Messages in this thread | | | Date | Tue, 7 Jan 2014 21:37:52 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving |
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On Tue, Jan 07, 2014 at 03:16:32PM +0000, Morten Rasmussen wrote: > From a load perspective wouldn't it be better to pick the least loaded > cpu in the group? It is not cheap to implement, but in theory it should > give less balancing within the group later an less unfairness until it > happens.
I tried that; see 04f733b4afac5dc93ae9b0a8703c60b87def491e for why it doesn't work.
> Rotating the cpu is probably good enough for most cases and certainly > easier to implement.
Indeed.
> The bias continues after they first round of load balance by the other > cpus?
The cost, yes. Even when perfectly balanced, we still get to iterate the entire machine computing s[gd]_lb_stats to find out we're good and don't need to move tasks about.
> Pulling everything to one cpu is not ideal from a performance point of > view. You loose some available cpu cycles until the balance settles. > However, it is not easy to do better and maintain scalability at the > same time.
Right, its part of the cost we pay for scaling better. And rotating this cost around a bit would alleviate the obvious bias.
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