Re: [PATCH 04/12] Export fragmentation index via /proc/pagetypeinfo

[KOSAKI Motohiro]

> Fragmentation index is a value that makes sense when an allocation of a
> given size would fail. The index indicates whether an allocation failure is
> due to a lack of memory (values towards 0) or due to external fragmentation
> (value towards 1).  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/pagetypeinfo.
> 
> The index is normally calculated as a value between 0 and 1 which is
> obviously unsuitable within the kernel. Instead, the first three decimal
> places are used as a value between 0 and 1000 for an integer approximation.

Hmmm..

I haven't understand why admin need to know two metrics (unusable-index
and fragmentation-index). they have very similar meanings and easy confusable
imho.

Can we make just one user friendly metrics?


> 
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> ---
>  Documentation/filesystems/proc.txt |   11 ++++++
>  mm/vmstat.c                        |   63 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 74 insertions(+), 0 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 0968a81..06bf53c 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -618,6 +618,10 @@ Unusable free space index at order
>  Node    0, zone      DMA                         0      0      0      2      6     18     34     67     99    227    485
>  Node    0, zone    DMA32                         0      0      1      2      4      7     10     17     23     31     34
>  
> +Fragmentation index at order
> +Node    0, zone      DMA                        -1     -1     -1     -1     -1     -1     -1     -1     -1     -1     -1
> +Node    0, zone    DMA32                        -1     -1     -1     -1     -1     -1     -1     -1     -1     -1     -1
> +
>  Number of blocks type     Unmovable  Reclaimable      Movable      Reserve      Isolate
>  Node 0, zone      DMA            2            0            5            1            0
>  Node 0, zone    DMA32           41            6          967            2            0
> @@ -639,6 +643,13 @@ value between 0 and 1000. The higher the value, the more of free memory is
>  unusable and by implication, the worse the external fragmentation is. The
>  percentage of unusable free memory can be found by dividing this value by 10.
>  
> +The fragmentation index, is only meaningful if an allocation would fail and
> +indicates what the failure is due to. A value of -1 such as in the example
> +states that the allocation would succeed. If it would fail, the value is
> +between 0 and 1000. A value tending towards 0 implies the allocation failed
> +due to a lack of memory. A value tending towards 1000 implies it failed
> +due to external fragmentation.
> +
>  If min_free_kbytes has been tuned correctly (recommendations made by hugeadm
>  from libhugetlbfs http://sourceforge.net/projects/libhugetlbfs/), one can
>  make an estimate of the likely number of huge pages that can be allocated
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index d05d610..e2d0cc1 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -494,6 +494,35 @@ static void fill_contig_page_info(struct zone *zone,
>  }
>  
>  /*
> + * A fragmentation index only makes sense if an allocation of a requested
> + * size would fail. If that is true, the fragmentation index indicates
> + * whether external fragmentation or a lack of memory was the problem.
> + * The value can be used to determine if page reclaim or compaction
> + * should be used
> + */
> +int fragmentation_index(struct zone *zone,
> +				unsigned int order,
> +				struct contig_page_info *info)
> +{
> +	unsigned long requested = 1UL << order;
> +
> +	if (!info->free_blocks_total)
> +		return 0;
> +
> +	/* Fragmentation index only makes sense when a request would fail */
> +	if (info->free_blocks_suitable)
> +		return -1;
> +
> +	/*
> +	 * Index is between 0 and 1 so return within 3 decimal places
> +	 *
> +	 * 0 => allocation would fail due to lack of memory
> +	 * 1 => allocation would fail due to fragmentation
> +	 */
> +	return 1000 - ( (1000+(info->free_pages * 1000 / requested)) / info->free_blocks_total);
> +}

Dumb question. I haven't understand why this calculation represent
fragmentation index. Do this have theorical background? if yes, can you
please tell me the pointer?




> +
> +/*
>   * Return an index indicating how much of the available free memory is
>   * unusable for an allocation of the requested size.
>   */
> @@ -516,6 +545,39 @@ static int unusable_free_index(struct zone *zone,
>  
>  }
>  
> +static void pagetypeinfo_showfragmentation_print(struct seq_file *m,
> +					pg_data_t *pgdat, struct zone *zone)
> +{
> +	unsigned int order;
> +
> +	/* Alloc on stack as interrupts are disabled for zone walk */
> +	struct contig_page_info info;
> +
> +	seq_printf(m, "Node %4d, zone %8s %19s",
> +				pgdat->node_id,
> +				zone->name, " ");
> +	for (order = 0; order < MAX_ORDER; ++order) {
> +		fill_contig_page_info(zone, order, &info);
> +		seq_printf(m, "%6d ", fragmentation_index(zone, order, &info));
> +	}
> +
> +	seq_putc(m, '\n');
> +}
> +
> +/*
> + * Display fragmentation index for orders that allocations would fail for
> + * XXX: Could be a lot more efficient, but it's not a critical path
> + */
> +static int pagetypeinfo_showfragmentation(struct seq_file *m, void *arg)
> +{
> +	pg_data_t *pgdat = (pg_data_t *)arg;
> +
> +	seq_printf(m, "\nFragmentation index at order\n");
> +	walk_zones_in_node(m, pgdat, pagetypeinfo_showfragmentation_print);
> +
> +	return 0;
> +}
> +
>  static void pagetypeinfo_showunusable_print(struct seq_file *m,
>  					pg_data_t *pgdat, struct zone *zone)
>  {
> @@ -657,6 +719,7 @@ static int pagetypeinfo_show(struct seq_file *m, void *arg)
>  	seq_putc(m, '\n');
>  	pagetypeinfo_showfree(m, pgdat);
>  	pagetypeinfo_showunusable(m, pgdat);
> +	pagetypeinfo_showfragmentation(m, pgdat);
>  	pagetypeinfo_showblockcount(m, pgdat);
>  
>  	return 0;
> -- 
> 1.6.5
> 



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