linux-kernel - Re: [PATCH 09/10] sched/fair: disable stealing if too many NUMA nodes

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Message-ID: <20181022220509.GC3109@worktop.c.hoisthospitality.com>
Date:   Tue, 23 Oct 2018 00:05:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     mingo@...hat.com, subhra.mazumdar@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH 09/10] sched/fair: disable stealing if too many NUMA nodes

On Mon, Oct 22, 2018 at 03:21:20PM -0400, Steven Sistare wrote:
> On 10/22/2018 2:47 PM, Steven Sistare wrote:
> > On 10/22/2018 1:06 PM, Peter Zijlstra wrote:
> >> On Mon, Oct 22, 2018 at 07:59:40AM -0700, Steve Sistare wrote:
> >>> The STEAL feature causes regressions on hackbench on larger NUMA systems,
> >>> so disable it on systems with more than sched_steal_node_limit nodes
> >>> (default 2).
> >>
> >> How come? From a quick read the stealing is per LLC, where do we steal
> >> across nodes?
> > 
> > See the complete explanation in this patch.  It is deeper than can be gleaned
> > from a quick read.
> 
> I should have said a bit more.  Your quick take on stealing is correct, we do
> not steal across nodes.  However, stealing reduces average run queue length which 
> influences wake_affine migrations.  Now see the complete explanation.

Right; read a bit more just now.

hackbench is a fairly poor benchmark for numa performance. One that
comes to mind is multi wharehouse specjbb stuff (assuming you have numa
balance enabled of course).