linux-kernel - Re: [PATCH RESEND] autonuma: Fix scan period updating

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Message-Id: <20190729072845.GC7168@linux.vnet.ibm.com>
Date:   Mon, 29 Jul 2019 12:58:45 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...e.de>, jhladky@...hat.com,
        lvenanci@...hat.com, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH RESEND] autonuma: Fix scan period updating

> >> 
> >> if (lr_ratio >= NUMA_PERIOD_THRESHOLD)
> >>     slow down scanning
> >> else if (sp_ratio >= NUMA_PERIOD_THRESHOLD) {
> >>     if (NUMA_PERIOD_SLOTS - lr_ratio >= NUMA_PERIOD_THRESHOLD)
> >>         speed up scanning
> 
> Thought about this again.  For example, a multi-threads workload runs on
> a 4-sockets machine, and most memory accesses are shared.  The optimal
> situation will be pseudo-interleaving, that is, spreading memory
> accesses evenly among 4 NUMA nodes.  Where "share" >> "private", and
> "remote" > "local".  And we should slow down scanning to reduce the
> overhead.
> 
> What do you think about this?

If all 4 nodes have equal access, then all 4 nodes will be active nodes.

>From task_numa_fault()

	if (!priv && !local && ng && ng->active_nodes > 1 &&
				numa_is_active_node(cpu_node, ng) &&
				numa_is_active_node(mem_node, ng))
		local = 1;

Hence all accesses will be accounted as local. Hence scanning would slow
down.

-- 
Thanks and Regards
Srikar Dronamraju