On 09/05/19 12:46, Peter Zijlstra wrote: > On Thu, Sep 05, 2019 at 10:45:27AM +0100, Patrick Bellasi wrote: > > > > From just reading the above, I would expect it to have the range > > > [-20,19] just like normal nice. Apparently this is not so. > > > > Regarding the range for the latency-nice values, I guess we have two > > options: > > > > - [-20..19], which makes it similar to priorities > > downside: we quite likely end up with a kernel space representation > > which does not match the user-space one, e.g. look at > > task_struct::prio. > > > > - [0..1024], which makes it more similar to a "percentage" > > > > Being latency-nice a new concept, we are not constrained by POSIX and > > IMHO the [0..1024] scale is a better fit. > > > > That will translate into: > > > > latency-nice=0 : default (current mainline) behaviour, all "biasing" > > policies are disabled and we wakeup up as fast as possible > > > > latency-nice=1024 : maximum niceness, where for example we can imaging > > to turn switch a CFS task to be SCHED_IDLE? > > There's a few things wrong there; I really feel that if we call it nice, > it should be like nice. Otherwise we should call it latency-bias and not > have the association with nice to confuse people. > > Secondly; the default should be in the middle of the range. Naturally > this would be a signed range like nice [-(x+1),x] for some x. but if you > want [0,1024], then the default really should be 512, but personally I > like 0 better as a default, in which case we need negative numbers. > > This is important because we want to be able to bias towards less > importance to (tail) latency as well as more importantance to (tail) > latency. > > Specifically, Oracle wants to sacrifice (some) latency for throughput. > Facebook OTOH seems to want to sacrifice (some) throughput for latency.
Another use case I'm considering is using latency-nice to prefer an idle CPU if latency-nice is set otherwise go for the most energy efficient CPU. Ie: sacrifice (some) energy for latency. The way I see interpreting latency-nice here as a binary switch. But maybe we can use the range to select what (some) energy to sacrifice mean here. Hmmm. -- Qais Yousef