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

[Claudio Fontana]

On 3/7/22 11:32 AM, Dr. David Alan Gilbert wrote:
> * Claudio Fontana (cfontana@suse.de) wrote:
>> On 3/5/22 2:20 PM, 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>
>>>
>>>
>>> git bisect start
>>> # bad: [5c8463886d50eeb0337bd121ab877cf692731e36] Merge remote-tracking 
>>> branch 'remotes/kraxel/tags/kraxel-20220304-pull-request' into staging
>>> git bisect bad 5c8463886d50eeb0337bd121ab877cf692731e36
>>> # good: [6cdf8c4efa073eac7d5f9894329e2d07743c2955] Update version for 4.2.1 
>>> release
>>> git bisect good 6cdf8c4efa073eac7d5f9894329e2d07743c2955
>>> # good: [b0ca999a43a22b38158a222233d3f5881648bb4f] Update version for 
>>> v4.2.0 release
>>> git bisect good b0ca999a43a22b38158a222233d3f5881648bb4f
>>> # skip: [e2665f314d80d7edbfe7f8275abed7e2c93c0ddc] target/mips: Alias MSA 
>>> vector registers on FPU scalar registers
>>> git bisect skip e2665f314d80d7edbfe7f8275abed7e2c93c0ddc
>>> # good: [4762c82cbda22b1036ce9dd2c5e951ac0ed0a7d3] tests/docker: Install 
>>> static libc package in CentOS 7
>>> git bisect good 4762c82cbda22b1036ce9dd2c5e951ac0ed0a7d3
>>> # bad: [d4127349e316b5c78645f95dba5922196ac4cc23] Merge remote-tracking 
>>> branch 'remotes/berrange-gitlab/tags/crypto-and-more-pull-request' into 
>>> staging
>>> git bisect bad d4127349e316b5c78645f95dba5922196ac4cc23
>>> # bad: [d90f154867ec0ec22fd719164b88716e8fd48672] Merge remote-tracking 
>>> branch 'remotes/dg-gitlab/tags/ppc-for-6.1-20210504' into staging
>>> git bisect bad d90f154867ec0ec22fd719164b88716e8fd48672
>>> # good: [dd5af6ece9b101d29895851a7441d848b7ccdbff] tests/docker: add a 
>>> test-tcg for building then running check-tcg
>>> git bisect good dd5af6ece9b101d29895851a7441d848b7ccdbff
>>> # bad: [90ec1cff768fcbe1fa2870d2018f378376f4f744] target/riscv: Adjust 
>>> privilege level for HLV(X)/HSV instructions
>>> git bisect bad 90ec1cff768fcbe1fa2870d2018f378376f4f744
>>> # good: [373969507a3dc7de2d291da7e1bd03acf46ec643] migration: Replaced 
>>> qemu_mutex_lock calls with QEMU_LOCK_GUARD
>>> git bisect good 373969507a3dc7de2d291da7e1bd03acf46ec643
>>> # good: [4083904bc9fe5da580f7ca397b1e828fbc322732] Merge remote-tracking 
>>> branch 'remotes/rth-gitlab/tags/pull-tcg-20210317' into staging
>>> git bisect good 4083904bc9fe5da580f7ca397b1e828fbc322732
>>> # bad: [009ff89328b1da3ea8ba316bf2be2125bc9937c5] vl: allow passing JSON to 
>>> -object
>>> git bisect bad 009ff89328b1da3ea8ba316bf2be2125bc9937c5
>>> # bad: [50243407457a9fb0ed17b9a9ba9fc9aee09495b1] qapi/qom: Drop deprecated 
>>> 'props' from object-add
>>> git bisect bad 50243407457a9fb0ed17b9a9ba9fc9aee09495b1
>>> # bad: [1b507e55f8199eaad99744613823f6929e4d57c6] Merge remote-tracking 
>>> branch 'remotes/berrange-gitlab/tags/dep-many-pull-request' into staging
>>> git bisect bad 1b507e55f8199eaad99744613823f6929e4d57c6
>>> # bad: [24e13a4dc1eb1630eceffc7ab334145d902e763d] chardev: reject use of 
>>> 'wait' flag for socket client chardevs
>>> git bisect bad 24e13a4dc1eb1630eceffc7ab334145d902e763d
>>> # good: [8becb36063fb14df1e3ae4916215667e2cb65fa2] monitor: remove 
>>> 'query-events' QMP command
>>> git bisect good 8becb36063fb14df1e3ae4916215667e2cb65fa2
>>> # bad: [8af54b9172ff3b9bbdbb3191ed84994d275a0d81] machine: remove 
>>> 'query-cpus' QMP command
>>> git bisect bad 8af54b9172ff3b9bbdbb3191ed84994d275a0d81
>>> # bad: [cbde7be900d2a2279cbc4becb91d1ddd6a014def] migrate: remove QMP/HMP 
>>> commands for speed, downtime and cache size
>>> git bisect bad cbde7be900d2a2279cbc4becb91d1ddd6a014def
>>> # first bad commit: [cbde7be900d2a2279cbc4becb91d1ddd6a014def] migrate: 
>>> remove QMP/HMP commands for speed, downtime and cache size
>>>
>>>
>>> Are there some obvious settings / options I am missing to regain the savevm 
>>> performance after this commit?
>>
>> Answering myself: 
> 
> <oops we seem to have split this thread into two>
> 
>> this seems to be due to a resulting different default xbzrle cache size 
>> (probably interactions between libvirt/qemu versions?).
>>
>> When forcing the xbzrle cache size to a larger value, the performance is 
>> back.
> 
> That's weird that 'virsh save' is ending up using xbzrle.

virsh save (or qemu savevm..) seems to me like it uses a subset of the 
migration code and migration parameters but not all..

> 
>>>
>>> I have seen projects attempting to improve other aspects of performance 
>>> (snapshot performance, etc), is there something going on to improve the 
>>> transfer of RAM in savevm too?
>>
>>
>> Still I would think that we should be able to do better than 600ish Mb/s , 
>> any ideas, prior work on this,
>> to improve savevm performance, especially looking at RAM regions transfer 
>> speed?
> 
> My normal feeling is ~10Gbps for a live migrate over the wire; I rarely
> try virsh save though.
> If you're using xbzrle that might explain it; it's known to eat cpu -
> but I'd never expect it to have been used with 'virsh save'.

some valgrind shows it among the top cpu eaters;

I wonder why we are able to do more than 2x better for actual live migration, 
compared with virsh save /dev/null ...

Thanks,

Claudio