Re: [PATCH 2/8] qapi/migration: Introduce vcpu-dirty-limit parameters

[Hyman]

在 2022/8/18 6:07, Peter Xu 写道:
> On Sat, Jul 23, 2022 at 03:49:14PM +0800, huangy81@chinatelecom.cn wrote:
> > From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> >
> > Introduce "vcpu-dirty-limit" migration parameter used
> > to limit dirty page rate during live migration.
> >
> > "vcpu-dirty-limit" and "x-vcpu-dirty-limit-period" are
> > two dirty-limit-related migration parameters, which can
> > be set before and during live migration by qmp
> > migrate-set-parameters.
> >
> > This two parameters are used to help implement the dirty
> > page rate limit algo of migration.
> >
> > Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> > ---
> >   migration/migration.c | 14 ++++++++++++++
> >   monitor/hmp-cmds.c    |  8 ++++++++
> >   qapi/migration.json   | 18 +++++++++++++++---
> >   3 files changed, 37 insertions(+), 3 deletions(-)
> >
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 7b19f85..ed1a47b 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -117,6 +117,7 @@
> >   #define DEFAULT_MIGRATE_ANNOUNCE_STEP    100
> >   
> >   #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD     500     /* ms */
> > +#define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT            1       /* MB/s */
>
> This default value also looks a bit weird.. why 1MB/s?  Thanks,
Indeed, it seems kind of weired, the reason to set default dirty limit 
to 1MB/s is that we want to keep the dirty limit working until vcpu 
dirty page rate drop to 1MB/s once dirtylimit capability enabled during 
migration. In this way, migration has the largest chance to get 
converged before vcpu dirty page rate drop to 1MB/s。 If we set default
dirty limit greater than 1MB/s, the probability of success for migration 
may be reduced, and the default behavior of migration is try the best to 
become sucessful.
>