On Thu, Sep 05, 2019 at 12:30:02 +0100, Peter Zijlstra wrote...
> On Thu, Sep 05, 2019 at 12:13:47PM +0100, Qais Yousef wrote: >> On 09/05/19 12:46, Peter Zijlstra wrote: > >> > 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. > > It cannot be binary, per definition is must be ternary, that is, <0, ==0 > and >0 (or middle value if you're of that persuasion). > > In your case, I'm thinking you mean >0, we want to lower the latency. > > Anyway; there were a number of things mentioned at OSPM that we could > tie into this thing and finding sensible mappings is going to be a bit > of trial and error I suppose. > > But as patrick said; we're very much exporting a BIAS knob, not a set of > behaviours. Right, although I think behaviours could still be exported but via a different and configurable interface, using thresholds. Either at compile time or via procfs maybe we can expose and properly document what happen in the scheduler if/when a task has a "latency niceness" crossing a given threshold. For example, by setting something like: /proc/sys/kernel/sched_cfs_latency_idle = 1000 we state that the task is going to be scheduled according to the SCHED_IDLE policy. ( ( (tomatoes target here) ) ) Not sure also if we wanna commit to user-space APIs how we internally map/translate a "latency niceness" value into a scheduler behaviour bias. Maybe better not at least at the very beginning. Best, Patrick -- #include <best/regards.h> Patrick Bellasi
