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 <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> > > > 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
