linux-kernel - Re: [PATCH 09/14] Add /proc trigger for memory compaction

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Message-Id: <20100406170555.1efe35b0.akpm@linux-foundation.org>
Date:	Tue, 6 Apr 2010 17:05:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 09/14] Add /proc trigger for memory compaction

On Fri,  2 Apr 2010 17:02:43 +0100
Mel Gorman <mel@....ul.ie> wrote:

> This patch adds a proc file /proc/sys/vm/compact_memory. When an arbitrary
> value is written to the file,

Might be better if "when the number 1 is written...".  That permits you
to add 2, 3 and 4 later on.

> all zones are compacted. The expected user
> of such a trigger is a job scheduler that prepares the system before the
> target application runs.
> 

Ick.  The days of multi-user computers seems to have passed.

> ...
>
> +/* Compact all zones within a node */
> +static int compact_node(int nid)
> +{
> +	int zoneid;
> +	pg_data_t *pgdat;
> +	struct zone *zone;
> +
> +	if (nid < 0 || nid >= nr_node_ids || !node_online(nid))
> +		return -EINVAL;
> +	pgdat = NODE_DATA(nid);
> +
> +	/* Flush pending updates to the LRU lists */
> +	lru_add_drain_all();
> +
> +	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
> +		struct compact_control cc;
> +
> +		zone = &pgdat->node_zones[zoneid];
> +		if (!populated_zone(zone))
> +			continue;
> +
> +		cc.nr_freepages = 0;
> +		cc.nr_migratepages = 0;
> +		cc.zone = zone;

It would be better to do

	struct compact_control cc = {
		.nr_freepages = 0,
		etc

because if you later add more fields to compact_control, everything
else works by magick.  That's served us pretty well with
writeback_control, scan_control, etc.
	
> +		INIT_LIST_HEAD(&cc.freepages);
> +		INIT_LIST_HEAD(&cc.migratepages);
> +
> +		compact_zone(zone, &cc);
> +
> +		VM_BUG_ON(!list_empty(&cc.freepages));
> +		VM_BUG_ON(!list_empty(&cc.migratepages));
> +	}
> +
> +	return 0;
> +}
> +
> +/* Compact all nodes in the system */
> +static int compact_nodes(void)
> +{
> +	int nid;
> +
> +	for_each_online_node(nid)
> +		compact_node(nid);

What if a node goes offline?

> +	return COMPACT_COMPLETE;
> +}
> +
>
> ...
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