linux-kernel - Re: [PATCH 3/3] writeback: Reporting dirty thresholds in /proc/vmstat

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Date:	Fri, 20 Aug 2010 11:16:48 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Michael Rubin <mrubin@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, jack@...e.cz, riel@...hat.com,
	akpm@...ux-foundation.org, david@...morbit.com, npiggin@...e.de,
	hch@....de, axboe@...nel.dk
Subject: Re: [PATCH 3/3] writeback: Reporting dirty thresholds in
 /proc/vmstat

On Thu, Aug 19, 2010 at 01:57:27PM -0700, Michael Rubin wrote:
> The kernel already exposes the desired thresholds in /proc/sys/vm with
> dirty_background_ratio and background_ratio. Instead the kernel may
> alter the number requested without giving the user any indication that
> is the case.

You mean the 5% lower bound in global_dirty_limits()? Let's rip it :)

> Knowing the actual ratios the kernel is honoring can help app developers
> understand how their buffered IO will be sent to the disk.
> 
> 	$ grep threshold /proc/vmstat
> 	nr_pages_dirty_threshold 409111
> 	nr_pages_dirty_background_threshold 818223

It's redundant to have _pages in the names. /proc/vmstat has the
tradition to use nr_dirty instead of nr_pages_dirty.

They do look like useful counters to export, especially when we do
dynamic dirty limits in future.

> Signed-off-by: Michael Rubin <mrubin@...gle.com>
> ---
>  include/linux/mmzone.h |    2 ++
>  mm/vmstat.c            |    8 ++++++++
>  2 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index f160481..7c4a3bf 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -114,6 +114,8 @@ enum zone_stat_item {
>  #endif
>  	NR_PAGES_ENTERED_WRITEBACK, /* number of times pages enter writeback */
>  	NR_FILE_PAGES_DIRTIED,      /* number of times pages get dirtied */
> +	NR_PAGES_DIRTY_THRESHOLD,   /* writeback threshold */
> +	NR_PAGES_DIRTY_BG_THRESHOLD,/* bg writeback threshold */

s/_PAGES//

>  	NR_VM_ZONE_STAT_ITEMS };
>  
>  /*
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index e177a40..8b5bc78 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -17,6 +17,7 @@
>  #include <linux/vmstat.h>
>  #include <linux/sched.h>
>  #include <linux/math64.h>
> +#include <linux/writeback.h>
>  
>  #ifdef CONFIG_VM_EVENT_COUNTERS
>  DEFINE_PER_CPU(struct vm_event_state, vm_event_states) = {{0}};
> @@ -742,6 +743,8 @@ static const char * const vmstat_text[] = {
>  #endif
>  	"nr_pages_entered_writeback",
>  	"nr_file_pages_dirtied",
> +	"nr_pages_dirty_threshold",
> +	"nr_pages_dirty_background_threshold",

s/_pages//

>  #ifdef CONFIG_VM_EVENT_COUNTERS
>  	"pgpgin",
> @@ -901,6 +904,7 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
>  #ifdef CONFIG_VM_EVENT_COUNTERS
>  	unsigned long *e;
>  #endif
> +	unsigned long dirty_thresh, dirty_bg_thresh;
>  	int i;
>  
>  	if (*pos >= ARRAY_SIZE(vmstat_text))
> @@ -918,6 +922,10 @@ static void *vmstat_start(struct seq_file *m, loff_t *pos)
>  		return ERR_PTR(-ENOMEM);
>  	for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++)
>  		v[i] = global_page_state(i);
> +
> +	get_dirty_limits(&dirty_thresh, &dirty_bg_thresh, NULL, NULL);

2.6.36-rc1 will need this:

        global_dirty_limits(v + NR_DIRTY_THRESHOLD, v + NR_DIRTY_BG_THRESHOLD);

Thanks,
Fengguang

> +	v[NR_PAGES_DIRTY_THRESHOLD] = dirty_thresh;
> +	v[NR_PAGES_DIRTY_BG_THRESHOLD] = dirty_bg_thresh;
>  #ifdef CONFIG_VM_EVENT_COUNTERS
>  	e = v + NR_VM_ZONE_STAT_ITEMS;
>  	all_vm_events(e);
> -- 
> 1.7.1
> 
> --
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