Re: [PATCH 2/2] writeback: Adding pages_dirtied and pages_entered_writeback

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  6 Aug 2010 09:18:59 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> > On Thu, Aug 5, 2010 at 4:56 PM, KOSAKI Motohiro
> > <kosaki.motohiro@jp.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.

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..