[Qemu-devel] 回复: 回复: 答复: migrate_set_speed has no effect if the guest is using hugepages.

[Lin Ma]

Hi Dave,

May I ask that do you have any update about the fix?

Thanks,
Lin

> -----邮件原件-----
> 发件人: Qemu-devel <qemu-devel-bounces+lma=address@hidden> 代
> 表 Lin Ma
> 发送时间: 2019年7月15日 17:43
> 收件人: Dr. David Alan Gilbert <address@hidden>
> 抄送: address@hidden
> 主题: [Qemu-devel] 回复: 答复: migrate_set_speed has no effect if the guest
> is using hugepages.
> 
> 
> 
> > -----邮件原件-----
> > 发件人: Dr. David Alan Gilbert <address@hidden>
> > 发送时间: 2019年7月12日 20:34
> > 收件人: Lin Ma <address@hidden>
> > 抄送: address@hidden
> > 主题: Re: 答复: [Qemu-devel] migrate_set_speed has no effect if the guest
> > is using hugepages.
> >
> > * Lin Ma (address@hidden) wrote:
> > >
> > >
> > > > -----邮件原件-----
> > > > 发件人: Dr. David Alan Gilbert <address@hidden>
> > > > 发送时间: 2019年7月11日 18:24
> > > > 收件人: Lin Ma <address@hidden>
> > > > 抄送: address@hidden
> > > > 主题: Re: [Qemu-devel] migrate_set_speed has no effect if the guest
> > > > is using hugepages.
> > > >
> > > > * Lin Ma (address@hidden) wrote:
> > > > > Hi all,
> > > >
> > > > Hi Lin,
> > >
> > > Hi Dave,
> > > >
> > > > > When I live migrate a qemu/kvm guest, If the guest is using huge
> > > > > pages, I found that the migrate_set_speed command had no effect
> > > > > during
> > > > stage 2.
> > > >
> > > > Can you explain what you mean by 'stage 2'?
> > > We know that the live migration contains 3 stages:
> > > Stage 1: Mark all of RAM dirty.
> > > Stage 2: Keep sending dirty RAM pages since last iteration Stage 3:
> > > Stop guest, transfer remaining dirty RAM, device state (Please refer
> > > to
> > > https://developers.redhat.com/blog/2015/03/24/live-migrating-qemu-kv
> > > m- virtual-machines/#live-migration for further details)
> >
> > OK, yeh the numbering is pretty arbitrary so it's not something I
> > normally think about like that.
> >
> > >
> > > > > It was caused by commit 4c011c3 postcopy: Send whole huge pages
> > > > >
> > > > > I'm wondering that is it by design or is it a bug waiting for fix?
> > > >
> > > > This is the first report I've seen for it.  How did you conclude
> > > > that
> > > > 4c011c3 caused it?  While I can see it might have some effect on
> > > > the bandwidth management, I'm surprised it has this much effect.
> > >
> > > While digging into the bandwidth issue, Git bisect shows that this
> > > commit was
> > the first bad commit.
> >
> > OK.
> >
> > > > What size huge pages are you using - 2MB or 1GB?
> > >
> > > When I hit this issue I was using 1GB huge page size.
> > > I tested this issue with 2MB page size today On Gigabit LAN,
> > > Although the bandwidth control looks a little better than using 1GB, But 
> > > not
> too much.
> > Please refer to the below test result.
> >
> > OK, I can certainly see why this might happen with 1GB huge pages; I
> > need to have a think about a fix.
> >
> > > > I can imagine we might have a problem that since we only do the
> > > > sleep between the hugepages, if we were using 1GB hugepages then
> > > > we'd see <big chunk of
> > > > data>[sleep]<big chunk of data>[sleep] which isn't as smooth as it
> > > > data>used to
> > be.
> > > >
> > > > Can you give me some more details of your test?
> > >
> > > Live migration bandwidth management testing with 2MB hugepage size:
> > > sles12sp4_i440fx is a qemu/kvm guest with 6GB memory size.
> > > Note: the throughput value is approximating value.
> > >
> > > Terminal 1:
> > > virsh migrate-setspeed sles12sp4_i440fx $bandwidth && virsh migrate
> > > --live sles12sp4_i440fx qemu+tcp://5810f/system
> > >
> > > Terminal 2:
> > > virsh qemu-monitor-command sles12sp4_i440fx --hmp "info migrate"
> > >
> > > bandwidth=5
> > > throughput: 160 mbps
> > >
> > > bandwidth=10
> > > throughput: 167 mbps
> > >
> > > bandwidth=15
> > > throughput: 168 mbps
> > >
> > > bandwidth=20
> > > throughput: 168 mbps
> > >
> > > bandwidth=21
> > > throughput: 336 mbps
> > >
> > > bandwidth=22
> > > throughput: 336 mbps
> > >
> > > bandwidth=25
> > > throughput: 335.87 mbps
> > >
> > > bandwidth=30
> > > throughput: 335 mbps
> > >
> > > bandwidth=35
> > > throughput: 335 mbps
> > >
> > > bandwidth=40
> > > throughput: 335 mbps
> > >
> > > bandwidth=45
> > > throughput: 504.00 mbps
> > >
> > > bandwidth=50
> > > throughput: 500.00 mbps
> > >
> > > bandwidth=55
> > > throughput: 500.00 mbps
> > >
> > > bandwidth=60
> > > throughput: 500.00 mbps
> > >
> > > bandwidth=65
> > > throughput: 650.00 mbps
> > >
> > > bandwidth=70
> > > throughput: 660.00 mbps
> >
> > OK, so migrate-setspeed takes a bandwidth in MBytes/sec and I guess
> > you're throughput is in MBit/sec - so at the higher end it's about
> > right, and at the lower end it's way off.
> >
> > Let me think about a fix for this.
> >
> > What are you using to measure throughput?
> 
> I use 'watch' command to observe the output of qemu hmp command 'info
> migrate', calculate the average value of throughput field during stage 2 of 
> live
> migration.
> 
> Thanks for taking time to dig into this issue, Lin