From: Dario Faggioli <dfaggioli@suse.com>
To: George Dunlap <George.Dunlap@citrix.com>,
Andrew Cooper <Andrew.Cooper3@citrix.com>
Cc: Juergen Gross <jgross@suse.com>,
Charles Arnold <carnold@suse.com>,
Tomas Mozes <hydrapolic@gmail.com>, Glen <glenbarney@gmail.com>,
Jan Beulich <jbeulich@suse.com>, Sarah Newman <srn@prgmr.com>,
xen-devel <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle
Date: Thu, 12 Mar 2020 16:10:35 +0100 [thread overview]
Message-ID: <23d68fd3cefbcdd0766a21973184ca4cf2fabfd2.camel@suse.com> (raw)
In-Reply-To: <49F5E83F-DB7A-4756-A1C5-DDF47C9312F3@citrix.com>
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On Thu, 2020-03-12 at 14:40 +0000, George Dunlap wrote:
> > On Mar 12, 2020, at 1:55 PM, Andrew Cooper <
> > Andrew.Cooper3@citrix.com> wrote:
> >
> > Certainly as far as XenServer is concerned, we haven't seen
> > symptoms
> > like this in a decade of running credit1.
>
> One reason could be because the symptoms are different. On credit1,
> credits and “priority” are separated; it’s not possible in credit1
> for a vcpu to end up with a lower priority than the idle domain, and
> no matter how low the credits become, a vcpu will always end up with
> some “peers” at the same priority level, meaning it always has a
> chance at some cpu.
>
Indeed. Under many respects, Credit1 is basically a slightly more
powerful variant of Round-Robin.
The actual time vCPUs spend executing influences the scheduling
decisions a lot less than in Credit2 (well, it influences the
scheduling decisions a lot less than in any scheduler of any OS or
hypervisor conceived and implemented in the last decade, or maybe even
more).
As a matter of fact, this property happens to have shield it from the
nastiest effects of the issue at hand.
> What Dario is saying (if I understand him correctly) is that the
> *proximate* cause (allowing a vcpu to have an effective priority of
> less than idle) is certainly credit2-only; but the *deeper* cause
> (vcpus racking up massive amounts of negative credit) is not.
>
Yes, that's exactly what I meant. :-)
Regards
--
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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next prev parent reply other threads:[~2020-03-12 15:11 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 13:44 [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits Dario Faggioli
2020-03-12 13:44 ` [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle Dario Faggioli
2020-03-12 13:55 ` Andrew Cooper
2020-03-12 14:40 ` George Dunlap
2020-03-12 15:10 ` Dario Faggioli [this message]
2020-03-12 14:58 ` Dario Faggioli
2020-03-12 14:45 ` George Dunlap
2020-03-12 17:03 ` Dario Faggioli
2020-03-12 15:26 ` Jan Beulich
2020-03-12 16:00 ` Jürgen Groß
2020-03-12 16:59 ` Dario Faggioli
2020-03-12 16:11 ` Dario Faggioli
2020-03-12 16:36 ` Jan Beulich
2020-03-12 13:44 ` [Xen-devel] [PATCH 2/2] xen: credit2: fix credit reset happening too few times Dario Faggioli
2020-03-12 15:08 ` [Xen-devel] [PATCH 0/2] xen: credit2: fix vcpu starvation due to too few credits Roger Pau Monné
2020-03-12 17:02 ` Dario Faggioli
2020-03-12 17:59 ` Roger Pau Monné
2020-03-13 6:19 ` Dario Faggioli
2020-03-12 15:51 ` Jürgen Groß
2020-03-12 16:27 ` Andrew Cooper
2020-03-13 7:26 ` Dario Faggioli
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