Re: [PATCH v2 00/10] sched: Flatten the pick

[Tejun Heo <tj@kernel.org>]

Hello, Peter.

On Mon, May 18, 2026 at 09:14:56AM +0200, Peter Zijlstra wrote:
...
> So the current scheme will inflate the part of A to be double the weight
> (of B), giving them 2 out of 3 parts on the contended CPUs, but then B
> will still get complete / uncontested access to those extra 128 CPUs,
> resulting in a 2:4 weight distribution.
> 
> Which also isn't as straight forward as one might think.

Right, the current behavior isn't quite what people would expect intuitively
either.

...
> So for the one contended CPU A gets 256 out of 257 parts, while B gets
> the full CPU for the remaining 255 CPUs, for a:
> 
>   256    1        257
>   --- : --- + 255*--- = 256:65535 ~ 1:256
>   257   257       257
> 
> distribution. While with the new scheme it would be:
> 
>  1   1       2
>  - : - + 255*- = 1:511
>  2   2       2
> 
> Which, realistically isn't all that different, except the old scheme has
> this really large weight to deal with.
> 
> So from where I'm sitting, yes different, but it behaves better.

I see. Thread cardinality and affinity problems make weight based
distribution such a pain. I wonder whether this can be better solved by
turning it into a two-layer allocation problem - groups to CPUs and then
timeshare on CPUs as necessary. That comes with a lot of its own problems
but it can, aspirationally at least, approximate global weight distribution
and would have better locality properties.

Thanks.

-- 
tejun