Re: [PATCH v1 5/8] migration: Export dirty-limit time info

[Hyman Huang <huangy81@chinatelecom.cn>]

在 2022/10/7 23:09, Markus Armbruster 写道:
> Hyman Huang <huangy81@chinatelecom.cn> writes:
> 
>> 在 2022/10/2 2:31, Markus Armbruster 写道:
>>> huangy81@chinatelecom.cn writes:
>>>
>>>> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>>>>
>>>> Export dirty limit throttle time and estimated ring full
>>>> time, through which we can observe the process of dirty
>>>> limit during live migration.
>>>>
>>>> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>>> [...]
>>>
>>>> diff --git a/qapi/migration.json b/qapi/migration.json
>>>> index bc4bc96..c263d54 100644
>>>> --- a/qapi/migration.json
>>>> +++ b/qapi/migration.json
>>>> @@ -242,6 +242,12 @@
>>>>    #                   Present and non-empty when migration is blocked.
>>>>    #                   (since 6.0)
>>>>    #
>>>> +# @dirty-limit-throttle-us-per-full: Throttle time (us) during the period of
>>>> +#                                    dirty ring full (since 7.0)
>>>> +#
>>>> +# @dirty-limit-us-ring-full: Estimated periodic time (us) of dirty ring full.
>>>> +#                            (since 7.0)
>>>> +#
>>>
>>> Can you explain what is measured here a bit more verbosely?
>>
>> The two fields of migration info aims to export dirty-limit throttle time so that upper apps can check out the process of live migration,
>> like 'cpu-throttle-percentage'.
>>
>> The commit "tests: Add migration dirty-limit capability test" make use of the 'dirty-limit-throttle-us-per-full' to checkout if dirty-limit has
>> started, the commit "tests/migration: Introduce dirty-limit into guestperf" introduce the two field so guestperf tools also show the
>> process of dirty-limit migration.
>>
>> And i also use qmp_query_migrate to observe the migration by checkout these two fields.
>>
>> I'm not sure if above explantation is what you want exactly, please be free to start any discussion about this features.
> 
> You explained use cases, which is always welcome.
> 
> I'm trying to understand the two new members' meaning, i.e. what exactly
> is being measured.

dirty-limit-throttle-us-per-full：
Means the time vCPU should sleep once it's dirty ring get full, since we 
set limit on vCPU every time it returns to Qemu for the 
KVM_EXIT_DIRTY_RING_FULL reason, the sleep time may also changes everty 
time dirty ring get full. 'dirty-limit-throttle-us-per-full' can be 
simplified as 'throttle time(us) every time vCPU's dirty ring full get 
full'. The 'dirty-limit' is just the prefix to mark that parameter is 
dirty-limit-related.

dirty-limit-us-ring-full:
It is an estimated value which means the time a vCPU's dirty ring get 
full. It depends on the vCPU's dirty page rate, the higher the rate is, 
the smaller dirty-limit-us-ring-full is.

dirty-limit-throttle-us-per-full / dirty-limit-us-ring-full * 100 is 
kind of like 'cpu-throttle-percentage'.

Thanks,

Yong

> 
> For existing @cpu-throttle-percentage, the doc comment tells me:
> "percentage of time guest cpus are being throttled during
> auto-converge."
> 
> For the your new members, the doc comment tries to tell me, but it
> doesn't succeed.  If you explain what is being measured more verbosely,
> we may be able to improve the doc comment.
> 

-- 
Best regard

Hyman Huang(黄勇)