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:
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..
Thanks,
Claudio
