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

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Date:   Mon, 22 Oct 2018 15:21:20 -0400
From:   Steven Sistare <steven.sistare@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
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
Subject: Re: [PATCH 09/10] sched/fair: disable stealing if too many NUMA nodes

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.

- Steve