Hi Meng and Dario, Thank you both very very much for the explanations! It really help a lot for understanding RT and what I am doing wrong here! I will do more research and reconfigure the experiments. Hope to have some good news soon!
Thanks again! On Sun, Nov 29, 2015 at 8:44 AM, Meng Xu <xumengpa...@gmail.com> wrote: > 2015-11-29 11:27 GMT-05:00 Dario Faggioli <dario.faggi...@citrix.com>: > > On Sun, 2015-11-29 at 10:38 -0500, Meng Xu wrote: > >> > >> > >> 2015-11-29 7:46 GMT-05:00 Yu-An(Victor) Chen <chen...@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? > > Ah, let me clarify... > It is not that simple. ;-) I just simplify it, hoping it can simplify > the problem and highlight the possible reason. > > > I mean, if the in- > > guest scheduling algorithm is global (e.g., global-EDF), the task could > > migrate, couldn't it? > > Yes. If these partial VCPUs happen to be scheduled "sequentially", the > OS inside VM can migrate the task and make the task keep running. But > that is not the worst-case for the OS. > > The worst case for the OS to schedule one task is that all of these > VCPUs are released at the same time, are schedulable as early as > possible in the first period and scheduled as late as possible in the > second period, which will create the largest starvation period to the > VM. > So in the worst case, you won't be able to schedule a task with > utilization U on k VCPUs with utilzation U, no matter how large k is. > (Think about that these k VCPUs are always scheduled at the same > time.) > > The detailed illustration of the worst case scenario is at Arvind's > paper: http://link.springer.com/article/10.1007%2Fs11241-009-9073-x > My latest journal paper > (http://link.springer.com/article/10.1007%2Fs11241-015-9223-2) tighten > the resource supply bound function of the MPR model. I believe the > equations are too boring to most of people in the mailing list. > > So let's avoid the complex equations here. ;-) > > To Yu-An, > The basic idea is, as Dario mentioned in the previous email, that the > configuration of VCPUs are important to provide the schedulability of > tasks inside VM. Especially, if you are doing research in RT, you need > to (maybe have to) know the RT scheduling theory. :-) > > > > > 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)? > No. In the worst case, it is. > Because the VCPUs are created with the "similar" release time, they > may very likely be scheduled concurrently and fall into the so-called > "worst case" (which is just worse case for sequential task) in an idle > system. > > > > > > Anyway... > > > > OK. I can understand... Hope my explanation won't cause more confusion. :-D > > Best, > > Meng > > ----------- > Meng Xu > PhD Student in Computer and Information Science > University of Pennsylvania > http://www.cis.upenn.edu/~mengxu/ >
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