Re: [PATCH 2/3] sched/numa: Allow a floating imbalance between NUMA nodes

[Mel Gorman]

On Tue, Nov 17, 2020 at 03:24:56PM +0100, Vincent Guittot wrote:
> On Tue, 17 Nov 2020 at 14:42, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Currently, an imbalance is only allowed when a destination node
> > is almost completely idle. This solved one basic class of problems
> > and was the cautious approach.
> >
> > This patch revisits the possibility that NUMA nodes can be imbalanced
> > until 25% of the CPUs are occupied. The reasoning behind 25% is somewhat
> > superficial -- it's half the cores when HT is enabled.  At higher
> > utilisations, balancing should continue as normal and keep things even
> > until scheduler domains are fully busy or over utilised.
> 
> This reminds me previous discussions on the same topic: how much
> imbalance is allowed that will not screw up the bandwidth of the node
> I'm worried that there is no topology insight in the decision like
> hyperthreading, or number of cpus in the LLC
> 

We still don't have a good answer for that. It could be a tunable I guess
but it would be horrible to tune properly.

-- 
Mel Gorman
SUSE Labs