Re: [PATCH QEMU v8 4/9] migration: Introduce dirty-limit capability

[Markus Armbruster <armbru@redhat.com>]

Yong Huang <yong.huang@smartx.com> writes:

> On Thu, Jul 13, 2023 at 8:44 PM Markus Armbruster <armbru@redhat.com> wrote:
>
>> ~hyman <hyman@git.sr.ht> writes:
>>
>> > From: Hyman Huang(黄勇) <yong.huang@smartx.com>
>> >
>> > 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) <yong.huang@smartx.com>
>> > Acked-by: Peter Xu <peterx@redhat.com>
>> > Reviewed-by: Juan Quintela <quintela@redhat.com>
>>
>> [...]
>>
>> > diff --git a/qapi/migration.json b/qapi/migration.json
>> > index e43371955a..031832cde5 100644
>> > --- a/qapi/migration.json
>> > +++ b/qapi/migration.json
>> > @@ -497,6 +497,15 @@
>> >  #     are present.  'return-path' capability must be enabled to use
>> >  #     it.  (since 8.1)
>> >  #
>> > +# @dirty-limit: If enabled, migration will use the dirty-limit
>> > +#     algorithm to throttle down guest instead of auto-converge
>> > +#     algorithm. This algorithm only works when vCPU's dirtyrate
>>
>> Two spaces after sentence-ending punctuation, please.
>>
>> "dirty rate" with a space, because that's how we spell it elsewhere.
>>
>> > +#     greater than 'vcpu-dirty-limit', read processes in guest os
>> > +#     aren't penalized any more, so the algorithm can improve
>> > +#     performance of vCPU during live migration. This is an optional
>> > +#     performance feature and should not affect the correctness of the
>> > +#     existing auto-converge algorithm. (since 8.1)
>> > +#
>>
>> I'm still confused.
>>
>> The text suggests there are two separate algorithms "to throttle down
>> guest": "auto converge" and "dirty limit", and we get to pick one.
>> Correct?
>>
> Yes, indeed !
>
>>
>> If it is correct, then the last sentence feels redundant: picking
>> another algorithm can't affect the algorithm we're *not* using.  What
>> are you trying to express here?
>>
> What i want to express is that the new algorithm implementation does
> not affect the original algorithm, leaving it in the comments seems
> redundant indeed.  I'll drop this in the next version.

Works for me.

>> When do we use "auto converge", and when do we use "dirty limit"?
>>
>> What does the user really need to know about these algorithms?  Enough
>> to pick one, I guess.  That means advantages and disadvantages of the
>> two algorithms.  Which are?
>
> 1. The implementation of dirty-limit is based on dirty-ring, which is
> qualified
>    to big systems with huge memories and can improve huge guest VM
>     responsiveness remarkably during live migration. As a consequence,
> dirty-limit
>     is recommended on platforms with huge guest VMs as is the way with
> dirty-ring.
> 2. dirty-limit convergence algorithm does not affect the "read-process" in
> guest
>    VM, so guest VM gains the equal read performance nearly as it runs on
> host
>    during the live migration. As a result, dirty-limit is recommended if
> the guest
>     VM requires a stable read performance.
> The above explanation is about the recommendation of dirty-limit, please
> review,
> if it's ok, i'll place it in the comment of the dirty-limit capability.

Yes, please.  But before that, I have still more questions.  "This
algorithm only works when vCPU's dirtyrate greater than
'vcpu-dirty-limit'" is a condition: "FEATURE only works when CONDITION".
What happens when the condition is not met?  How can the user ensure the
condition is met?

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