Re: The issues about architecture of the COLO checkpoint

[Daniel Cho <danielcho@qnap.com>]

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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 <chen.zhang@intel.com> 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 <chen.zhang@intel.com> 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 <dgilbert@redhat.com> 於 2020年2月13日 週四 下午6:38寫道：
>>
>>> * 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
>>>
>>>

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