linux-kernel - Re: I/O statistics per process

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Date:	Thu, 28 Sep 2006 12:09:52 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Jay Lan <jlan@...r.sgi.com>
Cc:	roland <devzero@....de>, Fengguang Wu <fengguang.wu@...il.com>,
	linux-kernel@...r.kernel.org, lserinol@...il.com
Subject: Re: I/O statistics per process

On Thu, 28 Sep 2006 11:55:38 -0700
Jay Lan <jlan@...r.sgi.com> wrote:

> Andrew Morton wrote:
> > On Wed, 27 Sep 2006 23:22:02 +0200
> > "roland" <devzero@....de> wrote:
> > 
> >>thanks. tried to contact redflag, but they don`t answer. maybe support is 
> >>being on holiday.... !?
> >>
> >>linux kernel hackers - there is really no standard way to watch i/o metrics 
> >>(bytes read/written) at process level?
> > 
> > The patch csa-accounting-taskstats-update.patch in current -mm kernels
> > (whcih is planned for 2.6.19) does have per-process chars-read and
> > chars-written accounting ("Extended accounting fields").  That's probably
> > not waht you really want, although it might tell you what you want to know.
> > 
> >>it`s extremly hard for the admin to track down, what process is hogging the 
> >>disk - especially if there is more than one task consuming cpu.
> 
> Rolend,
> 
> The per-process chars-read and chars-writeen accounting is made
> available through taskstats interface (see Documentation/accounting/
> taskstats.txt) in 2.6.18-mm1 kernel. Unfortunately, the user-space CSA
> package is still a few months away. You may, for now, write your
> own taskstats application or go a long way to port the in-kernel
> implementation of pagg/job/csa.
> 
> However, the "Externded acocunting fields" patch does not provide you
> straight forward answer. The patch provides accounting data only at
> process termination (just like the BSD accounting) and it seems that
> you want to see which run-away application (ie, alive) eating up your
> disk. The taskstats interface offers a query mode (command-response),
> but currently only delayacct uses that mode. We would need to make
> those data available in the query mode in order for application to
> see accounting data of live processes.

ow.  That is a rather important enhancement to have.

> > 
> > csa-accounting-taskstats-update.patch makes that information available to
> > userspace.
> > 
> > But it's approximate, because
> > 
> > - it doesn't account for disk readahead
> > 
> > - it doesn't account for pagefault-initiated reads (althought it easily
> >   could - Jay?)
> > 
> > - it overaccounts for a process writing to an already-dirty page.
> > 
> >   (We could fix this too: nuke the existing stuff and do
> > 
> > 	current->wchar += PAGE_CACHE_SIZE;
> > 
> >    in __set_page_dirty_[no]buffers().) (But that ends up being wrong if
> >    someone truncates the file before it got written)
> > 
> > - it doesn't account for file readahead (although it easily could)
> > 
> > - it doesn't account for pagefault-initiated readahead (it could)
> > 
> > 
> > hm.  There's actually quite a lot we could do here to make these fields
> > more accurate and useful.  A lot of this depends on what the definition of
> > these fields _is_.  Is is just for disk IO?  Is it supposed to include
> > console IO, or what?

I'd be interested in your opinions on all the above, please.

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