Re: I/O statistics per process

From: Jay Lan
Date: Thu Sep 28 2006 - 18:02:50 EST

Andrew Morton wrote:
> On Thu, 28 Sep 2006 11:55:38 -0700
> Jay Lan <jlan@xxxxxxxxxxxx> wrote:
>> Andrew Morton wrote:
>>> On Wed, 27 Sep 2006 23:22:02 +0200
>>> "roland" <devzero@xxxxxx> 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.

Yes, it is needed to provide accounting on live processes. Both
BSD and CSA traditionally focused on completed processes. I guess
that was the difference between a system accounting and system

I certainly can make this enhancement. :)

>>> 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)

Mmm, i am not a true FS I/O person. The data collection patches i
submitted in Nov 2004 was the code i inherited and has been
used in production system by our CSA customers. We lost a bit in
contents and accuracy when CSA was ported from IRIX to Linux. I am
sure there is room for improvement without much overhead. Maybe FS
I/O guys can chip in?

>>> 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?

Yes, the char_read and char_written are only for disk I/O.

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

Sorry i can not answer you on data colleciton code.

- jay


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