linux-kernel - Re: [PATCH 09/10] mm: expose BDI statistics in sysfs.

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Message-Id: <20070421025528.03105b60.akpm@linux-foundation.org>
Date:	Sat, 21 Apr 2007 02:55:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, nikita@...sterfs.com,
	trond.myklebust@....uio.no, yingchao.zhou@...il.com
Subject: Re: [PATCH 09/10] mm: expose BDI statistics in sysfs.

On Fri, 20 Apr 2007 17:52:03 +0200 Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> Expose the per BDI stats in /sys/block/<dev>/queue/*
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  block/ll_rw_blk.c |   32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
> 
> Index: linux-2.6-mm/block/ll_rw_blk.c
> ===================================================================
> --- linux-2.6-mm.orig/block/ll_rw_blk.c
> +++ linux-2.6-mm/block/ll_rw_blk.c
> @@ -3976,6 +3976,15 @@ static ssize_t queue_max_hw_sectors_show
>  	return queue_var_show(max_hw_sectors_kb, (page));
>  }
>  
> +static ssize_t queue_nr_reclaimable_show(struct request_queue *q, char *page)
> +{
> +	return sprintf(page, "%lld\n", bdi_stat(&q->backing_dev_info, BDI_RECLAIMABLE));
> +}

We try to present memory statistics to userspace in bytes or kbytes rather
than number-of-pages.  Because page-size varies between architectures and
between .configs.  Displaying number-of-pages is just inviting people to write
it-broke-when-i-moved-it-to-ia64 applications.

Plus kbytes is a bit more user-friendly, particularly when the user will
want to compare these numbers to /proc/meminfo, for example.

Using %llu might be more appropriate than %lld.

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