linux-kernel - Re: [RFC][PATCH 14/26] sched, numa: Numa balancer

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Date:	Sat, 07 Jul 2012 14:26:39 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dan Smith <danms@...ibm.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 14/26] sched, numa: Numa balancer

On 03/16/2012 10:40 AM, Peter Zijlstra wrote:

> +/*
> + * Assumes symmetric NUMA -- that is, each node is of equal size.
> + */
> +static void set_max_mem_load(unsigned long load)
> +{
> +	unsigned long old_load;
> +
> +	spin_lock(&max_mem_load.lock);
> +	old_load = max_mem_load.load;
> +	if (!old_load)
> +		old_load = load;
> +	max_mem_load.load = (old_load + load) >> 1;
> +	spin_unlock(&max_mem_load.lock);
> +}

The above in your patch kind of conflicts with this bit
from patch 6/26:

+	/*
+	 * Migration allocates pages in the highest zone. If we cannot
+	 * do so then migration (at least from node to node) is not
+	 * possible.
+	 */
+	if (vma->vm_file &&
+		gfp_zone(mapping_gfp_mask(vma->vm_file->f_mapping))
+								< policy_zone)
+			return 0;

Looking at how the memory load code is used, I wonder
if it would make sense to count "zone size - free - inactive
file" pages instead?

> +			/*
> +			 * Avoid migrating ne's when we'll know we'll push our
> +			 * node over the memory limit.
> +			 */
> +			if (max_mem_load &&
> +			    imb->mem_load + mem_moved + ne_mem > max_mem_load)
> +				goto next;

> +static void numa_balance(struct node_queue *this_nq)
> +{
> +	struct numa_imbalance imb;
> +	int busiest;
> +
> +	busiest = find_busiest_node(this_nq->node, &imb);
> +	if (busiest == -1)
> +		return;
> +
> +	if (imb.cpu <= 0 && imb.mem <= 0)
> +		return;
> +
> +	move_processes(nq_of(busiest), this_nq, &imb);
> +}

You asked how and why Andrea's algorithm converges.
After looking at both patch sets for a while, and asking
for clarification, I think I can see how his code converges.

It is not yet clear to me how and why your code converges.

I see some dual bin packing (CPU & memory) heuristics, but
it is not at all clear to me how they interact, especially
when workloads are going active and idle on a regular basis.

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