On Tue, Jul 18, 2023 at 7:04 PM Markus Armbruster <arm...@redhat.com> wrote:
> Yong Huang <yong.hu...@smartx.com> writes: > > > On Thu, Jul 13, 2023 at 8:44 PM Markus Armbruster <arm...@redhat.com> > wrote: > > > >> ~hyman <hy...@git.sr.ht> writes: > >> > >> > From: Hyman Huang(黄勇) <yong.hu...@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.hu...@smartx.com> > >> > Acked-by: Peter Xu <pet...@redhat.com> > >> > Reviewed-by: Juan Quintela <quint...@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". > I failed to express my meaning again : ( . "Throttle algo only works when vCPU's dirtyrate greater than 'vcpu-dirty-limit' " should change to "vCPU throttle only works when vCPU's dirtyrate greater than 'vcpu-dirty-limit'". Not the whole "algo" ! > What happens when the condition is not met? How can the user ensure the > condition is met? > > [...] > > -- Best regards
