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

[Dr. David Alan Gilbert]

* 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-kvm-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 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?

Dave

> 
> Thanks,
> Lin
> 
> 
> > Dave
> > 
> > >
> > > Thanks,
> > > Lin
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK