Messages in this thread | | | Subject | Re: Usecases for the per-task latency-nice attribute | From | Parth Shah <> | Date | Fri, 20 Sep 2019 16:15:37 +0530 |
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On 9/19/19 8:13 PM, Qais Yousef wrote: > On 09/18/19 18:11, Parth Shah wrote: >> Hello everyone, >> >> As per the discussion in LPC2019, new per-task property like latency-nice >> can be useful in certain scenarios. The scheduler can take proper decision >> by knowing latency requirement of a task from the end-user itself. >> >> There has already been an effort from Subhra for introducing Task >> latency-nice [1] values and have seen several possibilities where this type of >> interface can be used. >> >> From the best of my understanding of the discussion on the mail thread and >> in the LPC2019, it seems that there are two dilemmas; >> >> 1. Name: What should be the name for such attr for all the possible usecases? >> ============= >> Latency nice is the proposed name as of now where the lower value indicates >> that the task doesn't care much for the latency and we can spend some more >> time in the kernel to decide a better placement of a task (to save time, >> energy, etc.) >> But there seems to be a bit of confusion on whether we want biasing as well >> (latency-biased) or something similar, in which case "latency-nice" may >> confuse the end-user. >> >> 2. Value: What should be the range of possible values supported by this new >> attr? >> ============== >> The possible values of such task attribute still need community attention. >> Do we need a range of values or just binary/ternary values are sufficient? >> Also signed or unsigned and so the length of the variable (u64, s32, etc)? > > IMO the main question is who is the intended user of this new knob/API? > > If it's intended for system admins to optimize certain workloads on a system > then I like the latency-nice range. > > If we want to support application writers to define the latency requirements of > their tasks then I think latency-nice would be very confusing to use. > Especially when one has to consider they lack a pre-knowledge about the system > they will run on; and what else they are sharing the resources with. >
Yes, valid point. But from my view, this will most certainly be for system admins who can optimize certain workloads from the systemd, tuned or similar OS daemons.
>> >> >> >> This mail is to initiate the discussion regarding the possible usecases of >> such per task attribute and to come up with a specific name and value for >> the same. >> >> Hopefully, interested one should plot out their usecase for which this new >> attr can potentially help in solving or optimizing it. >> >> >> Well, to start with, here is my usecase. >> >> ------------------- >> **Usecases** >> ------------------- >> >> $> TurboSched >> ==================== >> TurboSched [2] tries to minimize the number of active cores in a socket by >> packing an un-important and low-utilization (named jitter) task on an >> already active core and thus refrains from waking up of a new core if >> possible. This requires tagging of tasks from the userspace hinting which >> tasks are un-important and thus waking-up a new core to minimize the >> latency is un-necessary for such tasks. >> As per the discussion on the posted RFC, it will be appropriate to use the >> task latency property where a task with the highest latency-nice value can >> be packed. >> But for this specific use-cases, having just a binary value to know which >> task is latency-sensitive and which not is sufficient enough, but having a >> range is also a good way to go where above some threshold the task can be >> packed. > > > $> EAS > ==================== > The new knob can help EAS path to switch to spreading behavior when > latency-nice is set instead of packing tasks on the most energy efficient CPU. > ie: pick the most energy efficient idle CPU. >
+1
Thanks, Parth
> -- > Qais Yousef >
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