Date:	Tue,  3 Aug 2010 15:19:07 -0700
From:	Michael Rubin <mrubin@...gle.com>
To:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Cc:	jack@...e.cz, akpm@...ux-foundation.org, david@...morbit.com,
	hch@....de, axboe@...nel.dk, Michael Rubin <mrubin@...gle.com>
Subject: [PATCH 0/2] Adding four writeback files in /proc/sys/vm

Patch #1 sets up some helper functions for accounting.

Patch #2 adds some writeback files for visibility

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
   # cat /proc/sys/vm/dirty_threshold_kbytes
   816673
   # cat /proc/sys/vm/dirty_background_threshold_kbytes
   408336

The files fall into two groups.

pages_dirtied and pages_entered_writeback:

These two new files are necessary to give visibility into writeback
behaviour. We have /proc/diskstats which lets us understand the io in
the block layer. We have blktrace for more indepth understanding. We have
e2fsprogs and debugsfs to give insight into the file systems behaviour,
but we don't offer our users the ability understand what writeback is
doing. There is no non-debugfs way to know how active it is, if it's
falling behind or to quantify it's efforts on a system. With these values
exported users can easily see how much data applications are sending
through writeback and also at what rates writeback is processing this
data. Comparing the rates of change between the two allow developers
to see when writeback is not able to keep up with incoming traffic and
the rate of dirty memory being sent to the IO back end.  This allows
folks to understand their io workloads and track kernel issues. Non
kernel engineers at Google often use these counters to solve puzzling
performance problems.

dirty_threshold_kbytes and dirty_background_threshold kbytes:

We already expose these thresholds in /proc/sys/vm with
dirty_background_ratio and background_ratio. What's frustrating about
the ratio variables and the need for these are that they are not
honored by the kernel. Instead the kernel may alter the number
requested without giving the user any indication that is the case.
An app developer can set the ratio to 2% but end up with 5% as
get_dirty_limits makes sure it is never lower than 5% when set from
the ratio. Arguably that can be fixed too but the limits which decide
whether writeback is invoked to aggressively clean dirty pages is
dependent on changing page state retrieved in
determine_dirtyable_memory. It makes understanding when the kernel
decides to writeback data a moving target that no app can ever
determine. With these thresholds visible and collected over time it
gives apps a chance to know why writeback happened, or why it did not.
As systems get larger and larger RAM developers use the ratios to
predict when their workloads will see writeback invoked. Today there
is no way to accurately indicate what the kernel will use to kick off
writeback. Hence the need for these two new files.

Michael Rubin (2):
  mm: helper functions for dirty and writeback accounting
  writeback: Adding four read-only files to /proc/sys/vm

 Documentation/sysctl/vm.txt |   41 ++++++++++++++++++++++--
 drivers/base/node.c         |   14 ++++++++
 fs/ceph/addr.c              |    8 +---
 fs/nilfs2/segment.c         |    2 +-
 include/linux/mm.h          |    1 +
 include/linux/mmzone.h      |    2 +
 include/linux/writeback.h   |   17 ++++++++++
 kernel/sysctl.c             |   28 ++++++++++++++++
 mm/page-writeback.c         |   73 +++++++++++++++++++++++++++++++++++++++++--
 mm/vmstat.c                 |    2 +
 10 files changed, 174 insertions(+), 14 deletions(-)

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