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

[Dr. David Alan Gilbert]

* 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.

> > 
> > 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'.

Dave


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