Re: [Xen-devel] [PATCH 1/2] xen: credit2: avoid vCPUs to ever reach lower credits than idle

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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