Re: [PATCH v1 3/8] migration: Introduce dirty-limit capability

[Hyman Huang]

在 2022/9/5 17:32, Markus Armbruster 写道:
> Hyman Huang <huangy81@chinatelecom.cn> writes:
>
> > 在 2022/9/2 16:07, Markus Armbruster 写道:
> > > 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
> > > > migratioin-set-capabilities, and set the parameters
> > >
> > > migrate-set-capabilities
> > >
> > > > "x-vcpu-dirty-limit-period", "vcpu-dirty-limit" suitably
> > >
> > > "x-vcpu-dirty-limit"
> > >
> > > > to speed up convergence.
> > > >
> > > > Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
> > >
> > > Hmm.  You make dirty-limiting as a whole a stable interface (evidence:
> > > capability "dirty-limit" is stable), but keep its two parameters
> > > unstable.  Rationale behind that?
> > Thanks Markus's comments. :)
> >
> > x-vcpu-dirty-limit-period is an experimental parameter indeed, as to 
> > x-vcpu-dirty-limit, i think it's resonable to be a stable parameter.
> > These 2 parameters are introduced first time and none of them suffer heavily 
> > tests, so i also made vcpu-dirty-limit experimental last version.
> >
> > For dirty-limit interface, it improves the vCPU computational performance 
> > during migration indeed(see the test results in cover
> > letter), so it sounds ok to be an stable interface.
> >
> > The 'x-vcpu-dirty-limit-period' parameter can be dropped, IMHO, after being 
> > proved insignificant for migration in the future, and meanwhile,
> > x-vcpu-dirty-limit be made stable.
> >
> > Since I don't have much experience to introducing fresh new interface,
> > any suggestions are welcome.
>
> Is the new interface fit for purpose without use of any experimental
> parameter?
>  > If the answer is something like "command dirty-limit improves things
> even without use of experimental parameters, but using them may well
> improve things more (but we need more testing to know for sure)", then
> your current use of 'unstable' may make sense.
Yes, with the default value of parameter，the new interface works ok and 
improve performance.

For x-vcpu-dirty-limit, we provide it because user may not want virtual 
CPU throttle heavily, x-vcpu-dirty-limit is kind of like 
cpu-throttle-percentage, which is used to setup the threshold when 
making guest down.

For x-vcpu-dirty-limit-period, it is just as you said: "command 
dirty-limit improves things even without use of experimental parameters, 
but using them may wellimprove things more (but we need more testing to 
know for sure)"

So, should i make x-vcpu-dirty-limit non-experimental next version?
--
Best regard

Hyman Huang(黄勇)