> 於 2020年2月13日 週四 下午6:38寫道:
> >>>
> >>>> * Daniel Cho (address@hidden) 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 <address@hidden> 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' <address@hidden>; Daniel Cho
> >>>> <
> >>>> > > address@hidden>
> >>>> > > *Cc:* Dr. David Alan Gilbert <address@hidden>;
> >>>> address@hidden
> >>>> > > *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 <address@hidden>
> >>>> > > *Sent:* Thursday, February 13, 2020 9:45 AM
> >>>> > > *To:* Daniel Cho <address@hidden>
> >>>> > > *Cc:* Dr. David Alan Gilbert <address@hidden>;
> >>>> address@hidden;
> >>>> > > Zhang, Chen <address@hidden>
> >>>> > > *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:address@hidden <address@hidden>]
> >>>> > > *Sent:* Wednesday, February 12, 2020 4:37 PM
> >>>> > > *To:* Zhang, Chen <address@hidden>
> >>>> > > *Cc:* Zhanghailiang <address@hidden>; Dr. David
> >>>> Alan
> >>>> > > Gilbert <address@hidden>; address@hidden
> >>>> > > *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 <address@hidden> 於 2020年2月12日 週三 下午1:45寫道:
> >>>> > >
> >>>> > >
> >>>> > >
> >>>> > > > -----Original Message-----
> >>>> > > > From: Zhanghailiang <address@hidden>
> >>>> > > > Sent: Wednesday, February 12, 2020 11:18 AM
> >>>> > > > To: Dr. David Alan Gilbert <address@hidden>; Daniel Cho
> >>>> > > > <address@hidden>; Zhang, Chen <address@hidden>
> >>>> > > > Cc: address@hidden
> >>>> > > > 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:address@hidden]
> >>>> > > > Sent: Wednesday, February 12, 2020 1:48 AM
> >>>> > > > To: Daniel Cho <address@hidden>; address@hidden;
> >>>> > > > Zhanghailiang <address@hidden>
> >>>> > > > Cc: address@hidden
> >>>> > > > Subject: Re: The issues about architecture of the COLO checkpoint
> >>>> > > >
> >>>> > > >
> >>>> > > > cc'ing in COLO people:
> >>>> > > >
> >>>> > > >
> >>>> > > > * Daniel Cho (address@hidden) 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 / address@hidden / Manchester, UK
> >>>> > >
> >>>> > >
> >>>> --
> >>>> Dr. David Alan Gilbert / address@hidden / Manchester, UK
> >>>>
> >>>>
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK




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