On 05/09/2019 12:18, Patrick Bellasi wrote: >> 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. > > Right, we have this dualism to deal with and current mainline behaviour > is somehow in the middle. > > BTW, the FB requirement is the same we have in Android. > We want some CFS tasks to have very small latency and a low chance > to be preempted by the wake-up of less-important "background" tasks. > > I'm not totally against the usage of a signed range, but I'm thinking > that since we are introducing a new (non POSIX) concept we can get the > chance to make it more human friendly. > > Give the two extremes above, would not be much simpler and intuitive to > have 0 implementing the FB/Android (no latency) case and 1024 the > (max latency) Oracle case? >
For something like latency-<whatever>, I don't see the point of having such a wide range. The nice range is probably more than enough - and before even bothering about the range, we should probably agree on what the range should represent. If it's niceness, I read it as: positive latency-nice value means we're nice to latency, means we reduce it. So the further up you go, the more you restrict your wakeup scan. I think it's quite easy to map that into the code: current behaviour at 0, with a decreasing scan mask size as we go towards +19. I don't think anyone needs 512 steps to tune this. I don't know what logic we'd follow for negative values though. Maybe latency-nice -20 means always going through the slowpath, but what of the intermediate values? AFAICT this RFC only looks at wakeups, but I guess latency-nice can be applied elsewhere (e.g. load-balance, something like task_hot() and its use of sysctl_sched_migration_cost). > Moreover, we will never match completely the nice semantic, give that > a 1 nice unit has a proper math meaning, isn't something like 10% CPU > usage change for each step? > > For latency-nice instead we will likely base our biasing strategies on > some predefined (maybe system-wide configurable) const thresholds. > > Could changing the name to "latency-tolerance" break the tie by marking > its difference wrt prior/nice levels? AFAIR, that was also the original > proposal [1] by PaulT during the OSPM discussion. > > Best, > Patrick > > [1] https://youtu.be/oz43thSFqmk?t=1302 >
