Re: [PATCH 1/4] xen: credit2: implement utilization cap

From: Anshul Makkar <anshul.makkar@citrix.com>
To: Dario Faggioli <dario.faggioli@citrix.com>,
	xen-devel@lists.xenproject.org
Cc: George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH 1/4] xen: credit2: implement utilization cap
Date: Thu, 15 Jun 2017 17:16:26 +0100	[thread overview]
Message-ID: <6ccdc293-26f6-efc9-817b-a5268f58200a@citrix.com> (raw)
In-Reply-To: <1497388423.26212.43.camel@citrix.com>

On 13/06/2017 22:13, Dario Faggioli wrote:
> On Tue, 2017-06-13 at 17:07 +0100, Anshul Makkar wrote:
>> On 12/06/2017 14:19, Dario Faggioli wrote:
>>>>> @@ -92,6 +92,82 @@
>>>>>   */
>>
>> what I want to ask is that if the budget of the domain is
>> replenished,
>> but credit for the vcpus of that domain is not available, then what
>> happens.
>>
> Yes, but the point is that budget can be available or not, while
> credits are always available. There's no such thing as credit not being
> available at all.
>
> The amount of credits each vcpu has decides which vcpu will run, in the
> sense that it will be the one that has the highest amount of credits.
> The others will indeed wait, but because they've got less credit than
> the one that runs, not because they don't have credits available.
>
Ok, as discussed, credits are resetted if it reaches 0. In that sense 
its being considered "always available"..

>> I believe, vcpus won't be scheduled (even if they have budget_quota)
>> till they get their credit replenished.
>>
> Credits are not exhausted or replenished.
>>>
>>> But... it's already totally dynamic.
>>
>> csched2_dom_cntl()
>> {
>> svc->budget_quota = max(sdom->tot_budget / sdom->nr_vcpus,
>>                                          CSCHED2_MIN_TIMER);
>> }
>> If domain->tot_budge = 200
>> nr_cpus is 4, then each cpu gets 50%.
>> How this is dynamic allocation ? We are not considering vcpu
>> utilization
>> of other vcpus of domain before allocating budget_quota for some
>> vcpu.
>>
> Right. Well, what this means is that each vcpu will get budget in
> chunks of tot_budget/nr_vcpus. But then, how much budget each vcpu will
> actually be able to get and consume in each period, it's impossible to
> know in advance, as it will depend on overall system load, and the
> behavior of the various vcpus of the domain.
Yeah, the current implementation is dynamic in the sense that vcpu can 
execute in total more than its quota across multiple budget cycles, but 
its static in the sense that vcpu can't execute more than its budget 
quota in a single budget cycle.s
>
>>>> In runq candidate we have a code base
>
> Regards,
> Dario
>
Reviewed-by: Anshul Makkar<anshul.makkar@citrix.com>
Anshul


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