Re: [PATCH v4 13/14] migration: for SEV live migration bump downtime limit to 1s.

[Ashish Kalra via <qemu-devel@nongnu.org>]

On Fri, Sep 10, 2021 at 10:43:50AM +0100, Daniel P. Berrangé wrote:
> On Wed, Aug 04, 2021 at 11:59:47AM +0000, Ashish Kalra wrote:
> > From: Ashish Kalra <ashish.kalra@amd.com>
> > 
> > Now, qemu has a default expected downtime of 300 ms and
> > SEV Live migration has a page-per-second bandwidth of 350-450 pages
> > ( SEV Live migration being generally slow due to guest RAM pages
> > being migrated after encryption using the security processor ).
> > With this expected downtime of 300ms and 350-450 pps bandwith,
> > the threshold size = <1/3 of the PPS bandwidth = ~100 pages.
> > 
> > Now, this threshold size is the maximum pages/bytes that can be
> > sent in the final completion phase of Live migration
> > (where the source VM is stopped) with the expected downtime.
> > Therefore, with the threshold size computed above,
> > the migration completion phase which halts the source VM
> > and then transfers the leftover dirty pages,
> > is only reached in SEV live migration case when # of dirty pages are ~100.
> > 
> > The dirty-pages-rate with larger guest RAM configuration like 4G, 8G, etc.
> > is much higher, typically in the range of 300-400+ pages, hence,
> > we always remain in the "dirty-sync" phase of migration and never
> > reach the migration completion phase with above guest RAM configs.
> > 
> > To summarize, with larger guest RAM configs,
> > the dirty-pages-rate > threshold_size (with the default qemu expected downtime of 300ms).
> > 
> > So, the fix is to increase qemu's expected downtime.
> > 
> > This is a tweakable parameter which can be set using "migrate_set_downtime".
> > 
> > With a downtime of 1 second, we get a threshold size of ~350-450 pages,
> > which will handle the "dirty-pages-rate" of 300+ pages and complete
> > the migration process, so we bump the default downtime to 1s in case
> > of SEV live migration being active.
> > 
> > Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
> > ---
> >  migration/migration.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index daea3ecd04..c9bc33fb10 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -3568,6 +3568,10 @@ static void migration_update_counters(MigrationState *s,
> >      transferred = current_bytes - s->iteration_initial_bytes;
> >      time_spent = current_time - s->iteration_start_time;
> >      bandwidth = (double)transferred / time_spent;
> > +    if (memcrypt_enabled() &&
> > +        s->parameters.downtime_limit < 1000) {
> > +        s->parameters.downtime_limit = 1000;
> > +    }
> 
> I don't think we can be silently changing a value set by the mgmt
> app. If the app requests 300 ms downtime, then we *must* honour
> that, because it is driven by the SLA they need to privide to the
> guest user's workload. If it means the migration won't complete,
> it is up to the app to deal with that in some manner.
> 
> At most I think this is a documentation task to give guidance to
> mgmt apps about what special SEV-only things to consider whe tuning
> live migration.
> 

Yes, this makes sense to add this as part of SEV live migration
documentation. Also this is a tuneable parameter with
migrate-set-parameters, so this can be configured or adjusted
accordingly. 

Thanks,
Ashish

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