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

[Claudio Fontana]

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: 

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.


> 
> 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?

Thanks,

Claudio