Yong Huang <yong.hu...@smartx.com> writes: > 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" !
Let me paraphrase to make sure I got it... The vCPU is throttled as needed to keep its dirty rate within the limit set with set-vcpu-dirty-limit. Correct? What happens when I enable the dirty limit convergence algorithm without setting a limit with set-vcpu-dirty-limit? >> What happens when the condition is not met? How can the user ensure the >> condition is met? >> >> [...] >> >>
