[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xendevel] xen 4.5.0 rtds scheduler perform poorly with 2vms
On Fri, 20151127 at 08:36 0800, YuAn(Victor) Chen wrote: > Hi Dario, > Hi, > Thanks for the reply! > You're welcome. :) I'm adding Meng to Cc... > My goal for the experiment is to show that xen rtds scheduler is > better than credit scheduler when it comes to real time tasks. > so my set up is: > > for xencredit : 2vms sharing 8 cores (cpu 07) using credit > scheduler(both with weight of 800 and capacity of 400) > for xenrtds: 2 vms sharing 8 cores (cpu07) using RTDS (both with > period of 10000 and budget of 5000) > in both setup, dom0 is using 1 core from cpu 815 > I can't see you saying how many vCPUs each VM has. I presume 4? I'll assume that is the case in the rest of this email. > VM2 will run tasks that has utilization of 4 cores. VM1 will run > tasks that has utilization from 1 to 4, and I will record VM1's > schedulbility. > I'm not sure what you mean with "tasks that has utilization of 4 cores" (or, in general, "of X cores"). I'll assume it means that you run some kind of periodic tasks, each one with an utilization ratio, given by the computation time of their periodic instances over their period, and that the sum of all these ratios give 4 (or, in general, X). If this is not the case, please, clarify. > I am hoping to see that the VM1 using xenrtds will perform better > than the VM1 using xencredit, but what I am seeing is the opposite. > Ok, let's try some maths. Not realtime scheduling theory (for now :) ), just maths. let's take VM2. What I understand is that the "internal demand", from tasks running inside it, would be 400%. Assuming the VM has 4 vCPUs, since each vCPU has 5000/10000=50%, the VM has, in total, 200%. So, in this case, I don't see how a demand of 400% could be satisfied by a supply of 200%. If the VM has 8 vCPUs, each at 50%, that does mean that it could, in theory, serve a demand of 400%. But that does not "just" happens. Here it is where realtime scheduling theory comes into play, and I'll let Meng comment on this (if he's interested :D), as I don't recall the details of those formulas any longer! In a nutshell, budgets ad periods of tasks and VMs must satisfy certain condition, in order to the schedulability of the whole system to be guaranteed. Therefore, either because you have misconfigured the system, or because you are hitting some realtime scheduling "issues", it is well possible that, in this simple case, for a best effort scheduler to do a pretty fine job. There are indeed differences and specific use case and scenarios for Credit and RTDS. The one you described is just not one of them. :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) Attachment:
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