Re: bad qemu savevm to /dev/null performance (600 MiB/s max) (Was: 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 1:28 PM, Dr. David Alan Gilbert wrote:
> > * 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?
> 
> much faster apparently, 30 sec savevm vs 7 seconds for migration to a netcat socket sent to /dev/null.
> 
> nc -l -U /tmp/savevm.socket
> 
> virsh suspend centos7
> Domain centos7 suspended
> 
> virsh qemu-monitor-command --cmd '{ "execute": "migrate", "arguments": { "uri": "unix:///tmp/savevm.socket" } }' centos7
> 
> virt97:/mnt # virsh qemu-monitor-command --cmd '{ "execute": "query-migrate" }' centos7
> {"return":{"blocked":false,"status":"completed","setup-time":118,"downtime":257,"total-time":7524,"ram":{"total":32213049344,"postcopy-requests":0,"dirty-sync-count":3,"multifd-bytes":0,"pages-per-second":1057530,"page-size":4096,"remaining":0,"mbps":24215.572437483122,"transferred":22417172290,"duplicate":2407520,"dirty-pages-rate":0,"skipped":0,"normal-bytes":22351847424,"normal":5456994}},"id":"libvirt-438"}
> 
> virt97:/mnt # virsh qemu-monitor-command --cmd '{ "execute": "query-migrate-parameters" }' centos7
> {"return":{"cpu-throttle-tailslow":false,"xbzrle-cache-size":67108864,"cpu-throttle-initial":20,"announce-max":550,"decompress-threads":2,"compress-threads":8,"compress-level":0,"multifd-channels":8,"multifd-zstd-level":1,"announce-initial":50,"block-incremental":false,"compress-wait-thread":true,"downtime-limit":300,"tls-authz":"","multifd-compression":"none","announce-rounds":5,"announce-step":100,"tls-creds":"","multifd-zlib-level":1,"max-cpu-throttle":99,"max-postcopy-bandwidth":0,"tls-hostname":"","throttle-trigger-threshold":50,"max-bandwidth":9223372036853727232,"x-checkpoint-delay":20000,"cpu-throttle-increment":10},"id":"libvirt-439"}
> 
> 
> I did also a run with multifd-channels:1 instead of 8, if it matters:

I suspect you haven't actually got multifd enabled ( check
query-migrate-capabilities ?).
> 
> virt97:/mnt # virsh qemu-monitor-command --cmd '{ "execute": "query-migrate" }' centos7
> {"return":{"blocked":false,"status":"completed","setup-time":119,"downtime":260,"total-time":8601,"ram":{"total":32213049344,"postcopy-requests":0,"dirty-sync-count":3,"multifd-bytes":0,"pages-per-second":908820,"page-size":4096,"remaining":0,"mbps":21141.861157274227,"transferred":22415264188,"duplicate":2407986,"dirty-pages-rate":0,"skipped":0,"normal-bytes":22349938688,"normal":5456528}},"id":"libvirt-453"}
> 
> virt97:/mnt # virsh qemu-monitor-command --cmd '{ "execute": "query-migrate-parameters" }' centos7
> {"return":{"cpu-throttle-tailslow":false,"xbzrle-cache-size":67108864,"cpu-throttle-initial":20,"announce-max":550,"decompress-threads":2,"compress-threads":8,"compress-level":0,"multifd-channels":1,"multifd-zstd-level":1,"announce-initial":50,"block-incremental":false,"compress-wait-thread":true,"downtime-limit":300,"tls-authz":"","multifd-compression":"none","announce-rounds":5,"announce-step":100,"tls-creds":"","multifd-zlib-level":1,"max-cpu-throttle":99,"max-postcopy-bandwidth":0,"tls-hostname":"","throttle-trigger-threshold":50,"max-bandwidth":9223372036853727232,"x-checkpoint-delay":20000,"cpu-throttle-increment":10},"id":"libvirt-454"}
> 
> 
> Still we are in the 20 Gbps range, or around 2560 MiB/s, way faster than savevm which does around 600 MiB/s when the wind is in its favor..

Yeh that's what I'd hope for off a decent CPU; hmm there's not that much
savevm specific is there?

Dave

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