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

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Message-Id: <20100806093312.31F9.A69D9226@jp.fujitsu.com>
Date:	Fri,  6 Aug 2010 09:44:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com, Michael Rubin <mrubin@...gle.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, jack@...e.cz, david@...morbit.com, hch@....de,
	axboe@...nel.dk
Subject: Re: [PATCH 2/2] writeback: Adding pages_dirtied and  pages_entered_writeback

> On Fri,  6 Aug 2010 09:18:59 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > > On Thu, Aug 5, 2010 at 4:56 PM, KOSAKI Motohiro
> > > <kosaki.motohiro@...fujitsu.com> wrote:
> > > > /proc/vmstat already have both.
> > > >
> > > > cat /proc/vmstat |grep nr_dirty
> > > > cat /proc/vmstat |grep nr_writeback
> > > >
> > > > Also, /sys/devices/system/node/node0/meminfo show per-node stat.
> > > >
> > > > Perhaps, I'm missing your point.
> > > 
> > > These only show the number of dirty pages present in the system at the
> > > point they are queried.
> > > The counter I am trying to add are increasing over time. They allow
> > > developers to see rates of pages being dirtied and entering writeback.
> > > Which is very helpful.
> > 
> > Usually administrators get the data two times and subtract them. Isn't it sufficient?
> > 
> 
> Nope.  The existing nr_dirty is "number of pages dirtied since boot"
> minus "number of pages cleaned since boot".  If you do the
> wait-one-second-then-subtract thing on nr_dirty, the result is
> dirtying-bandwidth minus cleaning-bandwidth, and can't be used to
> determine dirtying-bandwidth.

Technically, yes. I meant, _now_, typical administrators are using the 
subtraction.
Do you mean this is wrong? or do you mean you have another use case?

Just curious.


> I can see that a graph of dirtying events versus time could be an
> interesting thing.  I don't see how it could be obtained using the
> existing instrumentation.  tracepoints, probably..

I think it depend on frequency of the usecase. If the usecase is enouth
major, convenience way (e.g. /proc/vmstat) is very helpful.

probably, I haven't understand the usecase of this feature.



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