Re: [PATCH] sched, fair: Allow a small load imbalance between low utilisation SD_NUMA domains v4

[Mel Gorman <mgorman@techsingularity.net>]

On Fri, Jan 17, 2020 at 02:08:13PM +0100, Vincent Guittot wrote:
> > > This patch allows a fixed degree of imbalance of two tasks to exist
> > > between NUMA domains regardless of utilisation levels. In many cases,
> > > this prevents communicating tasks being pulled apart. It was evaluated
> > > whether the imbalance should be scaled to the domain size. However, no
> > > additional benefit was measured across a range of workloads and machines
> > > and scaling adds the risk that lower domains have to be rebalanced. While
> > > this could change again in the future, such a change should specify the
> > > use case and benefit.
> > >
> >
> > Any thoughts on whether this is ok for tip or are there suggestions on
> > an alternative approach?
> 
> I have just finished to run some tests on my system with your patch
> and I haven't seen any noticeable any changes so far which was a bit
> expected. The tests that I usually run, use more than 4 tasks on my 2
> nodes system;

This is indeed expected. With more active tasks, normal load balancing
applies.

> the only exception is perf sched  pipe and the results
> for this test stays the same with and without your patch.

I never saw much difference with perf sched pipe either. It was
generally within the noise.

> I'm curious
> if this impacts Phil's tests which run LU.c benchmark with some
> burning cpu tasks

I didn't see any problem with LU.c whether parallelised by openMPI or
openMP but an independent check would be nice.

-- 
Mel Gorman
SUSE Labs