Re: xen 4.5.0 rtds scheduler perform poorly with 2vms

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Sun, 2015-11-29 at 10:38 -0500, Meng Xu wrote:
> 
> 
> 2015-11-29 7:46 GMT-05:00 Yu-An(Victor) Chen <chen116@usc.edu>:
> > Hi Meng,
> > 
> Hi, 
>  
> > 
> > So I will rewrite my setup here again, but this time I shorten the
> > period and budget for RTDS like you suggested:
> > 
> Nice! :-)
> 
>  
> > -----------------------------------------------------------------
> > ------------------------------------------------------------
> > 
> (I like this line, BTW. :-D) 
> > 
> > 
> > for xen-credit : 2vms (both vm are given 8 vCPUs) sharing 8 cores
> > (cpu 0-7) using credit scheduler(both with weight of 800 and
> > capacity of 400)
> > for xen-rtds: 2 vms (both vm are given 8 vCPUs) sharing 8 cores
> > (cpu0-7) using RTDS (both with period of 4000(4ms) and budget of
> > 2000(2ms))) 
> > In both setup, dom0 is using 1 core from cpu 8-15
> > 
> > In both setup:
> > 
> > I loaded VM2 with constant running task with total utilization of 4
> > cores.
> > and in VM1 I run iterations of tasks of total utilization rate of 1
> > cores, 2 cores, 3 cores, 4 cores, and then record their
> > schedulbility.
> > -----------------------------------------------------------------
> > ------------------------------------------------------------
> > 
> > So st_jobs_stats for the missed deadline jobs are:
> > 
> > trial #1 composed of 2 tasks: total tasks utilization rate = 1
> > 
> > (period, exe, deadline)=(21ms,12.023ms,21ms) -> miss all deadline
> > (period, exe, deadline)=(100ms,37.985ms,100ms) -> no miss
> > 
> yes, this is the information I need and we can solve the mystery
> now... 
> Let's look at this task:
> (period, exe, deadline)=(21ms,12.023ms,21ms) -> miss all deadline
> Its utilization is 12.023 / 21 ~= 0.5614;
> Your VCPU utilization is only 2ms / 4ms = 0.5 
> So even when this task is pinned to one VCPU, it will still miss
> deadline because it has only one thread. :-)
>
Mmmm... As I said many times, I don't remember much of all those RT
schedulability formulas, but, is really that simple? I mean, if the in-
guest scheduling algorithm is global (e.g., global-EDF), the task could
migrate, couldn't it?

Is it really the case that you can never schedule tasks with U greater
than the smaller U of the various vCPUs (which seems to me to be what
you're implying)?

Anyway...

> So basically, it is because your configuration is incorrect. That's
> also why you need to read some literatures in real-time scheduling
> theory. ;-) 
> 
... I totally agree with this! ;-D

Regards,
Dario
>  
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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