Re: [PATCH v4 06/10] migration: Introduce dirty-limit capability

[Markus Armbruster <armbru@redhat.com>]

huangy81@chinatelecom.cn writes:

> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>
> Introduce migration dirty-limit capability, which can
> be turned on before live migration and limit dirty
> page rate durty live migration.
>
> Introduce migrate_dirty_limit function to help check
> if dirty-limit capability enabled during live migration.
>
> Meanwhile, refactor vcpu_dirty_rate_stat_collect
> so that period can be configured instead of hardcoded.
>
> dirty-limit capability is kind of like auto-converge
> but using dirty limit instead of traditional cpu-throttle
> to throttle guest down. To enable this feature, turn on
> the dirty-limit capability before live migration using
> migrate-set-capabilities, and set the parameters
> "x-vcpu-dirty-limit-period", "vcpu-dirty-limit" suitably
> to speed up convergence.
>
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> Acked-by: Peter Xu <peterx@redhat.com>

[...]

> diff --git a/qapi/migration.json b/qapi/migration.json
> index d33cc2d582..b7a92be055 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -477,6 +477,8 @@
>  #                    will be handled faster.  This is a performance feature and
>  #                    should not affect the correctness of postcopy migration.
>  #                    (since 7.1)
> +# @dirty-limit: Use dirty-limit to throttle down guest if enabled.
> +#               (since 8.0)

Feels too terse.  What exactly is used and how?  It's not the capability
itself (although the text sure sounds like it).  I guess it's the thing
you set with command set-vcpu-dirty-limit.

Is that used only when the capability is set?

>  #
>  # Features:
>  # @unstable: Members @x-colo and @x-ignore-shared are experimental.
> @@ -492,7 +494,7 @@
>             'dirty-bitmaps', 'postcopy-blocktime', 'late-block-activate',
>             { 'name': 'x-ignore-shared', 'features': [ 'unstable' ] },
>             'validate-uuid', 'background-snapshot',
> -           'zero-copy-send', 'postcopy-preempt'] }
> +           'zero-copy-send', 'postcopy-preempt', 'dirty-limit'] }
>  
>  ##
>  # @MigrationCapabilityStatus:

[...]