Hyman Huang <huang...@chinatelecom.cn> writes:
> 在 2023/3/24 20:11, Markus Armbruster 写道:
>> huang...@chinatelecom.cn writes:
>>
>>> From: Hyman Huang(黄勇) <huang...@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) <huang...@chinatelecom.cn>
>>> Acked-by: Peter Xu <pet...@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?
>
> Yes, live migration set "dirty-limit" value when that capability is set,
> the comment changes to "Apply the algorithm of dirty page rate limit to
> throttle down guest if capability is set, rather than auto-converge".
>
> Please continue to polish the doc if needed. Thanks.
Let's see whether I understand.
Throttling happens only during migration.
There are two throttling algorithms: "auto-converge" (default) and
"dirty page rate limit".
The latter can be tuned with set-vcpu-dirty-limit.
Correct?
What happens when migration capability dirty-limit is enabled, but the
user hasn't set a limit with set-vcpu-dirty-limit, or canceled it with
cancel-vcpu-dirty-limit?
>>> #
>>> # 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:
>> [...]
>>
