Re: [RFC PATCH v2 00/17] Core scheduling v2

[Mel Gorman]

On Fri, Apr 26, 2019 at 11:37:11AM -0700, Subhra Mazumdar wrote:
> > > So we avoid a maybe 0.1% scheduler placement overhead but inflict 5-10%
> > > harm on the workload, and also blow up stddev by randomly co-scheduling
> > > two tasks on the same physical core? Not a good trade-off.
> > > 
> > > I really think we should implement a relatively strict physical core
> > > placement policy in the under-utilized case, and resist any attempts to
> > > weaken this for special workloads that ping-pong quickly and benefit from
> > > sharing the same physical core.
> > > 
> > It's worth a shot at least. Changes should mostly be in the wake_affine
> > path for most loads of interest.
>
> Doesn't select_idle_sibling already try to do that by calling
> select_idle_core? For our OLTP workload we infact found the cost of
> select_idle_core was actually hurting more than it helped to find a fully
> idle core, so a net negative.
> 

select_idle_sibling is not guarnateed to call select_idle_core or avoid
selecting HT sibling whose other sibling is !idle but yes, in that path,
the search cost is a general concern which is why any change there is
tricky at best.

-- 
Mel Gorman
SUSE Labs