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Date: Tue, 16 Feb 2010 16:59:05 +0900 (JST)
From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To: Mel Gorman <mel@....ul.ie>
Cc: kosaki.motohiro@...fujitsu.com,
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>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 04/12] Export fragmentation index via /proc/pagetypeinfo
> 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@....ul.ie>
> ---
> 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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