Re: starting to look at qemu savevm performance, a first regression detected

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

* Claudio Fontana (cfontana@suse.de) wrote:
> On 3/7/22 1:20 PM, Daniel P. Berrangé wrote:
> > On Mon, Mar 07, 2022 at 01:09:55PM +0100, Claudio Fontana wrote:
> >> On 3/7/22 1:00 PM, Daniel P. Berrangé wrote:
> >>> On Mon, Mar 07, 2022 at 12:19:22PM +0100, Claudio Fontana wrote:
> >>>> On 3/7/22 10:51 AM, Daniel P. Berrangé wrote:
> >>>>> On Mon, Mar 07, 2022 at 10:44:56AM +0100, Claudio Fontana wrote:
> >>>>>> Hello Daniel,
> >>>>>>
> >>>>>> On 3/7/22 10:27 AM, Daniel P. Berrangé wrote:
> >>>>>>> On Sat, Mar 05, 2022 at 02:19:39PM +0100, Claudio Fontana wrote:
> >>>>>>>>
> >>>>>>>> Hello all,
> >>>>>>>>
> >>>>>>>> I have been looking at some reports of bad qemu savevm performance in large VMs (around 20+ Gb),
> >>>>>>>> when used in libvirt commands like:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> virsh save domain /dev/null
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> I have written a simple test to run in a Linux centos7-minimal-2009 guest, which allocates and touches 20G mem.
> >>>>>>>>
> >>>>>>>> With any qemu version since around 2020, I am not seeing more than 580 Mb/Sec even in the most ideal of situations.
> >>>>>>>>
> >>>>>>>> This drops to around 122 Mb/sec after commit: cbde7be900d2a2279cbc4becb91d1ddd6a014def .
> >>>>>>>>
> >>>>>>>> Here is the bisection for this particular drop in throughput:
> >>>>>>>>
> >>>>>>>> commit cbde7be900d2a2279cbc4becb91d1ddd6a014def (HEAD, refs/bisect/bad)
> >>>>>>>> Author: Daniel P. Berrangé <berrange@redhat.com>
> >>>>>>>> Date:   Fri Feb 19 18:40:12 2021 +0000
> >>>>>>>>
> >>>>>>>>     migrate: remove QMP/HMP commands for speed, downtime and cache size
> >>>>>>>>     
> >>>>>>>>     The generic 'migrate_set_parameters' command handle all types of param.
> >>>>>>>>     
> >>>>>>>>     Only the QMP commands were documented in the deprecations page, but the
> >>>>>>>>     rationale for deprecating applies equally to HMP, and the replacements
> >>>>>>>>     exist. Furthermore the HMP commands are just shims to the QMP commands,
> >>>>>>>>     so removing the latter breaks the former unless they get re-implemented.
> >>>>>>>>     
> >>>>>>>>     Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> >>>>>>>>     Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> >>>>>>>
> >>>>>>> That doesn't make a whole lot of sense as a bisect result.
> >>>>>>> How reliable is that bisect end point ? Have you bisected
> >>>>>>> to that point more than once ?
> >>>>>>
> >>>>>> I did run through the bisect itself only once, so I'll double check that.
> >>>>>> The results seem to be reproducible almost to the second though, a savevm that took 35 seconds before the commit takes 2m 48 seconds after.
> >>>>>>
> >>>>>> For this test I am using libvirt v6.0.0.
> >>>
> >>> I've just noticed this.  That version of libvirt is 2 years old and
> >>> doesn't have full support for migrate_set_parameters.
> >>>
> >>>
> >>>> 2022-03-07 10:47:20.145+0000: 134386: info : qemuMonitorIOWrite:452 : QEMU_MONITOR_IO_WRITE: mon=0x7fa4380028a0 buf={"execute":"migrate_set_speed","arguments":{"value":9223372036853727232},"id":"libvirt-19"}^M
> >>>>  len=93 ret=93 errno=0
> >>>> 2022-03-07 10:47:20.146+0000: 134386: info : qemuMonitorJSONIOProcessLine:240 : QEMU_MONITOR_RECV_REPLY: mon=0x7fa4380028a0 reply={"id": "libvirt-19", "error": {"class": "CommandNotFound", "desc": "The command migrate_set_speed has not been found"}}
> >>>> 2022-03-07 10:47:20.147+0000: 134391: error : qemuMonitorJSONCheckError:412 : internal error: unable to execute QEMU command 'migrate_set_speed': The command migrate_set_speed has not been found
> >>>
> >>> We see the migrate_set_speed failing and libvirt obviously ignores that
> >>> failure.
> >>>
> >>> In current libvirt migrate_set_speed is not used as it properly
> >>> handles migrate_set_parameters AFAICT.
> >>>
> >>> I think you just need to upgrade libvirt if you want to use this
> >>> newer QEMU version
> >>>
> >>> Regards,
> >>> Daniel
> >>>
> >>
> >> Got it, this explains it, sorry for the noise on this.
> >>
> >> I'll continue to investigate the general issue of low throughput with virsh save / qemu savevm .
> > 
> > BTW, consider measuring with the --bypass-cache flag to virsh save.
> > This causes libvirt to use a I/O helper that uses O_DIRECT when
> > saving the image. This should give more predictable results by
> > avoiding the influence of host I/O cache which can be in a differnt
> > state of usage each time you measure.  It was also intended that
> > by avoiding hitting cache, saving the memory image of a large VM
> > will not push other useful stuff out of host I/O  cache which can
> > negatively impact other running VMs.
> > 
> > Also it is possible to configure compression on the libvirt side
> > which may be useful if you have spare CPU cycles, but your storage
> > is slow. See 'save_image_format' in the /etc/libvirt/qemu.conf
> > 
> > With regards,
> > Daniel
> > 
> 
> Hi Daniel, thanks for these good info,
> 
> regarding slow storage, for these tests I am saving to /dev/null to avoid having to take storage into account
> (and still getting low bandwidth unfortunately) so I guess compression is out of the question.

What type of speeds do you get if you try a migrate to a netcat socket?

Dave

> Thanks!
> 
> Claudio
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK