Messages in this thread | | | Date | Fri, 13 Oct 2023 09:37:39 +0200 | From | Peter Zijlstra <> | Subject | Re: Re: [PATCH 03/15] sched/fair: Add lag based placement |
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On Thu, Oct 12, 2023 at 03:04:47PM +0800, Abel Wu wrote: > On 10/11/23 9:24 PM, Peter Zijlstra Wrote:
> > > > + * we should inflate the lag before placement such that the > > > > + * effective lag after placement comes out right. > > > > + * > > > > + * As such, invert the above relation for vl'_i to get the vl_i > > > > + * we need to use such that the lag after placement is the lag > > > > + * we computed before dequeue. > > > > + * > > > > + * vl'_i = vl_i - w_i*vl_i / (W + w_i) > > > > + * = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i) > > > > + * > > > > + * (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i > > > > + * = W*vl_i > > > > + * > > > > + * vl_i = (W + w_i)*vl'_i / W > > > > And then we obtain the scale factor: (W + w_i)/W, which is >1, right? > > Yeah, I see. But the scale factor is only for the to-be-placed entity. > Say there is an entity k on the tree: > > vl_k = V - v_k > > adding the to-be-placed entity i affects this by: > > define delta := w_i*vl_i / (W + w_i) > > vl'_k = V' - v_k > = V - delta - (V - vl_k) > = vl_k - delta > > hence for any entity on the tree, its lag is offsetted by @delta. So > I wonder if we should simply do offsetting rather than scaling.
I don't see the point, the result is the same and computing delta seems numerically less stable.
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