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    Subject[PATCH 0/5] writeback: kernel visibility
    Date
    Patch #1 sets up some helper functions for account_page_dirty and fixes
    a bug in ceph

    Patch #2 sets up some helper functions for account_page_writeback

    Patch #3 adds writeback visibility in /proc/vmstat

    To help developers and applications gain visibility into writeback
    behaviour this patch adds two counters to /proc/vmstat.

    # grep nr_dirtied /proc/vmstat
    nr_dirtied 3747
    # grep nr_written /proc/vmstat
    nr_written 3618

    These entries allow user apps to understand writeback behaviour over
    time and learn how it is impacting their performance. Currently there
    is no way to inspect dirty and writeback speed over time. It's not
    possible for nr_dirty/nr_writeback.

    These entries 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 in depth 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 way to know how active it is over the whole system,
    if it's falling behind or to quantify it's efforts. 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.

    Patch #4 adds a pernode vmstat file with nr_dirtied and nr_written

    Patch #5 add writeback thresholds to /proc/vmstat

    Currently these values are in debugfs. But they should be promoted to
    /proc since they are useful for developers who are writing databases
    and file servers and are not debugging the kernel.

    The output is as below:

    # grep threshold /proc/vmstat
    nr_pages_dirty_threshold 409111
    nr_pages_dirty_background_threshold 818223

    Michael Rubin (5):
    mm: exporting account_page_dirty
    mm: account_page_writeback added
    writeback: nr_dirtied and nr_written in /proc/vmstat
    writeback: Adding /sys/devices/system/node/<node>/vmstat
    writeback: Reporting dirty thresholds in /proc/vmstat

    drivers/base/node.c | 14 ++++++++++++++
    fs/ceph/addr.c | 8 +-------
    fs/nilfs2/segment.c | 2 +-
    include/linux/mm.h | 1 +
    include/linux/mmzone.h | 4 ++++
    mm/page-writeback.c | 16 +++++++++++++++-
    mm/vmstat.c | 7 +++++++
    7 files changed, 43 insertions(+), 9 deletions(-)



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