On 3/9/22 12:43 PM, Dr. David Alan Gilbert wrote: > * Claudio Fontana (cfont...@suse.de) wrote: >> On 3/7/22 1:28 PM, Dr. David Alan Gilbert wrote: >>> * Claudio Fontana (cfont...@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 <berra...@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 <dgilb...@redhat.com> >>>>>>>>>>>> Signed-off-by: Daniel P. Berrangé <berra...@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.. > qemu-1646827174 > Yeh that's what I'd hope for off a decent CPU; hmm there's not that much > savevm specific is there?
Hmm not sure. I have a hunch, is the difference just more threads being used to transfer the data? Is the migration path creating threads, while savevm doesn't? Hmm... Thanks, Claudio