On Fri,  2 Apr 2010 17:02:39 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

quoted text
> Unusable free space index is a measure of external fragmentation that
> takes the allocation size into account. For the most part, the huge page
> size will be the size of interest but not necessarily so it is exported
> on a per-order and per-zone basis via /proc/unusable_index.

I'd suggest /proc/sys/vm/unusable_index.  I don't know how pagetypeinfo
found its way into the top-level dir.

quoted text
> The index is a value between 0 and 1. It can be expressed as a
> percentage by multiplying by 100 as documented in
> Documentation/filesystems/proc.txt.
> 
> ...
> 
> +> cat /proc/unusable_index
> +Node 0, zone      DMA 0.000 0.000 0.000 0.001 0.005 0.013 0.021 0.037 0.037 0.101 0.230
> +Node 0, zone   Normal 0.000 0.000 0.000 0.001 0.002 0.002 0.005 0.015 0.028 0.028 0.054
> +
> +The unusable free space index measures how much of the available free
> +memory cannot be used to satisfy an allocation of a given size and is a
> +value between 0 and 1. The higher the value, the more of free memory is
> +unusable and by implication, the worse the external fragmentation is. This
> +can be expressed as a percentage by multiplying by 100.

That's going to hurt my brain.  Why didn't it report usable free blocks?

Also, the index is scaled by the actual amount of free memory in the
zones, yes?  So to work out how many order-N pages are available you
first need to know how many free pages there are?

Seems complicated.

quoted text
>  
> +
> +struct contig_page_info {
> +	unsigned long free_pages;
> +	unsigned long free_blocks_total;
> +	unsigned long free_blocks_suitable;
> +};
> +
> +/*
> + * Calculate the number of free pages in a zone, how many contiguous
> + * pages are free and how many are large enough to satisfy an allocation of
> + * the target size. Note that this function makes no attempt to estimate
> + * how many suitable free blocks there *might* be if MOVABLE pages were
> + * migrated. Calculating that is possible, but expensive and can be
> + * figured out from userspace
> + */
> +static void fill_contig_page_info(struct zone *zone,
> +				unsigned int suitable_order,
> +				struct contig_page_info *info)
> +{
> +	unsigned int order;
> +
> +	info->free_pages = 0;
> +	info->free_blocks_total = 0;
> +	info->free_blocks_suitable = 0;
> +
> +	for (order = 0; order < MAX_ORDER; order++) {
> +		unsigned long blocks;
> +
> +		/* Count number of free blocks */
> +		blocks = zone->free_area[order].nr_free;
> +		info->free_blocks_total += blocks;
> +
> +		/* Count free base pages */
> +		info->free_pages += blocks << order;
> +
> +		/* Count the suitable free blocks */
> +		if (order >= suitable_order)
> +			info->free_blocks_suitable += blocks <<
> +						(order - suitable_order);
> +	}
> +}
> +
> +/*
> + * Return an index indicating how much of the available free memory is
> + * unusable for an allocation of the requested size.
> + */
> +static int unusable_free_index(unsigned int order,
> +				struct contig_page_info *info)
> +{
> +	/* No free memory is interpreted as all free memory is unusable */
> +	if (info->free_pages == 0)
> +		return 1000;
> +
> +	/*
> +	 * Index should be a value between 0 and 1. Return a value to 3
> +	 * decimal places.
> +	 *
> +	 * 0 => no fragmentation
> +	 * 1 => high fragmentation
> +	 */
> +	return div_u64((info->free_pages - (info->free_blocks_suitable << order)) * 1000ULL, info->free_pages);
> +
> +}
> +
> +static void unusable_show_print(struct seq_file *m,
> +					pg_data_t *pgdat, struct zone *zone)
> +{
> +	unsigned int order;
> +	int index;
> +	struct contig_page_info info;
> +
> +	seq_printf(m, "Node %d, zone %8s ",
> +				pgdat->node_id,
> +				zone->name);
> +	for (order = 0; order < MAX_ORDER; ++order) {
> +		fill_contig_page_info(zone, order, &info);
> +		index = unusable_free_index(order, &info);
> +		seq_printf(m, "%d.%03d ", index / 1000, index % 1000);
> +	}
> +
> +	seq_putc(m, '\n');
> +}
> +
> +/*
> + * Display unusable free space index
> + * XXX: Could be a lot more efficient, but it's not a critical path
> + */
> +static int unusable_show(struct seq_file *m, void *arg)
> +{
> +	pg_data_t *pgdat = (pg_data_t *)arg;
> +
> +	/* check memoryless node */
> +	if (!node_state(pgdat->node_id, N_HIGH_MEMORY))
> +		return 0;
> +
> +	walk_zones_in_node(m, pgdat, unusable_show_print);
> +
> +	return 0;
> +}
> +
>  static void pagetypeinfo_showfree_print(struct seq_file *m,
>  					pg_data_t *pgdat, struct zone *zone)
>  {
> @@ -603,6 +703,25 @@ static const struct file_operations pagetypeinfo_file_ops = {
>  	.release	= seq_release,
>  };
>  
> +static const struct seq_operations unusable_op = {
> +	.start	= frag_start,
> +	.next	= frag_next,
> +	.stop	= frag_stop,
> +	.show	= unusable_show,
> +};
> +
> +static int unusable_open(struct inode *inode, struct file *file)
> +{
> +	return seq_open(file, &unusable_op);
> +}
> +
> +static const struct file_operations unusable_file_ops = {
> +	.open		= unusable_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= seq_release,
> +};
> +
>  #ifdef CONFIG_ZONE_DMA
>  #define TEXT_FOR_DMA(xx) xx "_dma",
>  #else
> @@ -947,6 +1066,7 @@ static int __init setup_vmstat(void)
>  #ifdef CONFIG_PROC_FS
>  	proc_create("buddyinfo", S_IRUGO, NULL, &fragmentation_file_operations);
>  	proc_create("pagetypeinfo", S_IRUGO, NULL, &pagetypeinfo_file_ops);
> +	proc_create("unusable_index", S_IRUGO, NULL, &unusable_file_ops);
>  	proc_create("vmstat", S_IRUGO, NULL, &proc_vmstat_file_operations);
>  	proc_create("zoneinfo", S_IRUGO, NULL, &proc_zoneinfo_file_operations);
>  #endif

All this code will be bloat for most people, I suspect.  Can we find a
suitable #ifdef wrapper to keep my cellphone happy?

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