Re: [PATCH,RFC] numa,sched: use group fault statistics in numa placement

[Rik van Riel <riel@redhat.com>]

On 08/01/2013 06:37 AM, Peter Zijlstra wrote:
> On Thu, Aug 01, 2013 at 02:23:19AM -0400, Rik van Riel wrote:
>> Subject: [PATCH,RFC] numa,sched: use group fault statistics in numa placement
>>
>> Here is a quick strawman on how the group fault stuff could be used
>> to help pick the best node for a task. This is likely to be quite
>> suboptimal and in need of tweaking. My main goal is to get this to
>> Peter & Mel before it's breakfast time on their side of the Atlantic...
>>
>> This goes on top of "sched, numa: Use {cpu, pid} to create task groups for shared faults"
>>
>> Enjoy :)
>>
>> +	/*
>> +	 * Should we stay on our own, or move in with the group?
>> +	 * The absolute count of faults may not be useful, but comparing
>> +	 * the fraction of accesses in each top node may give us a hint
>> +	 * where to start looking for a migration target.
>> +	 *
>> +	 *  max_group_faults     max_faults
>> +	 * ------------------ > ------------
>> +	 * total_group_faults   total_faults
>> +	 */
>> +	if (max_group_nid >= 0 && max_group_nid != max_nid) {
>> +		if (max_group_faults * total_faults >
>> +				max_faults * total_group_faults)
>> +			max_nid = max_group_nid;
>> +	}
>
> This makes sense.. another part of the problem, which you might already
> have spotted is selecting a task to swap with.
>
> If you only look at per task faults its often impossible to find a
> suitable swap task because moving you to a more suitable node would
> degrade the other task -- below a patch you've already seen but I
> haven't yet posted because I'm not at all sure its something 'sane' :-)

I did not realize you had not posted that patch yet, and was
actually building on top of it :)

I suspect that comparing both per-task and per-group fault weights
in task_numa_compare should make your code do the right thing in
task_numa_migrate.

I suspect there will be enough randomness in accesses that they
will never be exactly the same, so we might not need an explicit
tie breaker.

However, if numa_migrate_preferred fails, we may want to try
migrating to any node that has a better score than the current
one.  After all, if we have a group of tasks that would fit in
2 NUMA nodes, we don't want half of the tasks to not migrate
at all because the top node is full. We want them to move to
the #2 node at some point.

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