linux-kernel - Re: [PATCH 2/3] writeback: Adding pages_dirtied and pages_entered

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Message-ID: <20100820025111.GB5502@localhost>
Date:	Fri, 20 Aug 2010 10:51:11 +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 2/3] writeback: Adding pages_dirtied and
 pages_entered_writeback

On Thu, Aug 19, 2010 at 01:57:26PM -0700, Michael Rubin wrote:
> To help developers and applications gain visibility into writeback
> behaviour adding four read only sysctl files into /proc/sys/vm.
> These files allow user apps to understand writeback behaviour over time
> and learn how it is impacting their performance.
> 
>    # cat /proc/sys/vm/pages_dirtied
>    3747
>    # cat /proc/sys/vm/pages_entered_writeback
>    3618

As Rik said, /proc/sys is not a suitable place.

Frankly speaking I've worked on writeback for years and never felt
the need to add these counters. What I often do is:

$ vmmon -d 1 nr_writeback nr_dirty nr_unstable

     nr_writeback         nr_dirty      nr_unstable
            68738                0            39568
            66051                0            42255
            63406                0            44900
            60643                0            47663
            57954                0            50352
            55264                0            53042
            52592                0            55715
            49922                0            58385
That is what I get when copying /dev/zero to NFS.

You can find vmmon.c in Andrew Morton's ext3-tools package.
Also attached for your convenience.

I'm very interested in Google's use case for this patch, and why
the simple /proc/vmstat based vmmon tool is not enough.

Thanks,
Fengguang
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