linux-kernel - Re: [PATCH 2/4] sched,numa: weigh nearby nodes for task placement on complex NUMA topologies

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Date:	Fri, 09 May 2014 11:16:53 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org, mgorman@...e.de,
	chegu_vinod@...com
Subject: Re: [PATCH 2/4] sched,numa: weigh nearby nodes for task placement
 on complex NUMA topologies

On 05/09/2014 06:03 AM, Peter Zijlstra wrote:
> On Thu, May 08, 2014 at 01:23:29PM -0400, riel@...hat.com wrote:
>>   static inline unsigned long task_weight(struct task_struct *p, int nid)
>>   {
>> -	unsigned long total_faults;
>> +	unsigned long total_faults, score;
>>
>>   	if (!p->numa_faults_memory)
>>   		return 0;
>> @@ -940,15 +997,32 @@ static inline unsigned long task_weight(struct task_struct *p, int nid)
>>   	if (!total_faults)
>>   		return 0;
>>
>> -	return 1000 * task_faults(p, nid) / total_faults;
>> +	score = 1000 * task_faults(p, nid);
>> +	score += nearby_nodes_score(p, nid, true);
>> +
>> +	score /= total_faults;
>> +
>> +	return score;
>>   }
>
> So you add an O(nr_nodes) loop to task_weight(), but that in itself is
> already called from O(nr_nodes) loops, yielding a total complexity of
> O(nr_nodes^2).

However, it only does actual calculations for nodes that
are closer by than the furthest away nodes in the system.

Hopefully on even the largest systems, that will mean an
"island" of a handful of nodes, with everything else being
at the same large distance.

> This might be fine, but algorithmic complexity should very much be a
> part of the changelog I think.

Agreed, I do need to document this kind of thing better,
if only because it gives people a chance to verify my
assumptions.

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